ALBERT

Description: Abstract Understanding the mechanisms by which earthquake cycles produce folding and accommodate shortening is essential to quantify the seismic potential of active faults and integrate aseismic slip within our understanding of the physical mechanisms of the long‐term deformation. However, measuring such small deformation signals in mountainous areas is challenging with current space‐geodesy techniques, due to the low rates of motion relative to the amplitude of the noise. Here we successfully carry out a multitemporal Interferometric Synthetic Aperture Radar analysis over the North Qaidam fold‐thrust system in NE Tibet, where eight Mw〉 5.2 earthquakes occurred between 2003 and 2009. We report various cases of aseismic slip uplifting the thickened crust at short wavelengths. We provide a rare example of a steep, shallow, 13‐km‐long and 6‐km‐wide afterslip signal that coincides spatially with an anticline and that continues into 2011 in response to a Mw 6.3 event in 2003. We suggest that a buried seismic slip during the 2003 earthquake has triggered both plastic an‐elastic folding and aseismic slip on the shallow thrusts. We produce a first‐order two‐dimensional model of the postseismic surface displacements due to the 2003 earthquake and highlight a segmented slip on three fault patches that steepen approaching the surface. This study emphasizes the fundamental role of shallow aseismic slip in the long‐term and permanent deformation of thrusts and folds and the potential of Interferometric Synthetic Aperture Radar for detecting and characterizing the spatiotemporal behavior of aseismic slip over large mountainous regions.

Description: Abstract Structural details of the crust play an important role in controlling the distribution of volcanic activity in arc systems. In southwest Washington, several different regional structures associated with accretion and magmatism have been invoked to explain the broad distribution of Cascade volcanism in this region. In order to image these regional structures in the upper crust, Pg and Sg travel times from the imaging Magma Under St. Helens (iMUSH) active‐source seismic experiment are inverted for Vp, Vs, and Vp/Vs models in the region surrounding Mount St. Helens. Several features of these models provide new insights into the regional structure of the upper crust. A large section of the Southern Washington Cascades Conductor is imaged as a low Vp/Vs anomaly that is inferred to represent a broad sedimentary/metasedimentary sequence that composes the upper crust in this region. The accreted terrane Siletzia is imaged west of Mount St. Helens as north/south trending high Vp and Vp/Vs bodies. The Vp/Vs model shows relatively high Vp/Vs regions near Mount St. Helens and the Indian Heaven Volcanic Field, which could be related to the presence of magmatic fluids. Separating these two volcanic regions below 6‐km depth is a northeast trending series of high Vp and Vs bodies. These bodies have the same orientation as several volcanic/magmatic features at the surface, including Mount St. Helens and Mount Rainier, and it is argued that these high‐velocity features are a regional‐scale group of intrusive bodies associated with a crustal weak zone that focuses magma ascent.

Description: Abstract Seismicity of several intraplate seismic zones in the North American midcontinent is believed to be related to reactivation of ancient faults in Precambrian continental rifts by the contemporary stress field. Existence of such a rift system beneath the Wabash Valley Seismic Zone (WVSZ) is not clear. Here we obtained a crustal structural image along a 300‐km‐long profile across WVSZ using a dense linear seismic array. We first calculated teleseismic receiver functions of stations and applied the Common‐Conversion‐Point stacking method to image crustal interfaces and the Moho. We then used ambient noise cross correlation to obtain phase and group velocities of Rayleigh and Love waves. Finally, we jointly inverted the receiver function and surface wave dispersion data to determine shear wave velocity structure along the profile. The results show a thick (50‐ to 60‐km) crust with a typical Proterozoic crustal layering: a 1‐ to 2‐km thick Phanerozoic sedimentary layer, an upper crust ∼15 km thick, and a 30‐ to 40‐km‐thick lower crust. The unprecedented high‐resolution image also reveals a 50‐km‐wide high‐velocity body above an uplifted Moho and several velocity anomalies in the upper and middle crust beneath the La Salle Deformation Belt. We interpreted them as features produced by magmatic intrusions in a failed, immature continental rift during the end of Precambrian. Current seismicity in WVSZ is likely due to reactivation of ancient faults of the rift system by a combination of stress fields from the far‐field plate motion and prominent crustal and upper mantle heterogeneities in the region.

Description: Abstract The Charlevoix Seismic Zone (CSZ) is located along the early Paleozoic St. Lawrence rift zone in southeastern Quebec at the location of a major Devonian impact structure. The impact structure superimposed major, steeply dipping basement faults trending approximately N35°E. Approximately 250 earthquakes are recorded each year and are concentrated within and beneath the impact structure. Most M4+ earthquakes associated with the rift faults occurred outside the impact structure. Apart from the unique distribution of earthquakes, stress inversion of focal mechanisms shows stress rotations within the CSZ, and in the CSZ relative to the stress orientation determined from borehole breakouts. The primary goal of this research is to investigate the combined effects of the preexisting structures and regional stresses on earthquake activity and stress rotations in the CSZ. We approach this using PyLith, a finite‐element code for simulations of crustal deformation. Adopting the results from recent hypocenter relocation and 3‐D tomography studies, we modify the locations and dips of the rift faults and assess the effect of the new fault geometries on stress distributions. We also discuss the effects of resolved velocity anomalies. We find that the observed stress rotation is due to the combined effect of the rift faults and the impact structure. One‐dimensional velocity models of the CSZ with an embedded impact structure and a combination of 65°‐40°‐40° and constant 70° fault dip models with a very low friction coefficient of 0.3 and cohesion of 0 MPa can explain the observed seismicity and more than 50% of the stress rotations.

Description: Abstract Seismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench‐parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab‐driven corner flow. Its formation mechanism is currently unclear. Here we present high‐resolution 3‐D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench‐parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity 〉 vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab‐mantle decoupling at a common depth of ~70 km. On the basis of the high‐resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench‐parallel shear wave splitting measurements dominant in subduction zones globally.

Description: Abstract We investigate 3‐D seismic structures (Vp, Vs, and Poisson's ratio) and Vp azimuthal anisotropy in the source area of the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan. Its mainshock occurred at the edge of a high‐Vp (2–4%) seismogenic zone. Significant low‐Vs (−1% to −3%) and high Poisson's ratio (2–7%) anomalies are imaged in and below the source zone and extend to the upper surface of the subducting Pacific slab, most likely reflecting ascending fluids released by the slab dehydration. A high consistency between the fault plane and the low‐Vs and high Poisson's ratio anomalies indicates that the fluids may have entered the fault and affected the rupture nucleation. A high‐V (1–3%) anomaly is revealed in the fore‐arc mantle wedge and connects with the high‐V seismogenic zone, probably reflecting a lithospheric fragment and contributing to cool down the mantle wedge. Complex seismic anisotropy is revealed in the crust in and around the source area, which may reflect complicated stress regime and strong structural heterogeneities there.

Description: ABSTRACT Blended acquisition along with efficient spatial sampling is capable of providing high‐quality seismic data in a cost‐effective and productive manner. While deblending and data reconstruction conventionally accompany this way of data acquisition, the recorded data can be processed directly to estimate subsurface properties. We establish a workflow to design survey parameters that account for the source blending as well as the spatial sampling of sources and detectors. The proposed method involves an iterative scheme to derive the survey design leading to optimum reflectivity and velocity estimation via joint migration inversion. In the workflow, we extend the standard implementation of joint migration inversion to cope with the data acquired in a blended fashion along with irregular detector and source geometries. This makes a direct estimation of reflectivity and velocity models feasible without the need of deblending or data reconstruction. During the iterations, the errors in reflectivity and velocity estimates are used to update the survey parameters by integrating a genetic algorithm and a convolutional neural network. Bio‐inspired operators enable the simultaneous update of the blending and sampling operators. To relate the choice of survey parameters to the performance of joint migration inversion, we utilize a convolutional neural network. The applied network architecture discards suboptimal solutions among newly generated ones. Conversely, it carries optimal ones to the subsequent step, which improves the efficiency of the proposed approach. The resultant acquisition scenario yields a notable enhancement in both reflectivity and velocity estimation attributable to the choice of survey parameters.

Description: Abstract Recent laboratory evidence shows that compaction creep in porous rocks may develop through stages of acceleration, especially if the material is susceptible to strain localization. This paper provides a mechanical interpretation of compaction creep based on viscoplasticity and nonlinear dynamics. For this purpose, a constitutive operator describing the evolution of compaction creep is defined to evaluate the spontaneous accumulation of pore collapse within an active compaction band. This strategy enables the determination of eigenvalues associated with the stability of the response, i.e. able to differentiate decelerating from accelerating strain. This mathematical formalism was linked to a constitutive law able to simulate compaction localization. Material point simulations were then used to identify the region of the stress space where unstable compaction creep is expected, showing that accelerating strains correspond to pulses of inelastic strain rate. Such pulses were also found in full‐field numerical analyses of delayed compaction, revealing that they correspond to stages of inception and propagation of new bands across the volume of the simulated sample. These results illustrate the intimate relation between the spatial patterns of compaction and their temporal dynamics, showing that while homogeneous compaction develops with decaying rates of accumulation, localized compaction occurs through stages of accelerating deformation caused by the loss of strength taking place during the formation of a band. In addition, they provide a predictive modeling framework to simulate and explain the spatiotemporal dynamics of compaction in porous sedimentary formations.

Description: ABSTRACT Detailed P wave velocity and anisotropy structure of the uppermost mantle below the central United States is presented based on a tomographic inversion of Pn traveltimes for earthquakes in the range 2 to 14°. Dense raypath coverage throughout the northern Mississippi Embayment is obtained using the Northern Embayment Lithosphere Experiment and U.S. Transportable Array data sets. A detailed analysis of the trade‐off between velocity and anisotropy variations demonstrates that both are well resolved over most of the study area. Anomalously fast Pn velocities are identified below the northern Mississippi Embayment, centered on the New Madrid seismic zone. A prominent region of low velocity coincides with the southwestern margin of the Illinois basin. Pn anisotropy displays complex patterns and differs from absolute plate motion directions and SKS splitting directions. A circular pattern of fast anisotropy directions is centered on the New Madrid seismic zone and may be related to the presence of the mafic “rift pillow.”

Description: Abstract Sedimentary relative paleointensity (RPI) records are often carried by complex magnetic mineral mixtures, including detrital and biogenic magnetic minerals. Recent studies have demonstrated that magnetic inclusions within larger detrital silicate particles can make significant contributions to sedimentary paleomagnetic records. However, little is known about the role such inclusions play in sedimentary paleomagnetic signal recording. We analyzed paleomagnetic and mineral magnetic data for marine sediment core MD01‐2421 from the North Pacific Ocean, offshore of central Japan, to assess how magnetic inclusions and other detrital magnetic minerals record sedimentary paleomagnetic signals. Stratigraphic intervals in which abundant magnetic inclusions dominate the magnetic signal are compared with other intervals to assess quantitatively their contribution to sedimentary RPI signals. The normalized remanence record from core MD01‐2421 does not correlate clearly with global RPI stacks, which we attribute to a demonstrated lower paleomagnetic recording efficiency of magnetic inclusions compared to other detrital magnetic minerals. We also carried out the first laboratory redeposition experiments under controlled Earth‐like magnetic fields for particles with magnetic inclusions using material from core MD01‐2421. Our results confirm that such particles can be aligned by ambient magnetic fields but with a lower magnetic recording efficiency compared to other detrital magnetic minerals, which is consistent with normalized remanence data from core MD01‐2421. Our demonstration of the role of sedimentary magnetic inclusions should have wide applicability for understanding sedimentary paleomagnetic recording.

Description: Abstract The mechanical dynamics of volcanic systems can be better understood with detailed knowledge on strength of a volcanic edifice and subsurface. Previous work highlighting this on Mt. Etna has suggested that its carbonate basement could be a significant zone of widespread planar weakness. Here, we report new deformation experiments to better quantify such effects. We measure and compare key deformation parameters using Etna basalt (EB), which is representative of upper edifice lava flows, and Comiso limestone (CL), which is representative of the carbonate basement, under upper crustal conditions. These data are then used to derive empirical constitutive equations describing changes in rocks strength with pressure, temperature and strain rate. At a constant strain rate of 10‐5 s‐1 and an applied confining pressure of 50 MPa the brittle to ductile transitions were observed at 975 °C (EB) and 350 °C (CL). For the basaltic edifice of Mt. Etna, the strength is described with a Mohr‐coulomb failure criterion with μ ~0.704, C = 20 MPa. For the carbonate basement, strength is best described by a power law‐type flow in two regimes: a low‐T regime with stress exponent n ~5.4 and an activation energy Q ~ 170.6 kJ/mol and a high‐T regime with n~ 2.4 and Q ~ 293.4 kJ/mol. We show that extrapolation of these data to Etna's basement predicts a brittle to ductile transition that corresponds well with the generally observed trends of the seismogenic zone underneath Mt. Etna. This in turn may be useful for future numerical simulations of volcano‐tectonic deformation of Mt. Etna, and other volcanoes with limestone basements.

Description: Geophysical Prospecting, Volume 0, Issue ja, -Not available-.

Description: ABSTRACT Transverse isotropy with a vertical axis of symmetry is a common form of anisotropy in sedimentary basins, and it has a significant influence on the seismic amplitude variation with offset. Although exact solutions and approximations of the PP‐wave reflection coefficient for the transversely isotropic media with vertical axis of symmetry have been explicitly studied, it is difficult to apply these equations to amplitude inversion, because more than three parameters need to be estimated, and such an inverse problem is highly ill‐posed. In this paper, we propose a seismic amplitude inversion method for the transversely isotropic media with a vertical axis of symmetry based on a modified approximation of the reflection coefficient. This new approximation consists of only three model parameters: attribute A, the impedance (vertical phase velocity multiplied by bulk density); attribute B, shear modulus proportional to an anellipticity parameter (Thomsen's parameter ε−δ); and attribute C, the approximate horizontal P‐wave phase velocity, which can be well estimated by using a Bayesian‐framework‐based inversion method. Using numerical tests we show that the derived approximation has similar accuracy to the existing linear approximation and much higher accuracy than isotropic approximations, especially at large angles of incidence and for strong anisotropy. The new inversion method is validated by using both synthetic data and field seismic data. We show that the inverted attributes are robust for shale‐gas reservoir characterization: the shale formation can be discriminated from surrounding formations by using the crossplot of the attributes A and C, and then the gas‐bearing shale can be identified through the combination of the attributes A and B. We then propose a rock‐physics‐based method and a stepwise‐inversion‐based method to estimate the P‐wave anisotropy parameter (Thomsen's parameter ε). The latter is more suitable when subsurface media are strongly heterogeneous. The stepwise inversion produces a stable and accurate Thomsen's parameter ε, which is proved by using both synthetic and field data.

Description: Abstract On 20 April 2013, an Mw 6.6 Lushan earthquake occurred on the southwestern segment of the Longmen Shan fault belt, which is the tectonic block boundary between the eastern Tibetan plateau and the Sichuan basin. Seismic reflection profiles and aftershock relocation indicate that there exists a back thrust fault in the source region but whether it is ruptured during the Lushan earthquake remains controversial. Here the precise leveling data are firstly used together with Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and strong motion data to invert for the fault geometry and slip distribution associated with the earthquake. The joint inversion result shows that the Lushan earthquake occurred on a blind thrust fault with strike N208.5 °E and dip 42.1° to the NW and did not rupture the back reverse fault. The coseismic slip model reveals the Lushan earthquake involves the rupture of one major asperity. The coseismic slip is mainly concentrated on a steeply dipping fault plane. The coseismic rupture terminates on the southwestern side of the seismic gap between the Wenchuan and Lushan earthquakes. Topographic stress may be the dominant mechanism of coseismic rupture termination.

Description: Abstract Experimental data show that inelastic straining occurs even at very low pressure before and during “brittle” fracturing. This process is therefore investigated within the framework of elastoplasticity using 2D, 3‐layer FD modeling. The constitutive model includes both tensile and shear failure mechanisms coupled at the level of the strain softening law. The modeling results show that sets of parallel joints initiate as pure dilation bands, the narrow σ3‐normal bands of localized dilatant damage (inelastic deformation). The band thickness, length, and the initial strain softening degree within it are proportional to the ductility of the material, which increases with the effective stress level (σ1) or pressure. The strength reduction within the bands is accelerated at a certain stage, and the strength locally reaches zero resulting in fracture initiation. The initial fracture then propagates in mode I following the propagating band. The fracture (joint) appears thus as a band of damaged material with the increased porosity, which is maximum along the axial zone of the band where the material is completely broken. The damage is due to both tensile and shear mechanisms. The role of shear failure increases with the ductility (pressure) increase, which also leads to the band thickness increase. These processes can result in small (band thickness)‐scale shear fractures within the band, causing the increase in the roughness of fracture walls organized in plumose patterns typical of both natural and experimentally generated joints.

Description: ABSTRACT The subsurface media are not perfectly elastic, thus anelastic absorption, attenuation and dispersion (aka Q filtering) effects occur during wave propagation, diminishing seismic resolution. Compensating for anelastic effects is imperative for resolution enhancement. Q values are required for most of conventional Q‐compensation methods, and the source wavelet is additionally required for some of them. Based on the previous work of non‐stationary sparse reflectivity inversion , we evaluate a series of methods for Q‐compensation with/without knowing Q and with/without knowing wavelet. We demonstrate that if Q‐compensation takes the wavelet into account, it generates better results for the severely attenuated components, benefiting from the sparsity promotion. We then evaluate a two‐phase Q‐compensation method in the frequency domain to eliminate Q requirement. In phase 1, the observed seismogram is disintegrated into the least number of Q‐filtered wavelets chosen from a dictionary by optimizing a basis pursuit denoising problem, where the dictionary is composed of the known wavelet with different propagation times, each filtered with a range of possible values. The elements of the dictionary are weighted by the infinity norm of the corresponding column and further preconditioned to provide wavelets of different values and different propagation times equal probability to entry into the solution space. In phase 2, we derive analytic solutions for estimates of reflectivity and Q and solve an over‐determined equation to obtain the final reflectivity series and Q values, where both the amplitude and phase information are utilized to estimate the Q values. The evaluated inversion‐based Q estimation method handles the wave‐interference effects better than conventional spectral‐ratio‐based methods. For Q‐compensation, we investigate why sparsity promoting does matter. Numerical and field data experiments indicate the feasibility of the evaluated method of Q‐compensation without knowing Q but with wavelet given.

Description: Abstract Deciphering the relationship between lateral growth of faults and along‐strike deformation (i.e., shortening and uplift) in the Earth's upper crust remains a challenge. Here we gain insight into the relation between these processes by studying the Kashi anticline, an asymmetric, doubly plunging thrust‐fault‐related fold located in the southwest Tian Shan, China. We use seismic interpretation and field observations, together with 2‐D trishear and excess area methods, to quantify the distribution of shortening along this structure. The shortening distribution along strike of the Kashi anticline is nonlinear and has a peaked, asymmetric, bell shape, with a maximum value of 5.9 ± 0.2 km. After comparing the 3‐D structural model of the Kashi anticline and our trishear models, we propose that lateral propagation‐to‐maximum shortening ratio, initiation fault length, and lateral propagation rate control the lateral fault propagation process and the fold terminations. Moreover, the 3‐D fault morphology and the ages of the growth strata suggest that the Kashi anticline experienced two stages of lateral growth with propagation rates of 60 km/Ma between 1.4 ± 0.2 Myr and 0.9 ± 0.3 Ma, and ~67 km/Myr from 0.9 ± 0.3 Ma to present. These observations highlight the relation between the evolution of lateral fault growth and the along‐strike shortening distribution, allowing us to use the latter (which we can measure) to infer the former (which we cannot). These novel insights from the Kashi anticline can be used to understand lateral growth of thrust and normal faults worldwide.

Description: Abstract Seismically detected ultralow velocity zones (ULVZs) at the the core‐mantle boundary (CMB) reflect the dynamical state and geological evolution of the silicate‐metal frontier of Earth's deep interior. However, modeling the dynamical context of ULVZs is hampered by challenges, such as the necessity of fine scale resolution and the accurate treatment of large viscosity contrasts. Here we extend the treatment of ULVZs using a lubrication theory approach and apply it to numerical and analytical models relevant for mantle convection in the CMB region. A generic model of a thin and dense low viscosity ULVZ layer embedded between an overlying convecting viscous mantle and an underlying inviscid core can explain several features that are consistent with seismic inferences, such as the absence of ULVZs in some regions and a tabular shape where they are concentrated. The model explains how the topography of a ULVZ layer tends to saturate and flatten as it becomes thicker, due to a non‐linear feedback between viscous aggregation beneath upwelling mantle currents and gravitational spreading/relaxation. Implementation of the ULVZ equation in thermal convection models indicates that ULVZs are preferentially gathered beneath long‐lived plumes, and may not exist beneath newly formed plume roots where there is no source of layer material. The presence/absence of ULVZs and their detailed shapes may provide important insights into the dynamical state and convective instability of the lowermost mantle thermal boundary layer.

Description: Abstract We implement a Coulomb rate‐and‐state approach to explore the nonlinear relation between stressing rate and seismicity rate in the Groningen gas field. Coulomb stress rates are calculated, taking into account the 3‐D structural complexity of the field and including the poroelastic effect of the differential compaction due to fault offsets. The spatiotemporal evolution of the Groningen seismicity must be attributed to a combination of both (i) spatial variability in the induced stressing rate history and (ii) spatial heterogeneities in the rate‐and‐state model parameters. Focusing on two subareas of the Groningen field where the observed event rates are very contrasted even though the modeled seismicity rates are of similar magnitudes, we show that the rate‐and‐state model parameters are spatially heterogeneous. For these two subareas, the very low background seismicity rate of the Groningen gas field can explain the long delay in the seismicity response relative to the onset of reservoir depletion. The characteristic periods of stress perturbations, due to gas production fluctuations, are much shorter than the inferred intrinsic time delay of the earthquake nucleation process. In this regime the modeled seismicity rate is in phase with the stress changes. However, since the start of production and for two subareas of our analysis, the Groningen fault system is unsteady and it is gradually becoming more sensitive to the stressing rate.

Description: Abstract From 1963 to 1973 the U.S. Geological Survey (USGS) measured heat flow at 356 sites in the Amerasian Basin (Western Arctic Ocean) from a drifting ice island (T‐3). The resulting measurements, which are unevenly distributed on Alpha‐Mendeleev Ridge (AMR) and in Canada and Nautilus basins, greatly expand available heat flow data for the Arctic Ocean. Average T‐3 heat flow is ~54.7 ± 11.3 mW m‐2, and Nautilus Basin is the only well‐surveyed area (~13% of data) with significantly higher average heat flow (63.8 mW m‐2). Heat flow and bathymetry are not correlated at a large scale, and turbiditic surficial sediments (Canada and Nautilus basins) have higher heat flow than the sediments that blanket the AMR. Thermal gradients are mostly near‐linear, implying that conductive heat transport dominates and that near‐seafloor sediments are in thermal equilibrium with overlying bottom waters. Combining the heat flow data with modern seismic imagery suggests that some of the observed heat flow variability may be explained by local changes in sediment thickness or lithology or the presence of basement faults that channel circulating seawater. A numerical model that incorporates thermal conductivity variations along a profile from Canada Basin (thick sediment on mostly oceanic crust) to Alpha Ridge (thin sediment over thick magmatic units associated with the High Arctic Large Igneous Province) predicts heat flow lower than that observed on Alpha Ridge. This, along with other observations, implies that circulating fluids modulate conductive heat flow and contribute to high variability in the T‐3 dataset.

Description: Abstract The Early Cretaceous Ontong Java Plateau (OJP) in the southwestern Pacific Ocean is the largest oceanic plateau by volume on Earth, and a broad range of observations has been conducted to reveal its formation and evolution. However, because seafloor seismic observations of the OJP and surrounding areas have been insufficient so far, such experiments are capable of generating additional information regarding the crustal and mantle structure of the OJP. To image seismic velocity discontinuities from the crust to the uppermost mantle, we applied receiver function (RF) analysis to seismic records acquired by 17 broadband ocean bottom seismometers deployed across the region in and around the OJP and 3 broadband stations located on ocean islands in Micronesia (one: permanent, two: temporary). The results revealed mid‐crustal discontinuities and the Moho at depths of 10–20 km and 30–40 km (from the top of the basement), respectively, in the central OJP. Moreover, a mantle discontinuity was also imaged at the depth of 55–60 km (from the top of the basement) in the central OJP. These boundaries were not imaged outside the OJP, implying they are characteristic features of the OJP. In addition, RF images showed Moho signals at the depth of 20 km in the eastern OJP, where few previous seismic exploration surveys have been conducted. This depth is comparable with that found in the Manihiki and Hikurangi plateaus that were potentially separated from the OJP.

Description: Abstract Previous compilation of crustal structure in South America had large unsampled areas including the thin crust in the Sub‐Andean lowlands, largely estimated by gravity data, and the sparsely sampled Amazon Craton. A deployment of 35 seismic stations in Brazil, Bolivia, Paraguay, Argentina and Uruguay improved the coverage of the Pantanal Basin in Western Brazil, the intracratonic Paraná and the Chaco basins. Crustal thicknesses and Vp/Vs ratios were estimated with a modified H‐k method by producing three stacked traces to enhance the three Moho conversions (the direct Ps and the two multiples Ppps and Ppss). This modified method gives lower uncertainties than previous studies and shows more regional consistency between nearby stations. The temporary stations and the Brazilian network (RSBR) have characterized the crustal structure as follows. The Paraná Basin has a thick crust 40‐45 km, and average Vp/Vs ratio (1.71‐1.77), while the Chaco Basin has a slightly thinner crust (35‐40 km) and higher Vp/Vs ratio (1.75‐1.79). This confirms the lack of widespread magmatic underplating in the Paraná Basin that could be related to the origin of the flood basalts during the South Atlantic opening. A belt of thin crust (30‐35 km) with low Vp/Vs (〈1.74) is confined to the eastern edge of the Pantanal Basin. Normal crust (38‐43 km) is observed along the western edge of the Pantanal, from the southern part of the Amazon craton to the Rio Apa cratonic block. This study, combined with other published data, provides an updated crustal thickness map of South America.

Description: Abstract We consider fluid‐induced seismicity and present closed‐form expressions for the elastic displacements, strains and stresses resulting from injection into or production from a reservoir with displaced faults. We apply classic inclusion theory to two‐dimensional finite‐width and infinite‐width reservoir models. First we simplify the fault model to the bare minimum while still maintaining its essential features: a vertical fault in a homogeneous reservoir of infinite width in an infinite domain. We confirm and sharpen findings from earlier numerical studies and furthermore conclude that the development of infinitely large elastic shear stresses in a displaced fault, at the internal and external reservoir/fault corners, implies that even small amounts of injection or production will result in some amount of slip or other non‐elastic deformation. Another finding is that there is a marked difference between the shear stress patterns resulting from injection and production in a reservoir with a displaced fault. In both situations two slip patches emerge but at the start of injection some amount of slip occurs immediately in the overburden and underburden, whereas during production the slip may remain inside the reservoir region. Next we derive similar, but more complicated expressions for displaced inclined (normal or reverse) faults and conclude that our findings for vertical faults also apply to inclined faults.

Description: Abstract The collision of the Indian plate with Eurasia has played a major role in controlling the dynamics of central Asia leading to the world's largest continental deformation zone. In order to study the deformation within the Indian plate as well as the India‐Eurasia collision zone, we model the lithospheric stress field by calculating the two primary sources of stress, one arising due to topography and shallow lithospheric structure estimated by gravitational potential energy (GPE) differences and the other arising from basal tractions derived from density driven mantle convection. We use several tomography models to calculate horizontal tractions using the convection code HC for two radially varying viscosity structures. We also take into account lateral viscosity variations in the lithosphere model arising from stiff cratons, weak plate boundaries and strength variations due to old and young oceanic lithosphere. We do a quantitative comparison of our predicted deviatoric stresses, strain rates and plate velocities with surface observables and find that the regional tomography model of (A. Singh, Mercier, Ravi Kumar, Srinagesh, & Chadha, 2014) embedded in the global S‐wave model S40RTS does a remarkable job of fitting the observations of GPS velocities and strain rates as well as intraplate stress field from the World Stress Map.

Description: Abstract The eastern and northeastern Tibetan plateau is a key region to study the growth and expansion of the plateau and associated extrusion tectonics. We studied the seismic anisotropic structure in this region by shear‐wave splitting analysis of teleseismic records from a dense linear seismic array, to constrain the lithospheric deformation and processes. We detected small‐scale variations in anisotropy, including changes of splitting parameters around major faults and different anisotropy patterns among individual tectonic blocks and units but with consistent interior features. Our results combined with previous observations suggest that, in addition to the dominant effects of lateral extrusion induced by the India‐Eurasia collision, major faults and tectonic heterogeneity may have also exerted significant impacts on the deformation and thus anisotropic structure of the lithosphere. In particular, we constructed two‐layer anisotropy models for both the Longmenshan sub‐block in the easternmost Songpan‐Ganzi terrane and the Western Qinling orogen, indicating crust‐mantle decoupling in these areas. The lower anisotropic layer of both areas shows a general NW‐SE fast polarization direction (FPD). We attribute this feature to the large‐scale mantle deformation, due to the lateral extrusion of Tibet associated with the India‐Eurasia collision. The upper‐layer anisotropy in both areas features an optimal NEE‐SWW FPD. While in the Longmenshan sub‐block it may stem from crustal deformation under the combined effects of mid‐lower crustal flow, faulting and tectonic heterogeneity, that in the Western Qinling Orogen is probably resulted from shearing caused by upper‐crustal displacement along a mid‐crustal detachment.

Description: Abstract Cross‐correlation of fully diffuse wavefields averaged over time should converge to the Green's function; however, the ambient seismic field in the real Earth is not fully diffuse, which interferes with that convergence. We apply blind signal separation to reduce the effect of spurious non‐diffuse components on the cross‐correlation tensor of the ambient seismic field. We describe the diffuse component as having uncorrelated neighboring frequencies and equal intensity at all azimuths, and an independent (i.e., statistically uncorrelated) non‐diffuse component arising from a spatially isolated point source for which neighboring frequencies are correlated. Under the assumption of linear independence of the spurious non‐diffuse wave outside the stationary phase zone and the constructive interference of noise waves within that zone, we can suppress the spurious non‐diffuse component from the noise interferometry. Our numerical simulations show good separation of one spurious non‐diffuse noise source component for either non‐diffuse Rayleigh or Love waves. We apply this separation to the Rayleigh‐wave component of the Green's function for 136 cross‐correlation pairs from 17 stations in Southern California. We perform beamforming over different frequency bands for the cross‐correlations before and after the separation, and find that the reconstructed Rayleigh waves are more coherent. We also estimate the bias in Rayleigh wave phase velocity for each receiver pair due to the spurious non‐diffuse contribution.

Description: No abstract is available for this article.

Description: ABSTRACT We consider a transversely isotropic (TI) medium that is long‐wave equivalent to a stack of thin, parallel, isotropic layers and is obtained using the Backus average. In such media, we analyze the relations among anisotropy parameters; Thomsen parameters, ε, and δ, and a new parameter φ. We discuss the last parameter and show its essential properties; it equals to zero in the case of isotropy of equivalent medium and/or constant Lamé coefficient Λ in layers. The second property occurs to make φ sensitive to variations of Λ in thin‐bedded sequences. According to Gassmann, in isotropic media the variation of fluid content affects only the Lamé coefficient Λ, not μ; thus, the sensitivity to changes of Λ is an essential property in the context of possible detection of fluids. We show algebraically and numerically that φ is more sensitive to these variations than ε or δ. Nevertheless, each of these parameters is dependent on the changes of μ; to understand this influence, we exhibit comprehensive tables that illustrate the behavior of anisotropy parameters with respect to specific variations of Λ and μ. The changes of μ in layers can be presented by the Thomsen parameter γ that depends on them solely. Hence, knowing the values of elasticity coefficients of equivalent TI medium, we may compute φ and γ, and based on the aforementioned tables, we predict the expected variation of Λ; in this way, we propose a new method of possible fluid detection. Also, we show that the prior approach of possible detection of fluids, proposed by Berryman et al., may be unreliable in specific cases. To establish our results we use the Monte Carlo (MC) method; for the range and chosen variations of Lamé coefficients Λ and μ—relevant to sandstones—we generate these coefficients in thin layers and, after the averaging process, we obtain an equivalent TI medium. We repeat that process numerous times to get many equivalent TI media, and—for each of them—we compute their anisotropy parameters. We illustrate φ, ε, and δ in the form of cross‐plots that are relevant to the chosen variations of Λ and μ. Additionally, we present a table with the computed ranges of anisotropy parameters that correspond to different variations of Lamé coefficients. This article is protected by copyright. All rights reserved

Description: ABSTRACT Saltbodies are important subsurface structures that have significant implications for hydrocarbon accumulation and sealing in petroleum reservoirs, and accurate saltbody imaging and delineation is now greatly facilitated with the availability of three‐dimensional seismic surveying. However, with the growing demand for larger survey coverage and higher imaging resolution, the size of seismic data is increasing dramatically. Correspondingly, manual saltbody interpretation fails to offer an efficient solution, particularly in exploration areas of complicated salt intrusion history. Recently, artificial intelligence is attracting great attention from geoscientists that desire to utilize the popular machine learning (ML) technologies for evolving the interpretational tools capable of mimicking an experienced interpreter's intelligence. This study first implements two popular ML tools, the multi‐layer perceptron (MLP) and the convolutional neural network (CNN), for delineating seismic saltbodies at sample‐ and pattern‐levels, respectively, then compares their performance through applications to the synthetic SEAM seismic volume, and moreover tentatively investigates what contributes to the better CNN delineation. Specifically, the MLP scheme is capable of efficiently utilizing an interpreter's knowledge by selecting, pre‐conditioning, and integrating a set of seismic attributes that best highlight the target saltbodies, whereas the CNN scheme makes it possible for saltbody delineation directly from seismic amplitude and thus significantly reduces the dependency on attribute selection from interpreters. It is concluded that the better performance from the CNN scheme results from two factors. First, the CNN builds the mapping relationship between the seismic signals and the saltbodies using the original seismic amplitude instead of manually selected seismic attributes, so that the negative impact of using less representative attributes is virtually eliminated. Second and more importantly, the CNN defines, learns, and identifies the saltbodies by utilizing local seismic reflection patterns, so that the seismic noises and processing artifacts of distinct patterns are effectively identified and excluded. This article is protected by copyright. All rights reserved

Description: ABSTRACT In this paper, we deduced the corresponding first‐order velocity–stress equation for curvilinear coordinates from the first‐order velocity–stress equation based on the modified Biot/squirt model for a two‐dimensional two‐phase medium. The equations are then numerically solved by an optimized high‐order non‐staggered finite difference scheme, that is, the dispersion relation preserving/optimization MacCormack scheme. To implement undulating free‐surface topography, we derive an analytical relationship between the derivatives of the particle velocity components and use the compact finite‐difference scheme plus a traction‐image method. In the undulating free surface and the undulating subsurface interface of two‐phase medium, the complex reflected wave and transmitted wave can be clearly recognized in the numerical simulation results. The simulation results show that the curvilinear‐grid finite‐difference method, which uses a body‐conforming grid to describe the undulating surface, can accurately reduce the numerical scattering effect of seismic wave propagation caused by the use of ladder‐shaped grid to fit the surfaces when undulating topography is present in a two‐phase isotropic medium.

Description: ABSTRACT Seismic methods are becoming an established choice for deep mineral exploration after being extensively tested and employed for the past two decades. To investigate whether the early European mineral exploration datasets had potential for seismic imaging that was overlooked, we recover a low‐fold legacy seismic dataset from the Neves‐Corvo mine site in the Iberian Pyrite Belt in southern Portugal. This dataset is comprised of six, 4–6 km long, profiles acquired in 1996 for deep targeting. Using today's processing algorithms, the world‐class, ca. 150 Mt, Lombador massive sulphide and other smaller deposits were better imaged. Additionally, we also reveal a number of shallow but steeply dipping reflections that were absent in the original processing results. This study highlights that legacy seismic data are valuable and should be revisited regularly to take advantage of new processing algorithms and the experiences gained from processing such data in hard‐rock environments elsewhere. Remembering that an initial processing job in hard‐rock should always aim to first obtain an overall image of the subsurface and make reflections visible, and then subsequent goals of the workflow could be set to, for example, understanding relative amplitude ratios. The imaging of the known mineralization implies that this survey could likely have been among one of the pioneer studies in the world that demonstrated the capacity of directly imaging massive sulphide deposits using the seismic method. This article is protected by copyright. All rights reserved

Description: Abstract Top‐side reverberations off mantle discontinuities are commonly observed at long periods, but their interpretation is complicated because they include both near‐source and near‐receiver reflections. We have developed a method to isolate the station‐side reflectors in large data sets with many sources and receivers. Analysis of USArray transverse‐component data from 3200 earthquakes, using direct S as a reference phase, shows clear reflections off the 410‐ and 660‐km discontinuities, which can be used to map the depth and brightness of these features. Because our results are sensitive to the impedance contrast (velocity and density), they provide a useful complement to receiver‐function studies, which are primarily sensitive to the S velocity jump alone. In addition, reflectors in our images are more spread out in time than in receiver functions, providing good depth resolution. Our images show strong discontinuities near 410 and 660 km across the entire USArray footprint, with intriguing reflectors at shallower depths in many regions. Overall, the discontinuities in the east appear simpler and more monotonous with a uniform transition zone thickness of ~250 km compared to the western United States. In the west, we observe more complex discontinuity topography and small‐scale changes below the Great Basin and the Rocky Mountains, and a decrease in transition‐zone thickness along the western coast. We also observe a dipping reflector in the west that aligns with the top of the high‐velocity Farallon slab anomaly seen in some tomography models, but which also may be an artifact caused by near‐surface scattering of incoming S waves.

Description: Abstract Inelastic rheological behavior, such as viscoelasticity, is increasingly utilized in the modeling of volcanic ground deformation, as elevated thermal regimes induced by magmatic systems may necessitate the use of a mechanical model containing a component of time‐dependent viscous behavior. For the modeling of a given amplitude and footprint of ground deformation, incorporating a viscoelastic regime has been shown to reduce the magma reservoir overpressure requirements suggested by elastic models. This phenomenon, however, is restricted to pressure‐based analyses and the associated creep behavior. Viscoelastic materials exhibit additional constitutive time‐dependent behaviors, determined by the stress and strain states, that are yet to be analyzed in the context of volcanic ground deformation. By utilizing a mechanically homogeneous model space and distinct reservoir evolutions, we provide a comparison of three viscoelastic rheological models, including the commonly implemented Maxwell and Standard Linear Solid configurations, and their time‐dependent behaviors from a fundamental perspective. We also investigate the differences between deformation time series resulting from a pressurization or volume change, two contrasting approaches that are assumed to be equivalent through elastic modeling. Our results illustrate that the perceived influence of viscoelasticity is dependent on the mode of deformation, with stress‐based pressurization models imparting enhanced deformation relative to the elastic models, thus reducing pressure requirements. Strain‐based volumetric models, however, exhibit reduced levels of deformation and may produce episodes of apparent ground subsidence induced by source inflation or vice versa, due to the relaxation of crustal stresses, dependent on whether the reservoir is modeled to be expanding or contracting, respectively.

Description: Abstract Layer 2A, the porous and permeable uppermost igneous oceanic crust, permits the circulation of fluid within the crust, the exchange of dissolved mineral species between the ocean and crust, and the convective dissipation of heat from the crust. We examine the presence, temporal extent, thickness, and evolution of layer 2A using multichannel seismic data collected at 30°S in the South Atlantic across crustal age ranges of 0–70 Ma and half spreading rates of 12–31 mm/year. We observe the layer 2A/2B boundary in 0–48 Myr old crust but not in crust older than ~48 Ma. The thickness of layer 2A in the South Atlantic has substantial variability, with a mean of 760 m and a standard deviation of 290 m. Layer 2A has no systematic change in thickness with age in the South Atlantic, and thickness does not correlate with spreading rate. The crust in the South Atlantic is never fully sealed by sediment cover, which implies that the fluid circulation system in the upper crust never becomes fully closed and the thickness of layer 2A can work as a proxy for the depth at which significant circulation can occur. The disappearance of the layer 2A/2B boundary in older crust implies that fluid circulation within the upper crust continues to occur for at least ~48 Myr after crustal formation in the South Atlantic, after which layer 2A becomes indistinguishable from layer 2B in reflection images.

Description: Abstract There is growing evidence that outgassing through transient fracture networks exerts an important control on conduit processes and explosive‐effusive activity during silicic eruptions. Indeed, the first modern observations of rhyolitic eruptions have revealed that degassed lava effusion may depend upon outgassing during simultaneous pyroclastic venting. The outgassing is thought to occur as gas and pyroclastic debris are discharged through shallow fracture networks within otherwise low‐permeability, conduit‐plugging lava domes. However, this discharge is only transient, as these fractures become clogged and eventually blocked by the accumulation and sintering of hot, melt‐rich pyroclastic debris, drastically reducing their permeability and creating particle‐filled tuffisites. In this study we present the first published permeability measurements for rhyolitic tuffisites, using samples from the recent rhyolitic eruptions at Chaitén (2008‐2009) and Cordón Caulle (2011‐2012) in Chile. To place constraints on tuffisite permeability evolution, we combine (1) laboratory measurements of the porosity and permeability of tuffisites that preserve different degrees of sintering, (2) theoretical estimates on grainsize‐ and temperature‐dependent sintering timescales, and (3) H2O diffusion constraints on pressure‐time paths. The inferred timescales of sintering‐driven tuffisite compaction and permeability loss, spanning seconds (in the case of compaction‐driven sintering) to hours (surface tension‐driven sintering), coincide with timescales of diffusive degassing into tuffisites, observed vent pulsations during hybrid rhyolitic activity (extrusive behaviour coincident with intermittent explosions) and, more broadly, timescales of pressurisation accompanying silicic lava dome extrusion. We discuss herein the complex feedbacks between fracture opening, closing, and sintering, and their role in outgassing rhyolite lavas and mediating hybrid explosive‐effusive activity.

Description: Abstract We present the results of tomographic studies using seismic velocity and attenuation in the area of the Colima Volcanic complex (CVC). Our dataset comprises body waves from local earthquakes recorded by the temporary seismic stations of the CODEX network in the Colima area and a few stations of the regional Mapping the Rivera Subduction Zone (MARS) networks, both deployed in 2006–2008. We obtain three‐dimensional distributions of seismic velocities and attenuation in the crust beneath the CVC area. At shallow depths, we observe a large negative anomaly to the south of CVC, coinciding with the location of the Central Colima Graben. This anomaly may represent debris avalanche deposits, as well as shallow magma reservoirs feeding the eruptions of the presently active Volcán de Colima. In contrast, the volcano edifice of Nevado de Colima, which is built of rigid igneous rocks, is associated with high‐velocity and low‐attenuation anomalies at shallow depths. In the deeper section, a major anomaly with high Vp/Vs, low Vs, and high S wave attenuation corresponds to the location of the regional Tamazula fault. As this represents a mechanically weakened zone of the crust, it may form the pathway that feeds CVC. Both velocity and attenuation models show that the fault‐associated conduit brought magma from the mantle through the lower crust to a depth of 15 km. Then, a light fraction of magma may continue to ascend, forming shallow reservoirs beneath the southern flank of CVC.

Description: Abstract At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is the most active volcano of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the PROTEUS experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of 〉14,300 marine sound sources and 156 short period seismometers, covering an area 120 km by 45 km. Here, a P‐wave velocity model of the shallow, upper‐crustal structure (〈3 km depth), obtained using travel‐time tomography, is used to delineate fault zones, sedimentary basins, and tectono‐magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E‐W oriented basement faults within the Christiana basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi basins, are coincident with a region of extensive NE‐SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find regional magmatism has been localized in NE‐SW trending basin‐like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE‐SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation.

Description: Abstract The aftershock productivity is known to strongly vary for different mainshocks of the same magnitude, which cannot be simply explained by random fluctuations. In addition to variable source mechanisms, different rheological properties might be responsible for the observed variations. Here we show, for the subduction zone of northern Chile, that the aftershock productivity is linearly related to the degree of mechanical coupling along the subduction interface. Using the earthquake catalog of Sippl et al. (2018, https://doi.org/10.1002/2017JB015384), which consists of more than 100,000 events between 2007 and 2014, and three different coupling maps inferred from interseismic geodetic deformation data, we show that the observed aftershock numbers are significantly lower than expected from the Båth's law. Furthermore, the productivity decays systematically with depth in the uppermost 80 km, while the b value increases. We show that this lack of aftershocks and the observed depth dependence can be simply explained by a linear relationship between the productivity and the coupling coefficient, leading to Båth law only in the case of full coupling. Our results indicate that coupling maps might be useful to forecast aftershock productivity and vice versa.

Description: Abstract Spectral induced polarization spectra were carried out on three graphitic schists and two graphitic sandstones. The microstructural arrangement of graphite of two graphitic schists was studied with thin sections using transmitted and reflected light optical and electron microscopic methods. Chemical maps of selected areas confirm the presence of carbon. The complex conductivity spectra were measured in the frequency range 10 mHz to 45 kHz and in the temperature range +20 °C down to −15 °C. The measured spectra are fitted with a double Cole‐Cole complex conductivity model with one component associated with the polarization of graphite and the second component associated with the Maxwell‐Wagner polarization. The Cole‐Cole exponent and the chargeability are observed to be almost independent of temperature including in freezing conditions. The conductivity and relaxation time are dependent on the temperature in a predictable way. As long as the temperature decreases, the electrical conductivity decreases and the relaxation time increases. A finite element model is able to reproduce the observed results. In this model, we consider an intragrain polarization mechanism for the graphite and a change of the conductivity of the background material modeled with an exponential freezing curve. One of the core sample (a black schist), very rich in graphite, appears to be characterized by a very high conductivity (approximately 30 S/m). Two induced polarization profiles are discussed in the area of Thorens. The model is applied to the chargeability data to map the volumetric content of graphite.

Description: Abstract Receiver function analysis is widely used to image sharp structures in the Earth, such as the Moho or transition zone discontinuities. Standard procedures either rely on the assumption that underlying discontinuities are horizontal (common conversion point stacking) or are computationally expensive and usually limited to 2‐D geometries (reverse time migration and generalized Radon transform). Here, we develop a teleseismic imaging method that uses fast 3‐D traveltime calculations with minimal assumption about the underlying structure. This allows us to achieve high computational efficiency without limiting ourselves to 1‐D or 2‐D geometries. In our method, we apply acoustic Kirchhoff migration to transmitted and reflected teleseismic waves (i.e., receiver functions). The approach expands on the work of Cheng et al. (2016, https://doi.org/10.1093/gji/ggw062) to account for free surface multiples. We use an Eikonal solver based on the fast marching method to compute traveltimes for all scattered phases. Three‐dimensional scattering patterns are computed to correct the amplitudes and polarities of the three component input signals. We consider three different stacking methods (linear, phase weighted, and second root) to enhance the structures that are most coherent across scattering modes and find that second‐root stack is the most effective. Results from synthetic tests show that our imaging principle can recover scattering structures accurately with minimal artifacts. Application to real data from the Multidisciplinary Experiments for Dynamic Understanding of Subduction under the Aegean Sea experiment in the Hellenic subduction zone yields images that are similar to those obtained by 2‐D generalized Radon transform migration at no additional computational cost, further supporting the robustness of our approach.

Description: Abstract Forecasting the onset of a volcanic eruption from a closed system requires understanding its stress state and failure potential, which can be investigated through numerical modeling. However, the lack of constraints on model parameters, especially rheology, may substantially impair the accuracy of failure forecasts. Therefore, it is essential to know whether large variations and uncertainties in rock properties will preclude the ability of models to predict reservoir failure. A series of two‐dimensional, axisymmetric models are used to investigate sensitivities of brittle failure initiation to assumed rock properties. The numerical experiments indicate that the deformation and overpressure at failure onset simulated by elastic models will be much lower than the viscoelastic models, when the timescale of pressurization exceeds the viscoelastic relaxation time of the host rock. Poisson's ratio and internal friction angle have much less effect on failure forecasts than Young's modulus. Variations in Young's modulus significantly affect the prediction of surface deformation before failure onset when Young's modulus is 〈 40 GPa. Longer precursory volcano‐tectonic events may occur in weak host rock (E 〈 40 GPa) due to well‐developed Coulomb failure prior to dike propagation. Thus, combining surface deformation with seismicity may enhance the accuracy of eruption forecast in these situations. Compared to large and oblate magma systems, small and prolate systems create far less surface uplift prior to failure initiation, suggesting that more frequent measurements are necessary.

Description: Abstract Core‐mantle boundary (CMB) topography may provide useful hints on the deep mantle thermochemical structure, as clusters of thermal plumes and piles of chemically differentiated material, which are usually proposed as end‐member explanations for the large low shear‐wave velocity regions observed in the deep mantle, have different actions on this topography. CMB topography is further sensitive to several parameters, including mantle viscosity and its variations with thermal and compositional changes. Here we assess the influence of the postperovskite (pPv) phase viscosity on deep mantle dynamics and on CMB topography. We perform numerical simulations of thermal and thermochemical convection in spherical geometry, varying the ratio between pPv and bridgmanite viscosities, ΔηpPv, between 1 (regular pPv) and 10−3 (weak pPv). Thermochemical structures are dominated by smaller‐scale wavelengths (spherical harmonic degrees 3 to 6) and are more stable in weak than in regular pPv models. The amplitude of CMB topography is reduced by about a factor of 2 as ΔηpPv changes from 1 to 10−3, mostly due to a sharp drop in the depressions induced by downwellings reaching the CMB. By contrast, the topographies induced by plumes clusters and thermochemical piles are mostly unaffected. For all the values of ΔηpPv we tested, long‐wavelength CMB topography and reconstructed shear‐wave tomography are anticorrelated in purely thermal models, and correlated in thermochemical models with strong chemical density contrast (ΔρC = 140 kg/m3). In models with smaller density contrast (ΔρC = 90 kg/m3), topography and tomography are anticorrelated at ΔηpPv = 1, but correlated at ΔηpPv = 10−3.

Description: Abstract In this study, the micromechanical interparticle contact behavior of “De NoArtri” (DNA‐1A) grains is investigated, which is a lunar regolith simulant, using a custom‐built micromechanical loading apparatus, and the results on the DNA‐1A are compared with Ottawa sand which is a standard quartz soil. Material characterization is performed through several techniques. Based on microhardness intender and surface profiler analyses, it was found that the DNA‐1A grains had lower values of hardness and higher values of surface roughness compared to Ottawa sand grains. In normal contact micromechanical tests, the results showed that the DNA‐1A had softer behavior compared with Ottawa sand grains and that cumulative plastic displacements were observed for the DNA‐1A simulant during cyclic compression, whereas for Ottawa sand grains elastic displacements were dominant in the cyclic sequences. In tangential contact micromechanical tests, it was shown that the interparticle friction values of DNA‐1A were much greater than that of Ottawa sand grains, which was attributed to the softer contact response and greater roughness of the DNA‐1A grains. Widely used theoretical models both in normal and tangential directions were fitted to the experimental data to obtain representative parameters, which can be useful as input in numerical analyses which use the discrete element method.

Description: Abstract Volcanic plumes from small and moderate eruptions represent a challenge in the study of plume morphology due to eruption source parameter uncertainties and atmospheric influence. Sakurajima volcano, Japan, features such activity and due to its continuous eruptions in the recent years provides an ideal natural laboratory. A data set of 896 eruptions between 2009 and 2016 with well‐constrained plume heights, estimated erupted mass, and associated atmospheric conditions has been compiled. Plume heights ranged between 1,500 and 5,000 m and mainly developed under stable atmospheric stratification and low background wind speeds. The eruptions presented in the database were used to drive FPLUME, a 1‐D integral volcanic plume model, to study the simulated plume morphology. FPLUME was seen to provide consistent results under stable atmospheric stratification. A method for the real‐time monitoring of erupted mass used in the Sakurajima observatory was seen to provide appropriate first guess estimates for the eruptions, showing agreement with analytical and simulated mass flow rate calculations. Volcanic plumes from Sakurajima show significant influence by the atmospheric environment. The plume scaling parameter (Π) was used to characterize the expected degree of plume bending with results correlating well against modeled plume angles. The vertical wind profile was seen to have a significant impact on the resolved plume. Wind shear characteristics were seen to have a mechanical effect on the plume, aiding or inhibiting bending. Finally, potential issues were identified in simulations under unstable atmospheric conditions as the model either failed to provide a solution or overestimated the plume height.

Description: Abstract The physical mechanism of intermediate‐depth earthquakes is still uncertain. Dehydration embrittlement and thermal shear heating mechanisms are the leading hypotheses, and each has been supported both by observations and experiments. Slab character is likely to affect either mechanism. We apply uniform processing to data sets from the two main subduction zones in Japan: the older, colder, and faster‐subducting Pacific plate and the younger, warmer, and slower‐subducting Philippine Sea plate. We compare the stress drops and radiated efficiencies of intermediate‐depth earthquakes in these settings and find no significant differences between the scaling of source properties. In particular, we find both an increase of stress drop and apparent stress with increasing moment for the Pacific Plate subduction in Hokkaido and for the Philippine Sea Plate subduction in Kyushu. We suggest that this, along with apparent invariance of radiated efficiency, suggests that an embrittlement process is more important in these regions than a thermal shear mechanism.

Description: Abstract Temperature distribution at depth is of key importance for characterizing the crust, defining its mechanical behavior and deformation. Temperature can be retrieved by heat flow measurements in boreholes that are sparse, shallow, and have limited reliability, especially in active and recently active areas. Laboratory data and thermodynamic modeling demonstrate that temperature exerts a strong control on the seismic properties of rocks, supporting the hypothesis that seismic data can be used to constrain the crustal thermal structure. We use Rayleigh wave dispersion curves and receiver functions, jointly inverted with a transdimensional Monte Carlo Markov Chain algorithm, to retrieve the VS and VP/VS within the crust in the Italian peninsula. The high values (〉1.9) of VP/VS suggest the presence of filled‐fluid cracks in the middle and lower crust. Intracrustal discontinuities associated with large values of VP/VS are interpreted as the α−β quartz transition and used to estimate geothermal gradients. These are in agreement with the temperatures inferred from shear wave velocities and exhibit a behavior consistent with the known tectonic and geodynamic setting of the Italian peninsula. We argue that such methods, based on seismological observables, provide a viable alternative to heat flow measurements for inferring crustal thermal structure.

Description: Abstract Low‐δ26Mg basalts are commonly interpreted to represent melts derived from carbonated mantle sources. The mantle domain feeding low‐δ26Mg Cenozoic basalts in eastern China overlaps the so‐called Big Mantle Wedge (BMW) above the stagnant Pacific slab in the mantle transition zone, which indicates that the BMW is an important carbon reservoir generated by the slab. However, Mg isotopic composition in the nearby mantle beyond the BMW and, thus, the spatial extent of carbonated components in the mantle beneath eastern Asia have not yet been extensively characterized. Therefore, it remains largely unconstrained if additional or alternative carbon reservoirs exist. Here we carried out a geochemical study on Cenozoic Huihe nephelinites, which crop out ~500 km west of the present‐day BMW. These rocks are characterized by negative K, Zr, Hf, and Ti anomalies, high Zr/Hf, Ca/Al ratios, and low δ26Mg values, which suggest that they are derived from a carbonated mantle source. The composition of the nephelinites demonstrates that low δ26Mg mantle components exist at significant distances from the present‐day BMW, which highlights that in addition to the stagnant Pacific slab, other oceanic slab(s) also contribute(s) carbonate‐bearing crustal materials to the mantle sources of Cenozoic volcanism in eastern Asia.

Description: Abstract The 2016–2017 Central Apennines earthquake sequence is a recent example of how damages from subsequent aftershocks can exceed those caused by the initial mainshock. Recent studies reveal that physics‐based aftershock forecasts present comparable skills to their statistical counterparts, but their performance remains a controversial subject. Here we employ physics‐based models that combine the elasto‐static stress transfer with rate‐and‐state friction laws, and short‐term statistical Epidemic Type Aftershock Sequence (ETAS) models to describe the spatiotemporal evolution of the earthquake cascade. We then track the absolute and relative model performance using log‐likelihood statistics for a 1‐year horizon after the 24 August 2016 Mw = 6.0 Amatrice earthquake. We perform a series of pseudoprospective experiments by producing seven classes of Coulomb rate‐state (CRS) forecasts with gradual increase in data input quality and model complexity. Our goal is to investigate the influence of data quality on the predictive power of physics‐based models and to assess the comparative performance of the forecasts in critical time windows, such as the period following the 26 October Visso earthquakes leading to the 30 October Mw = 6.5 Norcia mainshock. We find that (1) the spatiotemporal performance of the basic CRS models is poor and progressively improves as more refined data are used, (2) CRS forecasts are about as informative as ETAS when secondary triggering effects from M3+ earthquakes are included together with spatially variable slip models, spatially heterogeneous receiver faults, and optimized rate‐and‐state parameters. After the Visso earthquakes, the more elaborate CRS model outperforms ETAS highlighting the importance of the static stress transfer for operational earthquake forecasting.

Description: Abstract We present a theoretical study focusing on exploring the possibility of controlling anthropogenic and natural seismicity. We actively control the pressure of injected fluids using a negative‐feedback control system. Our analysis is based on the spring‐slider model for modeling the earthquake instability. We use a general Coulomb‐type rheology for describing the frictional behavior of a fault system. This model leads to a nonautonomous system, whose steady state and stability are studied using a double‐scale asymptotic analysis. This approach renders the dominant order of the system time invariant. Established tools from the classical mathematical theory of control are used for designing a proper stabilizing controller. We show that the system is stabilizable by controlling fluid pressure. This is a central result for industrial operations. A stabilizing controller is then designed and tested. The controller regulates in real time the applied pressure in order to assure stability, avoid unwanted seismicity, and drive the system from unstable states of high potential energy, to stable ones of low energy. The controller performs well even in the absence of complete knowledge of the frictional properties of the system. Finally, we present two numerical examples (scenarios) and illustrate how anthropogenic and natural earthquakes could be, in theory, prevented.

Description: Abstract We integrate paleoseismic datasets along the Mt. Vettore‐Mt. Bove normal fault‐system (VBFS) rupturing at surface in the 30 October 2016 Norcia earthquake. Through the analysis of new trenches from this work and a review of the pre‐existing data, we correlate events among trench sites along antithetic and synthetic fault splays. We recognize seven M6.5, 2016 Norcia‐type (or larger) surface‐faulting events in the last ~22 kyr, including 2016. Before 2016, one event occurred in the past two millennia (260‐575 CE), and possibly corresponds to the event damaging Rome in 443 CE or 484/508 CE. Three previous events occurred between 10590 BCE and 415 BCE, whereas the two oldest ones date between 19820 BCE and 16540 BCE. The average recurrence time is 3360–3640 yrs for the last ~22 kyr, and 1220‐1970 yrs for the last ~4 kyr. We infer a minimum dip‐slip rate of 0.26‐0.38 mm/yr on the master fault in the central portion of the VBFS, and a dip‐slip rate of at least 0.10 mm/yr on the southernmost portion. We infer a Middle‐Late Pleistocene inception of the long‐term scarp of the investigated splays. The along‐strike variation of slip rates well reproduces the trend of the 2016 surface slip, thus the time window exposed in the trenches is representative for the present fault activity. Based on trenching data, different earthquake rupture scenarios should be also considered for local hazard assessment.

Description: ABSTRACT Differential compaction has long been used by seismic interpreters to infer subsurface geology using knowledge of the relative compaction of different types of sediments. We outline a method to infer the gross fraction of shale in an interval between two seismic horizons using sandstone and shale compaction laws. A key component of the method involves reconstruction of a smooth depositional horizon by interpolating decompacted thicknesses from well control. We derive analytic formulae for decompaction calculations using known porosity‐stress relations and do not employ discrete layer iterative methods; these formulae were found not only to depend uponthe gross fraction of shale but also on the clay content of the shales and the thickness of the interval. The relative merits of several interpolation options were explored, and found to depend upon the structural setting.The method was successfully applied to an oil sands project in Alberta, Canada. This article is protected by copyright. All rights reserved

Description: Abstract The mobile south flank of Kīlauea Volcano hosts two normal fault systems, the Koa'e fault system (KFS) and the Hilina fault system (HFS). In historical time, at least three M〉6.5 earthquakes have occurred on the basal detachment of the Kīlauea Volcano's south flank, with the most recent being the May 4, 2018 M6.9 earthquake. Here we analyze kinematic GPS data collected from 2001 to 2017, and InSAR data before, during and after the 2018 M6.9 earthquake to determine the crustal motion across the HFS and KFS faults. Our results indicate that the HFS faults did not significantly slip during the interseismic period from 2007 to 2011. Despite its substantial magnitude, InSAR shows that the 2018 M6.9 earthquake triggered sub‐cm level slip along sections of the previously mapped HFS branches. Up to 20 cm of offset occurred on what appears to be a newly formed (or previously unknown) fault near the eastern end of the HFS. During the 3 months following the M6.9 earthquake, up to more than 30 cm of slip occurred along the KFS, which helps accommodate rapid large‐scale subsidence of Kīlauea's summit region as large volumes of summit reservoir magma fed the lower East Rift Zone eruption. The HFS appears to activate only in concert with large earthquakes on the basal detachment. The KFS, on the other hand, moves both seismically during small local earthquakes, and aseismically in response to nearby earthquakes and caldera subsidence.

Description: Abstract Correlations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (MSWD = 0.69, probability = 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia, and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here, we suggest that both are part of the same large igneous province (~6.5 × 105 km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts, and to provide pivotal time‐constraints for stratigraphic correlations of Precambrian basins.

Description: Abstract Brucite, Mg (OH)2, is an important analog for studying the thermodynamics of hydrous silicate minerals in the deep Earth, as well as H/D isotope fractionation between minerals and water. In this study, we measured in situ Raman and Fourier transform infrared spectra for the natural and deuterated brucite samples, at high temperatures to 650 K, just before the dehydration of brucite at ambient pressure. All of the optical modes systematically shift to lower frequencies at elevated temperature, while deuterium substitution reduces the magnitudes of the temperature dependence. The isobaric mode Grüneisen parameters (γiP), as well as the intrinsic anharmonic parameters (ai), have been evaluated for the vibrational modes between Mg (OH)2 and Mg (OD)2. The anharmonic contribution to the thermodynamic properties (such as internal energy, isochoric and isothermal heat capacities, and entropy) is negative and severe at high temperature. The difference in the heat capacity is up to ~7% at 700 K due to the anharmonic effect. The deuterium isotopic effect on the thermodynamics is positive, and the magnitude of the isotopic effect is comparable to that from the anharmonic effect. On the other hand, the anharmonicity significantly increases the magnitude of the positive pressure dependence of the D/H fractionation β factor for brucite, and this correction could be more important at elevated temperature. At the temperature of 800 K, 103·(∂lnβ/∂P)T increases from +0.23 GPa−1 (for quasi‐harmonic approximation) to +0.44 GPa−1, due to the anharmonic correction.

Description: Abstract To evaluate the effect of melt viscosity on bubble nucleation, we formulated the homogeneous nucleation rate of water bubbles to explicitly include melt viscosity. The viscosity coefficient appears in the preexponential factor of the nucleation rate in terms of the Péclet number: the ratio of the bubble growth timescale by molecular diffusion and the viscous relaxation timescale. The preexponential factor is almost constant when viscosity is low (or a high Péclet number), whereas it linearly decreases with increasing viscosity (or a decreasing Péclet number) exceeding the crossover value of viscosity, under a given supersaturation. The crossover point depends on whether homogeneous or heterogeneous nucleation takes place. We numerically solved the evolution of bubble nucleation and growth processes in ascending magmas by using the new nucleation rate formula and a precise approximation of moment equations of the bubble size distribution function. The resultant bubble number density has two regimes, similar to the previous study, but the transition point between the diffusion‐controlled regime and the viscosity‐controlled regime moves to higher viscosity or higher decompression rates by 0.6 log units at the maximum. In the viscosity‐controlled regime, the effect of the better approximation of bubble size distribution moment equations reduces bubble number density by a few orders of magnitude compared with the previous study. As a result of compiling the past laboratory experimental data, it turned out that all the experiments are conducted under the conditions equivalent to the diffusion‐controlled regime. We propose an experimental condition to confirm the presence of the viscosity‐controlled regime.

Description: Abstract Seismic observations suggest (1) significant accumulation of subducted slabs above the 670‐km discontinuity in many subduction zones, (2) possible structure change at ~1,000‐km depth, and (3) the large low shear wave velocity provinces above the core‐mantle boundary in the African and Pacific lower mantle be associated with chemical heterogeneity. Global mantle convection models with realistic plate motion history reproduce most of these structures. However, it remains unclear how the convection models compare with seismic models at different spatial wavelengths and depths. By conducting quantitative analysis between mantle convection and seismic models, we found that mantle convective structures show significant correlations with seismic structures in the upper mantle and mantle transition zone for wavelengths up to spherical harmonic degree 20. However, the global correlation is weak at intermediate to short wavelengths (for degrees 4 and higher) in the lower mantle below ~1,000‐km depth. A weak layer beneath the spinel‐to‐postspinel phase change help consistently reproduce stagnant slabs in the western Pacific, while having insignificant effects elsewhere, that is, the large low shear wave velocity province structures. The cold slab structures and their correlations with the seismically fast anomalies are nearly identical for our convection models with and without the plumes, indicating that seismically fast anomalies in the mantle mainly result from the subducted slabs. Models with viscosity increase at 1,000‐km depth and the 670‐km depth phase change may reproduce seismic slab structures including the stagnant slabs in the mantle transition zone equally well as models with a thin weak layer below the 670‐km phase boundary.

Description: Abstract Orientations of natural fault systems are subject to large variations. They often contradict classical Coulomb failure theory as they are misoriented relative to the regional Andersonian stress field. This is ascribed to local effects of structural or stress heterogeneities and reorientations of structures or stresses on the long term. To better understand the relation between fault orientation and regional stresses, we simulate spontaneous fault growth and its effect on the stress field. Our approach incorporates earthquake rupture dynamics, viscoelastoplastic brittle deformation and a rate‐ and state‐dependent friction formulation in a continuum mechanics framework. We investigate how strike‐slip faults orient according to local and far‐field stresses during their growth. We identify two modes of fault growth, seismic and aseismic, distinguished by different fault angles and slip velocities. Seismic fault growth causes a significant elevation of dynamic stresses and friction values ahead of the propagating fault tip. These elevated quantities result in a greater strike angle relative to the maximum principal regional stress than that of a fault segment formed aseismically. When compared to the near‐tip time‐dependent stress field the fault orientations produced by both growth modes follow the classical failure theory. We demonstrate how the two types of fault growth may be distinguished in natural faults by comparing their angles relative to the original regional maximum principal stress. A stress field analysis of the Landers‐Kickapoo fault suggests that an angle greater than ∼25° between two faults indicates seismic fault growth.

Description: Abstract Earthquake ruptures dynamically activate coseismic off‐fault damage around fault cores. Systematic field observation efforts have shown the distribution of off‐fault damage around main faults, while numerical modeling using elastic‐plastic off‐fault material models has demonstrated the evolution of coseismic off‐fault damage during earthquake ruptures. Laboratory scale micro‐earthquake experiments have pointed out the enhanced high‐frequency radiation due to the coseismic off‐fault damage. However, the detailed off‐fault fracturing mechanisms, subsequent radiation and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics‐based dynamic earthquake rupture modeling framework, based on the combined finite‐discrete element method (FDEM), to investigate the fundamental mechanisms of coseismic off‐fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2‐D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off‐fault damage. We found the decrease in rupture velocity and the enhanced high‐frequency radiation in near‐field due to the coseismic off‐fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off‐fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. The present numerical framework for the dynamic earthquake rupture modeling thus provides the insight into the earthquake rupture dynamics with the coseismic off‐fault damage.

Description: Abstract Pulse‐like ruptures arise spontaneously in many elastodynamic rupture simulations and seem to be the dominant rupture mode along crustal faults. Pulse‐like ruptures propagating under steady state conditions can be efficiently analyzed theoretically, but it remains unclear how they can arise and how they evolve if perturbed. Using thermal pressurization as a representative constitutive law, we conduct elastodynamic simulations of pulse‐like ruptures and determine the spatiotemporal evolution of slip, slip rate, and pulse width perturbations induced by infinitesimal perturbations in background stress. These simulations indicate that steady state pulses driven by thermal pressurization are unstable. If the initial stress perturbation is negative, ruptures stop; conversely, if the perturbation is positive, ruptures grow and transition to either self‐similar pulses (at low background stress) or expanding cracks (at elevated background stress). Based on a dynamic dislocation model, we develop an elastodynamic equation of motion for slip pulses and demonstrate that steady state slip pulses are unstable if their accrued slip b is a decreasing function of the uniform background stress τb. This condition is satisfied by slip pulses driven by thermal pressurization. The equation of motion also predicts quantitatively the growth rate of perturbations and provides a generic tool to analyze the propagation of slip pulses. The unstable character of steady state slip pulses implies that this rupture mode is a key one determining the minimum stress conditions for sustainable ruptures along faults, that is, their “strength.” Furthermore, slip pulse instabilities can produce a remarkable complexity of rupture dynamics, even under uniform background stress conditions and material properties.

Description: ABSTRACT Reflection seismic data were acquired within two field campaigns in the Blötberget, Ludvika mining area of central Sweden, for deep imaging of iron‐oxide mineralization that were known to extend down to 800–850 m depth. The two surveys conducted in years 2015 and 2016, one employing a seismic landstreamer and geophones connected to wireless recorders, and another one using cabled geophones and wireless recorders, aimed to delineate the geometry and depth extent of the iron‐oxide mineralization for when mining commences in the area. Even with minimal and conventional processing approaches, the merged datasets provide encouraging information about the depth continuation of the mineralized horizons and the geological setting of the study area. Multiple sets of strong reflections represent a possible continuation of the known deposits that extend approximately 300 m further down‐dip than the known 850 m depth obtained from historical drilling. They show excellent correlation in shape and strength with those of the Blötberget deposits. Furthermore, several reflections in the footwall of the known mineralization can potentially be additional resources underlying the known ones. The results from these seismic surveys are encouraging for mineral exploration purposes given the good quality of the final section and fast seismic surveys employing a simple cost‐effective and easily available impact‐type seismic source.

Description: ABSTRACT In an acoustic transversely isotropic medium, there are two waves that propagate. One is the P‐wave and another one is the S‐wave (also known as S‐wave artefact). This paper is devoted to analyse the S‐wave in two‐dimensional acoustic transversely isotropic media with a tilted symmetry axis. We derive the S‐wave slowness surface and traveltime function in a homogeneous acoustic transversely isotropic medium with a tilted symmetry axis. The S‐wave traveltime approximations in acoustic transversely isotropic media with a tilted symmetry axis can be mapped from the counterparts for acoustic transversely isotropic media with a vertical symmetry axis. We consider a layered two‐dimensional acoustic transversely isotropic medium with a tilted symmetry axis to analyse the S‐wave moveout. We also illustrate the behaviour of the moveout for reflected S‐wave and converted waves.

Description: Abstract The Baiyun slide complex contains geological evidence for some of the largest landslide ever discovered in the continental slopes of the South China Sea. High‐resolution seismic data suggest that a variety of landslides with varied scales have occurred repeatedly in this area. The largest landslide reconstructed from bathymetric and seismic data has an estimated spatial coverage of ~5,500 km2 and a conservative volume of ~1,035 km3. Here, using geomorphological and geotechnical data, we construct a series of probable landslide scenarios and assess their tsunamigenic capacity. By treating the slides as deformable mudflows, we simulate the dynamics of landslide movements. The simulated landslide motions match the geophysical observations interpreted in previous studies. Particularly, we are able to reproduce the spatial distribution of observed runout, including the distance, shape, and deposit thickness, for the most credible slide scenario. We investigate tsunami impacts generated by different slide scenarios and highlight the importance of initial water depth, sliding direction, and nearshore bathymetry. The worst‐case scenario is capable of producing basin‐wide tsunami, with maximum wave amplitudes reaching ~5 m near Hong Kong and Macau, 1–3 m in western Philippines, and at least 1 m along central Vietnam, southeast Hainan, and southern Taiwan. The most noticeable phenomenon we observed is that the southern Chinese coast is the hardest‐hit region in all the simulated scenarios regardless of the diverse slide features. We conclude that the persistence of high tsunami impact is caused by the unique bathymetric feature of the wide continental shelf in front of southern China.

Description: Abstract ITSG‐Grace2018 is a new series of GRACE‐only gravity field solutions based on reprocessed GRACE observation data (L1B RL03) and the latest atmosphere and ocean dealiasing product (AOD1B RL06). It includes unconstrained monthly and constrained daily solutions, as well as a high‐resolution static gravity field. Compared to the previous ITSG release, we implemented a number of improvements within the processing chain and use updated background models. In an effort to better model all known error sources, we propagate synthetic orientation uncertainties of the star camera assembly to the antenna offset correction for intersatellite ranging observations. This enables the disentanglement of the stationary noise of the K‐Band system and the nonstationary noise of the antenna offset correction. We further incorporated uncertainties of the atmosphere and ocean dealiasing product to reduce temporal aliasing effects. To mitigate errors in the applied ocean tide model, we used constrained GRACE estimates of selected tidal constituents as an additional background model. Variability over quiet ocean areas suggests a 27% to 46% lower noise level compared to the current spherical harmonic solutions of the official processing centers (300 km Gaussian filter applied). To ensure that the low noise floor is not accompanied by signal loss, we examined drainage basin averages, which showed consistent amplitudes with the official GRACE time series. These evaluations lead to the conclusion that ITSG‐Grace2018 is a state‐of‐the‐art GRACE time series which exhibits an excellent signal‐to‐noise ratio.

Description: Abstract The northern Hikurangi plate boundary fault hosts a range of seismic behaviors, of which the physical mechanisms controlling seismicity are poorly understood, but often related to high pore fluid pressures and conditionally stable frictional conditions. Using 2D marine seismic streamer data, we employ full‐waveform inversion (FWI) to obtain a high‐resolution 2D P‐wave velocity model across the Hikurangi margin down to depths of ~2 km. The validity of the FWI velocity model is investigated through comparison with the pre‐stack depth migrated seismic reflection image, sonic well data, and the match between observed and synthetic waveforms. Our model reveals the shallow structure of the overriding plate, including the fault plumbing system above the zone of SSEs to theoretical resolution of a half seismic wavelength. We find that the hanging walls of thrust faults often have substantially higher velocities than footwalls, consistent with higher compaction. In some cases, intra‐wedge faults identified from reflection data are associated with low‐velocity anomalies, which may suggest they are high‐porosity zones acting as conduits for fluid flow. The continuity of velocity structure away from IODP drill site U1520 suggests that lithological variations in the incoming sedimentary stratigraphy observed at this site continue to the deformation front and are likely important in controlling seismic behavior. This investigation provides a high‐resolution insight into the shallow parts of subduction zones, which shows promise for the extension of modeling to 3D using a recently‐acquired, longer‐offset, seismic dataset.

Description: Abstract Studies of mechanical responses of the Earth crust to large earthquakes can provide us with unique insights into the processes of stress buildup and release. As a complement to geodetic methods that derive crustal strain dynamics from surface observations (e.g., GPS, InSAR), noise‐based seismic velocity monitoring directly probes the mechanical state of the crust, at depth and continuously in time. We investigate the responses of the crust to the Mw 9.0, 2011 Tohoku‐oki earthquake. In addition to the Hi‐net short‐period sensors, we use Hi‐net tiltmeters as long‐period seismometers (8–50 s) to sample the crust below 5 km in depth. The spatial distribution of the strong velocity decreases at short periods appears to be limited to the region of strong ground shaking induced by the 2011 Tohoku‐oki earthquake, while the long‐period velocity changes correlate well with the modeled static strain induced by viscoelastic relaxation and afterslip at depth. Amplitudes of coseismic velocity changes decrease with increasing depth. The temporal evolution of velocity changes in different period bands shows that the maximum drops in the velocity at long periods are delayed in time with respect to the occurrence of the Tohoku‐oki earthquake. The inversion of seismic velocity changes at depth illustrates how S wave velocities evolve down to 40 km at a regional scale after a major earthquake.

Description: Abstract Unconventional reservoirs comprise a growing portion of producible reserves due to increasing knowledge of their nature as well as significant advances in production technology. The development of advanced pore‐scale modeling techniques presents potential for better estimation of reservoir flow characteristics including relative permeability, saturation distributions, and capillary pressure. Although pore‐scale network models take into account the pore throat connections and the appropriate fluid properties, highly simplified pore cross‐sectional shapes are still employed when estimating the threshold capillary pressure for fluid‐fluid displacements in each pore element. As a result, there is a growing need for more realistic threshold capillary pressure estimates generated using pore geometries that honor the real pore topology. To this end, a semi‐analytical model is presented that allows the prediction of threshold capillary pressure as well as the capillary pressure vs. saturation relationship for piston‐like fluid displacements using images of unconventional reservoir rock samples. The model was validated on three different idealized pore geometries and compared against available analytical solutions, resulting in an error of less than 1% for all cases. The model was compared to experimental data using fluid occupancy maps obtained using an X‐ray nano‐CT scanner during an oil imbibition sequence into a miniature reservoir shale sample. The capillary pressure versus wetting phase saturation relationship was also determined for a 2D FIB‐SEM image slice. The presented model shows promise for enabling more advanced pore‐scale modeling of shale rock.

Description: ABSTRACT Transversely isotropic models with a tilted symmetry axis have become standard for imaging beneath dipping shale formations and in active tectonic areas. Here, we develop a methodology of wave‐equation‐based image‐domain tomography for acoustic tilted transversely isotropic media. We obtain the gradients of the objective function using an integral wave‐equation operator based on a separable dispersion relation that takes the symmetry‐axis tilt into account. In contrast to the more conventional differential solutions, the integral operator produces only the P‐wavefield without shear‐wave artifacts, which facilitates both imaging and velocity analysis. The model is parameterized by the P‐wave zero‐dip normal‐moveout velocity, the Thomsen parameter δ, anellipticity coefficient η, and the symmetry‐axis tilt θ. Assuming that the symmetry axis is orthogonal to reflectors, we study the influence of parameter errors on energy focusing in extended (space‐lag) common‐image gathers. Distortions in the anellipticity coefficient η introduce weak linear defocusing regardless of reflector dip, whereas δ influences both the energy focusing and depth scale of the migrated section. These results, which are consistent with the properties of the P‐wave time‐domain reflection moveout in tilted transversely isotropic media, provide important insights for implementation of velocity model‐building in the image‐domain. Then the algorithm is tested on a modified anticline section of the BP 2007 benchmark model. This article is protected by copyright. All rights reserved

Description: Abstract We study the 3‐D P wave velocity structure of the crust and mantle down to 1,000‐km depth beneath the central and eastern United States. A 3‐D velocity model is obtained by conducting a joint inversion of 236,670 arrival times of local earthquakes and 870,455 relative traveltime residuals of teleseismic events recorded by the EarthScope/USArray Transportable Array. Significant low‐velocity (low‐V) anomalies are revealed in the crust beneath the eastern arm of the Midcontinent Rift and the Triassic Basins along the East Coast, whereas a prominent high‐velocity (high‐V) anomaly is visible beneath the Llano Uplift in central Texas. The stable North American Craton exhibits high‐V anomalies at depths of 65–250 km. Low‐V anomalies exist along the eastern and southern margins of the craton, which may reflect relatively thin lithosphere there. A prominent low‐V anomaly is revealed at depths of 50–200 km beneath the New Madrid Seismic Zone, which is bounded by high‐V anomalies to its southeast and northwest. This feature reflects a weak lithosphere surrounded by relatively strong cratonic regions and stress concentration caused intraplate seismicity in the New Madrid region. Two high‐V bodies appear in the mantle transition zone (410‐ to 660‐km depths) beneath the Interior Low Plateaus, the central Great Plains, and the Central Lowland, which may reflect the subducted Farallon plate or delaminated lithosphere. At depths of 800–1,000 km, a high‐V anomaly is visible beneath the southeast United States, which may be the subducted Hess Rise conjugate.

Description: Abstract The 24 May 2013 earthquake beneath the Sea of Okhotsk (610 km, Mw 8.3) produced significant ground motion across the whole span of the Japanese islands, from 1,300‐4,200 km epicentral distance. The largest shaking was concentrated along the back‐arc side of the subduction zone, which is the opposite of the normal pattern for deep earthquakes in the Pacific slab. Observations from the dense Hi‐net and F‐net arrays across Japan show that the largest shaking in northern Japan (near 2,000 km epicentral distance) was caused by near‐caustic S waves, with triplication of upgoing and downgoing waves from the deep source and reflected waves from the 660 km discontinuity. 3‐D finite‐difference‐method simulations confirm that the anti‐waveguide effect of the high‐wavespeed slab is to push the zone of larger intensity 300 km further to south than might be expected. The S wavefont distorted by the slab has near‐critical incidence at the free surface producing large sP and generating shear‐coupled PL (s‐PL) waves with period 〉3 s. With increasing epicentral distance the S incident angle exceeds critical, then total sS reflection creates large ground motion at large distance (〉3,000 km) and even further (〉6,000 km) with sSS. The propagation of sS, sSS linking to sS‐PL and sSS‐PL wavetrains is very efficient in continental structures with thicker crust. The felt reports at large (4,000‐8000 km) distances from the 2013 Sea of Okhotsk earthquake can be explained by lengthy, long‐period ground motion in the continental environment with amplification in sedimentary basins and in tall buildings.

Description: Abstract Preparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high‐resolution seismicity catalogs framing the occurrence of 20 ML 〉 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.

Description: Abstract On 2 October 2016, a significant seismic swarm of long‐period events was recorded on Tenerife (Canary Islands, Spain). The swarm lasted more than 5 hr and consisted of at least 766 detected events. We found a positive correlation between the amplitude of each event and the preceding interevent time together with a stability of the spectral properties and waveform similarity during most of the swarm duration. Toward the end of the swarm, individual events merged into a continuous tremor. These observations can be explained by postulating an unsteady transonic choked flow within a crack‐like conduit as a source mechanism for this swarm. The flow resulted from a sudden discharge of magmatic fluids from a pressurized reservoir into the hydrothermal system of Tenerife. The injected fluids reached the surface starting about 1 month after the swarm, as evidenced by the macroscopic increase in the diffuse CO2 emissions from the crater of Teide volcano. The lack of ground deformation and the absence of relevant seismicity at depths greater than 10 km exclude the ascent of a basaltic magma batch as a causative source mechanism. Instead, we hypothesize the sudden release of fluids accumulated at the top of a magma chamber as a possible mechanism. Another possibility is the injection of a small batch of mafic magma into a cooling magma chamber, triggering a convective mixing. Both cases imply the presence of a magma chamber at depths greater than 8.6 km. These results have important implications for the development of the volcano monitoring system of Tenerife.

Description: Abstract The Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (〈1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system.

Description: Abstract The large magnitude of the 2011 Mw 9.0 Tohoku‐Oki earthquake, which occurred off the east coast of Japan, was not expected or predicted by any previous studies. One surprising observation was the sudden change in stress state; local earthquakes confirmed a compressional stress state before the main shock, whereas an extensional stress state was evident after the main shock. Using discrete element method modeling, this project attempts to reproduce the stress change after the main shock, and explores the conditions that can cause stress switching both onshore and offshore Tohoku. Our simulations demonstrate that rapid fault weakening can produce stress change and predominant normal‐fault earthquake mechanisms in the upper plate of Tohoku‐Oki. Several specific conditions seem to favor such stress switching; the megathrust fault must have been frictionally strong before the main shock, and comparable values of internal (μinternal) and basal friction (μbasal) are necessary to cause the formation of widespread normal faults within the wedge. Furthermore, dynamic extension during elastic unloading appears to play an important role in accommodating stress changes and wedge deformation in the Tohoku area; these cannot be explained solely based on Critical Coulomb Wedge models, but instead require dynamic unloading processes.

Description: Abstract Passive seismic methods for imaging the discontinuity structure of Earth have primarily focused on differences in vertically and radially polarized energy in the coda of earthquake‐generated body waves (e.g., receiver functions). To convert the timing of scattered wave arrivals to depth, three parameters must be known or inferred: depth or layer thickness (H), P‐wave velocity (VP), and S‐wave velocity (VS). A common way to solve for these parameters is through H‐κ stacking analysis, in which layer thickness and the ratio between VP and VS (κ) is calculated while holding one of the velocity parameters constant. However, this assumption biases estimates of layer properties and leads to uncertainties that are not appropriately quantified. As these results are commonly used as starting models for more complex seismic or geodynamic analyses, these assumptions can propagate much further than the initial study. In this study, we introduce independent observations from body‐wave autocorrelations that can help constrain this underdetermined problem. P‐wave autocorrelation allows for the recovery of the Moho‐reflected P‐wave phase from teleseismic earthquakes, which is removed during deconvolution in the calculation of receiver functions. As the Moho‐reflected P‐wave is independent of VS, this constraint allows us to create a system of equations that better quantifies the thickness, VP, and VS of a layer and produces a more appropriate estimation of associated uncertainties. We apply this to 88 seismic stations that are spatially distributed throughout the United States to obtain a model of crustal variability that is unbiased by a priori assumptions of velocity structure.

Description: Abstract Observations of shallow fault creep reveal increasingly complex time‐dependent slip histories that include quasi‐steady creep and triggered as well as spontaneous accelerated slip events. Here we report a recent slow slip event on the southern San Andreas fault (SSAF) triggered by the 2017 Mw8.2 Chiapas (Mexico) earthquake that occurred 3000 km away. Geodetic and geologic observations indicate that surface slip on the order of 10 mm occurred on a 40‐km‐long section of the SSAF between the Mecca Hills and Bombay Beach, starting minutes after the Chiapas earthquake and continuing for more than a year. Both the magnitude and the depth extent of creep vary along strike. We derive a high‐resolution map of surface displacements by combining Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) acquisitions from different lines of sight. InSAR‐derived displacements are in good agreement with the creepmeter data and field mapping of surface offsets. Inversions of surface displacement data using dislocation models indicate that the highest amplitudes of surface slip are associated with shallow (〈1 km) transient slip. We performed 2‐D simulations of shallow creep on a strike‐slip fault obeying rate‐and‐state friction to constrain frictional properties of the top few kilometers of the upper crust that can produce the observed behavior.

Description: Abstract Various factors influence particle breakage in the shear zone of silica sands, including particle shape characteristics, loading path, shear displacement, normal load, initial density and boundary conditions. The present study focuses on particle breakage occurring in the shear zone of a crushed silica sand under shear loading. Several ring shear tests were conducted to measure shear stress‐displacement response of a sand. Grain‐size distribution curves of the original sand (prior to shearing) and the sheared sand from the shear zone are then compared. The mechanism of particle breakage is determined by a thorough examination of particle damage under constant normal load (CNL) and constant volume (CV) tests, considering both loose and dense samples. The results show that void ratio is the dominant factor in particle breakage. On the other hand, it is observed that particle breakage resulting from particle shape properties accrue within the initial loading stages. It is observed that the boundary condition may also affect the magnitude of particle breakage, i.e. particle damage under CNL boundary condition is found to be significantly higher than that under CV condition for the same initial normal stress and void ratio.

Description: Abstract The gradual collapse of the subglacial Bárdarbunga caldera in 2014–2015 provided an opportunity to explore the geothermal signals produced by large‐scale volcanic events. In August 2014, four ice cauldrons (shallow depressions on the ice surface) formed on the caldera flank. These cauldrons reached their maximum volume rapidly and then shrank, indicating that they were created during minor subglacial eruptions. Several weeks after the start of the collapse, three cauldrons on the caldera rim grew in volume, with four smaller cauldrons forming in 2015–2017. The cauldrons reached volumes in the range of 1.0 ± 0.2 to 27 ± 3 million m3. HYDROTHERM numerical simulations of fluid flow and heat transport in the uppermost 1 km of the crust were performed to explore possible causes for these thermal signals and in particular assess the role of shallow magmatic intrusions. The heat transfer required to create the more rapidly formed caldera‐rim cauldrons can be reproduced with shallow intrusions into high‐permeability pathways, which greatly enhance the surface thermal signal. The preintrusion temperature of the surrounding bedrock has a major effect on heat transfer to the surface, with cold bedrock causing a buffering effect, whereas temperature conditions close to the boiling point of water produce far more efficient heat transfer due to the formation of steam plumes. Not all observed behavior is reproduced by our models, suggesting that geothermal reservoirs below 1‐km depth may play a significant role in the observed thermal anomalies.

Description: Abstract We describe a suite of repeating long‐period seismic events at Fuego volcano in Guatemala. These events, recorded on a temporary network over a period of 8 days during January 2012, did not occur with any visibly or audibly detectable emissions from the volcano. Events are separated into families based on different correlation coefficient thresholds. A correlation coefficient threshold of 0.70 yields two families with 123 events and 25 events, respectively. These two event families share enough common features that if the correlation coefficient threshold is 0.65, the families merge and grow to include an additional 226 events. The short duration and frequency content concentrated below 2 Hz of the second family allow us to create a phase‐weighted stack which we then inverted for source mechanism and location using unconstrained full‐waveform moment‐tensor inversion. The eigenvalue decompositions of the best‐fit models indicate the source is a crack with some volume change. The short duration of the modeled source time function and the slight variability of signal shape within the suite of repeating events indicate the events are caused by rapid pressurization of cracks or series of connected cracks. The interevent times of these events appear clustered, indicating driving processes more complex than continual degassing of magma in the conduit would allow. Better understanding of these events may shed light on processes not captured by real‐time seismic amplitude measurements or gas flux measurements alone.

Description: Abstract Natural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single‐domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5‐μm grains of titanomagnetite (Fe2.46Ti0.54O4) using variable‐temperature magnetic force microscopy. At room temperature, grains displayed single‐vortex and multivortex states. Upon heating, the single‐vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single‐vortex states. During cooling in a weak field (~0.1 mT), single‐vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single‐vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single‐vortex states occur in titanomagnetite and, like single‐domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.

Description: ABSTRACT Faced with the challenge of rapidly screening a huge expanse of frontier exploration acreage, often characterized by sparse vintage data, it is our experience that a combination of appropriate air‐ and ground‐based geophysical techniques contributes positively to the exploration value chain. Airborne gravity gradiometry in conjunction with conventional gravity and magnetic data, as well as geological knowledge, add significant value to the screening process. This combination can subsequently assist in optimizing the location of the more time‐consuming and expensive seismic programme. In addition, analysis and inversion of passive seismic data have also proven useful in providing depth to basement estimates, and results derived from all the techniques investigated have been consistent within several study areas. Following initial tests, where the data were independently analysed and cross‐checked for consistency (including comparisons with active source seismic data and well data, when available), the company now routinely adopts the integration of these techniques in our frontier exploration acreage to support sedimentary basin delineation and mapping. This allows the optimal positioning and focussing of the higher spend and higher footprint programmes, such as active reflection seismic.

Description: ABSTRACT The idea of curvature analysis has been widely used in subsurface structure interpretation from three‐dimensional seismic data (e.g., fault/fracture detection and geomorphology delineation) by measuring the lateral changes in the geometry of seismic events. However, such geometric curvature utilizes only the kinematic information (two‐way traveltime) of the available seismic signals. While analysing the dynamic information (waveform), the traditional approaches (e.g., complex trace analysis) are often trace‐wise and thereby fail to take into account the seismic reflector continuity and deviate from the true direction of geologic deposition, especially for steeply dipping formations. This study proposes extending the three‐dimensional curvature analysis to the waveforms in a seismic profile, here denoted as the waveform curvature, and investigates the associated implications for assisting seismic interpretation. Applications to the F3 seismic dataset over the Netherlands North Sea demonstrate the added values of the proposed waveform curvature analysis in four aspects. First, the capability of the curvature operator in differentiating convex and concave bending allows automatic decomposition of a seismic image by the reflector types (peaks, troughs and zero crossings), which can greatly facilitate computer‐aided horizon interpretation and modelling from three‐dimensional seismic data. Second, the signed minimum curvature offers a new analytical approach for estimating the fundamental and important reflector dip attribute by searching the orientation associated with least waveform variation. Third, the signed maximum curvature makes it possible to analyse the seismic signals along the normal direction of the reflection events. Finally, the curvature analysis promotes the frequency bands of the seismic signals and thereby enhances the apparent resolution on identifying and interpreting subtle seismic features.

Description: ABSTRACT To better image deformation structures within the inner accretionary wedge of the Nankai Trough, Japan, we apply common reflection angle migration to a legacy two‐dimensional seismic data set acquired with a 6 km streamer cable. In this region, many seismic surveys have been conducted to study the seismogenic zone related to plate subduction. However, the details of the accreted sediments beneath the Kumano forearc basin are still unclear due to the poor quality of seismic images caused by multiple reflections, highly attenuated signals, and possibly complex geological structures. Generating common image gathers in the subsurface local angle domain rather than the surface offset domain is more advantageous for imaging geological structures that involve complex wave paths and poor illumination. By applying this method, previously unseen structures are revealed in the thick accreted sediments. The newly imaged geometric features of reflectors, such as the folds in the shallow part of the section and the deep reflectors with stepwise discontinuities, imply deformation structures with multiple thrust faults. The reflections within the deep accreted sediments (approximately 5 km) are mainly mapped to far angles (30°–50°) in the common reflection angles, which correspond to the recorded offset distances greater than 4.5 km. This result indicates that the far offset/angle information is critical to image the deformation structures at depth. The new depth image from the common reflection angle migration provides seismic evidence of multiple thrust faults and their relationship with the megathrust fault that is essential for understanding the structure and evolution of the Nankai Trough seismogenic zone.

Description: ABSTRACT Potential field datasets are commonly broken down in the space domain into amplitude (total gradient, or analytic signal amplitude) and phase (tilt angle) components as part of the data processing and interpretation procedure. However, it is possible to reconstruct the data again in the space domain from the amplitude and phase, and if they have been modified then a filtered dataset will be produced. For example, modified derivatives and filters which are based on them (such as the tilt angle and the theta map) can be produced. In addition, the modification of the data amplitude prior to the reconstruction of the signal allows controllable automatic gain control filters to be designed. The procedures are demonstrated on aeromagnetic and gravity data from Southern Africa.

Description: ABSTRACT The seismic K‐Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K‐Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal‐to‐noise ratio. Our study aims to reveal high‐resolution seismic images of the K‐Horizon below a geothermal field in Southern Tuscany, using an advanced three‐dimensional seismic depth imaging approach. The key seismic pre‐processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre‐Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three‐dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal‐to‐noise ratio data set, the migration results with the heterogeneous three‐dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K‐Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K‐Horizon with better reflection coherency and higher signal‐to‐noise ratio than standard Kirchhoff Pre‐Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K‐Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K‐Horizon.

Description: ABSTRACT Over the last years, full‐waveform inversion has become an important tool in the list of processing and imaging technologies available to the industry. For marine towed‐streamer data, full‐waveform inversion is typically applied using an acoustic approximation because S‐waves do not propagate in water and elastic effects in recorded data are generally assumed to be small. We compare acoustic and elastic modelling and full‐waveform inversion for a field data set acquired offshore Angola over sediments containing a salt body with significant topology. Forward modelling tests reveal that such geological structures lead to significant mode conversions at interfaces and, consequently, to significant relative amplitude differences when elastically and acoustically modelled traces are compared. Using an acoustic approach for modelling in full‐waveform inversion therefore leads to problems matching the synthetic data with the field data, even for recorded pressure data and with trace normalization applied. Full‐waveform inversion is unable to find consistent model updates. Applying elastic full‐waveform inversion leads to more consistent and reliable model updates with less artefacts, at the expense of additional computation cost. Although two‐dimensional marine towed‐streamer data are least favourable for the application of full‐waveform inversion compared to three‐dimensional data or ocean‐bottom data, it is recommended to check on the existence of elastic effects before deciding on the final processing and imaging approach.

Description: ABSTRACT The hyperbolic Radon transform has a long history of applications in seismic data processing because of its ability to focus/sparsify the data in the transform domain. Recently, deconvolutive Radon transform has also been proposed with an improved time resolution which provides improved processing results. The basis functions of the (deconvolutive) Radon transform, however, are time‐variant, making the classical Fourier based algorithms ineffective to carry out the required computations. A direct implementation of the associated summations in the time–space domain is also computationally expensive, thus limiting the application of the transform on large data sets. In this paper, we present a new method for fast computation of the hyperbolic (deconvolutive) Radon transform. The method is based on the recently proposed generalized Fourier slice theorem which establishes an analytic expression between the Fourier transforms associated with the data and Radon plane. This allows very fast computations of the forward and inverse transforms simply using fast Fourier transform and interpolation procedures. These canonical transforms are used within an efficient iterative method for sparse solution of (deconvolutive) Radon transform. Numerical examples from synthetic and field seismic data confirm high performance of the proposed fast algorithm for filling in the large gaps in seismic data, separating primaries from multiple reflections, and performing high‐quality stretch‐free stacking.

Description: ABSTRACT A transmission + reflection wave‐equation traveltime and waveform inversion method is presented that inverts the seismic data for the anisotropic parameters in a vertical transverse isotropic medium. The simultaneous inversion of anisotropic parameters and ε is initially performed using transmission wave‐equation traveltime inversion method. Transmission wave‐equation traveltime only provides the low‐intermediate wavenumbers for the shallow part of the anisotropic model; in contrast, reflection wave‐equation traveltime estimates the anisotropic parameters in the deeper section of the model. By incorporating a layer‐stripping method with reflection wave‐equation traveltime, the ambiguity between the background‐velocity model and the depths of reflectors can be greatly mitigated. In the final step, multi‐scale full‐waveform inversion is performed to recover the high‐wavenumber component of the model.  We use a synthetic model to illustrate the local minima problem of full‐waveform inversion and how transmission and reflection wave‐equation traveltime can mitigate this problem. We demonstrate the efficacy of our new method using field data from the Gulf of Mexico.

Description: ABSTRACT This paper proposes a time‐domain fitting method for on‐site calibration of the air‐coil sensor. The air‐coil sensor has been widely used in transient electromagnetic exploration. Due to limited bandwidth of the coil, the output signal is distorted, causing a phenomenon known as the transition process. To accurately measure the magnetic field from the output signals, the relationship between the coil induced electromotive force and the output voltage must be confirmed by on‐site calibration, which requires high calibration accuracy and demands simple operation, portable equipment, and adaptability to the environment. Conventional frequency response methods, however, requires a uniform magnetic field with various frequencies to obtain the frequency response curve of the air‐coil sensor. The time to acquire the signal correlates with the number of test frequencies, and the equipment used to generate a uniform magnetic field must be tailored to the shape of the air‐coil sensor under test. This paper constructs a relationship between the calibration file and the zero‐input response of the air‐coil sensor and designs an optimization algorithm to suppress the soil eddy current effect. This on‐site calibration method lifts the dependence on the uniform calibration field and reduces significantly the time required for calibration. The calibration source can be generated by cutting off the voltage source in parallel to the calibration coil, which greatly reduces the cost of the signal generator and provides a better solution for realizing the embedded self‐test devices. Experimental results show that the proposed method effectively improves the calculation accuracy of the apparent resistivity.

Description: ABSTRACT A focussing function is a specially constructed field that focusses on to a purely downgoing pulse at a specified subsurface position upon injection into the medium. Such focussing functions are key ingredients in the Marchenko method and in its applications such as retrieving Green's functions, redatuming, imaging with multiples and synthesizing the response of virtual sources/receiver arrays at depth. In this study, we show how the focussing function and its corresponding focussed response at a specified subsurface position are heavily influenced by the aperture of the source/receiver array at the surface. We describe such effects by considering focussing functions in the context of time‐domain imaging, offering explicit connections between time processing and Marchenko focussing. In particular, we show that the focussed response radiates in the direction perpendicular to the line drawn from the centre of the surface data array aperture to the focussed position in the time‐imaging domain, that is, in time‐migration coordinates. The corresponding direction in the Cartesian domain follows from the sum (superposition) of the time‐domain direction and the directional change due to time‐to‐depth conversion. Therefore, the result from this study provides a better understanding of focussing functions and has implications in applications such as the construction of amplitude‐preserving redatuming and imaging, where the directional dependence of the focussed response plays a key role in controlling amplitude distortions.

Description: Geophysical Prospecting, Volume 67, Issue 2, Page 480-481, February 2019.

Description: ABSTRACT For airborne gravity gradiometry in rugged terrain, helicopters offer a significant advantage over fixed‐wing aircraft: their ability to maintain much lower ground clearances. Crucially, this provides both better signal‐to‐noise and better spatial resolution than is possible with a fixed‐wing survey in the same terrain. Comparing surveys over gentle terrain at Margaret Lake, Canada, and over rugged terrain at Mount Aso, Japan, demonstrates that there is some loss of spatial resolution in the more rugged terrain. The slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain. Transforming the curvature gradients measured by the Falcon gravity gradiometer into gravity and the complete set of tensor components is done by a Fourier method over gentle terrain and an equivalent source method for rugged terrain. The Fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths. The equivalent source method relies on a smooth model inversion, and the source distribution must be designed to suit the survey design.

Description: ABSTRACT Analysing S‐wave splitting has become a routine step in processing multicomponent data. Typically, this analysis leads to determining the principal directions of a transversely isotropic medium with a horizontal symmetry axis, which is assumed to be responsible for azimuthal anisotropy, and to the time delays between the fast and slow S‐waves. These parameters are commonly estimated layer‐by‐layer from the top. Errors in layer stripping occurring in shallow layers might propagate to deeper layers. We propose a method for S‐wave splitting analysis and compensation that consists of inverting interval values of splitting intensity to obtain a model of anisotropic parameters that vary with time and/or depth. Splitting intensity is a robust attribute with respect to structural variations and is commutative, which means that it can be summed along a ray (or throughout a sensitivity kernel volume) and can be linearly related to anisotropic perturbations at depth. Therefore, it is possible to estimate anisotropic properties within a geological formation (e.g. the reservoir) by analysing the differences of splitting intensity measured at the top and at the bottom of the layer. This allows us to avoid layer stripping, in particular, for shallow layers where anisotropic parameters are difficult to estimate due to poor coverage, and it makes S‐wave splitting analysis simpler to apply. We demonstrate this method on synthetic and real data. Because the splitting intensity attribute shows usefulness in S‐wave splitting analysis in transversely isotropic media, we extend the splitting intensity theory to lower symmetry classes. It enables the characterization of tilted transversely isotropic and tilted orthorhombic media, opening new opportunities for anisotropic model building.

Description: ABSTRACT Time‐lapse seismic data is useful for identifying fluid movement and pressure and saturation changes in a petroleum reservoir and for monitoring of CO2 injection. The focus of this paper is estimation of time‐lapse changes with uncertainty quantification using full‐waveform inversion. The purpose of also estimating the uncertainty in the inverted parameters is to be able to use the inverted seismic data quantitatively for updating reservoir models with ensemble‐based methods. We perform Bayesian inversion of seismic waveform data in the frequency domain by combining an iterated extended Kalman filter with an explicit representation of the sensitivity matrix in terms of Green functions (acoustic approximation). Using this method, we test different strategies for inversion of the time‐lapse seismic data with uncertainty. We compare the results from a sequential strategy (making a prior from the monitor survey using the inverted baseline survey) with a double difference strategy (inverting the difference between the monitor and baseline data). We apply the methods to a subset of the Marmousi2 P‐velocity model. Both strategies performed well and relatively good estimates of the monitor velocities and the time‐lapse differences were obtained. For the estimated time‐lapse differences, the double difference strategy gave the lowest errors.

Description: ABSTRACT We use Legendre polynomials to reparameterize geophysical inversions solved through a particle swarm optimization. The subsurface model is expanded into series of Legendre polynomials that are used as basis functions. In this framework, the unknown parameters become the series of expansion coefficients associated with each polynomial. The aim of this peculiar parameterization is threefold: efficiently decreasing the number of unknowns, inherently imposing a 1D spatial correlation to the recovered subsurface model and searching for maximally decoupled parameters. The proposed approach is applied to two highly non‐linear geophysical optimization problems: seismic‐petrophysical inversion and 1D elastic full‐waveform inversion. In this work, with the aim to maintain the discussion at a simple level, we limit the attention to synthetic seismic data. This strategy allows us to draw general conclusions about the suitability of this peculiar parameterization for solving geophysical problems. The results demonstrate that the proposed approach ensures fast convergence rates together with accurate and stable final model predictions. In particular, the proposed parameterization reveals to be effective in reducing the ill conditioning of the optimization problem and in circumventing the so‐called curse‐of‐dimensionality issue. We also demonstrate that the implemented algorithm greatly outperforms the outcomes of the more standard approach to global inversion in which each subsurface parameter is considered as an independent unknown.

Description: ABSTRACT The existence of aquifers extending from land beneath the sea floor up to a distance of several kilometres has been observed and examined all over the world. The coastal aquifer of Israel is a heavily used groundwater reservoir which has to be constantly monitored to ensure the drinking water supply. Former land‐based electromagnetic measurements show that it is, in several places, blocked to seawater intrusion and is consequently a candidate for submarine extension. Multicomponent long‐offset transient electromagnetic measurements were carried out offshore on the coast of Israel. We deployed a 400‐m‐long grounded dipole as transmitter and several electric and magnetic receivers on the sea floor up to a distance of 4.8 km from the coast. Altogether, we deployed 8 transmitter positions and received data sets at 14 receiver stations onshore and offshore, with offsets of mostly 400 and 800 m. In this paper, we present the survey and 1D Occam and Marquardt inversions of the offshore horizontal electric components in the broadside and inline configuration. In addition, the vertical magnetic component in the broadside position is also considered. Only single inversions, both single offset and single component, were used to detect the aquifer under sea sediments. We prove the submarine existence of the Israeli coastal aquifer up to a distance to the coast of approximately 3.2 to 3.6 km using all measured long‐offset transient electromagnetic components. In addition, we present modelling studies with synthetic data derived from a subsurface model adjusted to our survey area with very shallow water from 10 to 50 m. As part of the planning before the survey, a parameter study of the expected subsurface, the examination of the airwave phenomenon and the justification for our 1D inversion strategy are shown. More detailed eigen parameter analyses are added to explain the measured data.

Description: ABSTRACT In geophysical inverse problems the posterior model can be analytically assessed only in case of linear forward operators, Gaussian, Gaussian mixture, or generalized Gaussian prior models, continuous model properties, and Gaussian‐distributed noise contaminating the observed data. For this reason, one of the major challenges of seismic inversion is to derive reliable uncertainty appraisals in cases of complex prior models, non‐linear forward operators and mixed discrete‐continuous model parameters. We present two amplitude versus angle inversion strategies for the joint estimation of elastic properties and litho‐fluid facies from pre‐stack seismic data in case of non‐parametric mixture prior distributions and non‐linear forward modellings. The first strategy is a 2‐dimensional target‐oriented inversion that inverts the Amplitude Versus Angle responses of the target reflections by adopting the single‐interface full Zoeppritz equations. The second is an interval‐oriented approach that inverts the pre‐stack seismic responses along a given time interval using a 1‐dimensional convolutional forward modelling still based on the Zoeppritz equations. In both approaches the model vector includes the facies sequence and the elastic properties of P‐wave velocity, S‐wave velocity and density. The distribution of the elastic properties at each common‐mid‐point location (for the target‐oriented approach) or at each time‐sample position (for the time‐interval approach) is assumed to be multimodal with as many modes as the number of litho‐fluid facies considered. In this context, an analytical expression of the posterior model is no more available. For this reason, we adopt a Markov Chain Monte Carlo algorithm to numerically evaluate the posterior uncertainties. With the aim of speeding up the convergence of the probabilistic sampling, we adopt a specific recipe that includes multiple chains, a parallel tempering strategy, a delayed rejection updating scheme and hybridizes the standard Metropolis‐Hasting algorithm with the more advanced Differential Evolution Markov Chain method. For the lack of available field seismic data, we validate the two implemented algorithms by inverting synthetic seismic data derived on the basis of realistic subsurface models and actual well log data. The two approaches are also benchmarked against two analytical inversion approaches that assume Gaussian‐mixture distributed elastic parameters. The final predictions and the convergence analysis of the two implemented methods proved that our approaches retrieve reliable estimations and accurate uncertainties quantifications with a reasonable computational effort. This article is protected by copyright. All rights reserved

Description: Abstract We studied long‐term evolution of non‐transform discontinuities (NTDs) on the Mid‐Atlantic Ridge from 0 to ~20‐25 Ma crust using plate reconstructions of multibeam bathymetry, long‐range HMR1 sidescan sonar, residual mantle Bouguer gravity anomaly, and gravity‐derived crustal thickness. NTDs have propagated north and south with respect to flowlines of relative plate motion and both rapidly and slowly compared to the half spreading rate; at times they have been quasi‐stable. Fast, short‐term (〈2 m.y.) propagation is driven by reduced magma supply (increased extension) in the propagating ridge tip when NTD ridge‐axis offsets are small (〈~ 5 km). Propagation at larger offsets generally is slower and longer‐term. These NTDs can show classic structures of rift propagation including inner and outer pseudofaults and crustal blocks transferred between ridge flanks by discontinuous jumps of the propagating ridge tip. In all cases crustal transfer occurs within the NTD valley. Aside from ridge‐axis offset, the evolution of NTDs appears to be controlled by three factors: (1) Gross volume and distribution of magma supplied to ridge segments as controlled by 3D heterogeneities in mantle fertility and/or dynamic upwelling; this controls fundamental ridge segmentation. (2) The lithospheric plumbing system through which magma is delivered to the crust. (3) The consequent focusing of tectonic extension in magma‐poor parts of spreading segments, typically at segment ends, which can drive propagation. We also observe long‐wavelength (5‐10 m.y.) residual mantle Bouguer anomaly (RMBA) asymmetry between the conjugate ridge flanks, and we attribute this to asymmetric distribution of density anomalies in the upper mantle.

Description: Abstract In the parallel paper by Li et al. (2019), an effluent chemistry monitoring system was designed and used in core flood experiments to continuously measure the effluent concentration and study the evolution of the rock‐fluid system. In this study, the results from the parallel paper were used for interpretation and modeling of the dissolution and wormhole formation. Based on the behavior of the effluent concentration, two transient states and two quasi‐steady states were defined to describe the dissolution in the rock‐fluid system. Dimensional analysis was used to identify the controlling mechanisms of the dissolution and transport in the matrix and the wormholes. The dimensional analysis showed that the dissolution in the matrix was reaction‐controlled, while the dissolution in the wormholes was diffusion‐controlled. It also showed that the rock‐fluid system evolved from reaction‐controlled dissolution to diffusion‐controlled dissolution during the core flood tests. A continuum model and the extended Graetz solution were used to model the dissolution in the matrix and in the wormholes, respectively. In the continuum model, this study estimates the effective surface area as a function of the flow rate (injection flux), to account for the effect of flow conditions on dissolution. Finally, a semi‐empirical model combining the continuum model and the extended Graetz solution was developed to simulate the formation of wormholes and the evolution of the dissolution kinetics during core flood tests.

Description: Abstract Core flood tests were conducted to study the effect of flow rate on the dissolution of the gypsum rock matrix and the formation of wormholes. An effluent chemistry monitoring system (ECMS) was designed and integrated into a triaxial system to provide continuous effluent concentration measurements, in addition to the pressure and flow measurements during the core flood tests. X‐ray computed tomography (CT) was used to study the geometry of the wormholes after the tests. The core flood tests showed agreement with experiments reported in the literature regarding permeability evolution and wormhole breakthrough. By continuously monitoring the effluent concentration, the ECMS advanced the experimental study by showing how the dissolution kinetics evolved with the formation of wormholes. 3D topological and morphological algorithms were developed to analyze the CT data and provide quantitative descriptions for the wormhole geometry. The CT analysis showed that higher flow rates resulted in more complex wormhole geometries regarding the number of wormholes and branches.