ALBERT

Description: 〈span〉〈div〉Summary〈/div〉The strength of the lithosphere plays a key role in the formation and evolution of tectonic plate boundaries. Localized lithospheric deformation associated with plate tectonics requires a mechanism for weakening across the entire width of the lithosphere, including the strongest cold ductile region. We explore the microphysics of weakening of lithospheric materials, and in particular the coupled evolution of mineral grain size and intragranular defects and their control on lithospheric strength. We propose a model for the interaction between grain-boundaries and dislocation density to reduce the net free energy of grains during dynamic recrystallization (DRX). The driving forces for DRX arise from heterogeneity in dislocation density and grain boundary curvature. Our model shows that grain growth driven by variation in grain boundary curvature can be impeded by variation in dislocation density; this occurs because as the grains grow, to minimize their surface energy, their dislocation density and associated internal energy may increase and offset the driving forces for grain growth. The correlation between grain size and dislocation density can for example arise because the dislocation accumulation in smaller grains is suppressed due to the large stress that is needed to bend and elongate a short dislocation (as dictated by the small grain size), while the larger grains can have long dislocations and reach a steady state dislocation density dictated by the applied stress. In a lithospheric setting, slower grain growth means that it would require less mechanical work to establish weak localized shear zones through grain damage, and retard the healing of previously damaged zones. Furthermore, the competition of two different time-scales - that of grain growth and the dislocation kinetics - can lead to oscillating behavior over 1 to 10 years as the grain size and dislocation density advance towards their steady states. These oscillations are likely to have an effect on the rheology of lithospheric rocks, e.g. their strengthening and weakening through time, and have a potential application to geological processes such as postseismic creep in ductile shear zones.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Locating and monitoring passive seismic sources provides us important information for studying subsurface rock deformation, injected fluid migration, regional stress conditions as well as fault rupture mechanism. In this paper, we present a novel passive-source monitoring approach using vector-based elastic time-reversal imaging. By solving the elastic wave equation using observed multicomponent records as boundary conditions, we first compute back-propagated elastic wavefields in the subsurface. Then, we separate the extrapolated wavefields into compressional (P-wave) and shear (S-wave) modes using the vector Helmholtz decomposition. A zero-lag cross-correlation imaging condition is applied to the separated pure-mode vector wavefields to produce passive-source images. We compare imaging results using three implementations, i.e., dot-product, energy and power. Numerical experiments demonstrate that the power imaging condition gives us the highest resolution and is less sensitive to the presence of random noises. To capture the propagation of microseismic fracture and earthquake rupture, we modify the traditional zero-lag cross-correlation imaging condition by summing the multiplication of the separated P- and S-wavefields within local time windows, which enables us to capture the temporal and spatial evolution of earthquake rupture. 2D and 3D numerical examples demonstrate that the proposed method is capable of accurately locating point sources, as well as delineating dynamic propagation of hydraulic fracture and earthquake rupture.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Magnitudes of differential stress in the lithosphere, especially in the crust, are still disputed. Earthquake-based stress drop estimates indicate median values 〈 10 MPa whereas the lateral variation of gravitational potential energy per unit area (〈span〉GPE〈/span〉) across significant relief indicates stress magnitudes of ca. 100 MPa in average across a 100 km thick lithosphere between the Indian lowland and the Tibetan plateau. These standard 〈span〉GPE〈/span〉-based stress estimates correspond to membrane stresses, because they are associated with a deformation that is uniform with depth. We show here with new analytical results that lateral variations in 〈span〉GPE〈/span〉 can also cause bending moments and related bending stresses of several hundreds of MPa. Furthermore, we perform two-dimensional thermo-mechanical numerical simulations (1) to evaluate estimates for membrane and bending stresses based on 〈span〉GPE〈/span〉 variations, (2) to quantify minimum crustal stress magnitudes that are required to maintain the topographic relief between Indian lowland and Tibetan plateau for ca. 10 Ma and (3) to quantify the corresponding relative contribution of crustal strength to the total lithospheric strength. The numerical model includes viscoelastoplastic deformation, gravity and heat transfer. The model configuration is based on density fields from the CRUST1.0 data set and from a geophysically and petrologically constrained density model based on 〈span〉in situ〈/span〉 field campaigns. The numerical results indicate that values of differential stress in the upper crust must be 〉 ca. 180 MPa, corresponding to a friction angle of ca. 10°, to maintain the topographic relief between lowland and plateau for 〉 10 Ma. The relative contribution of crustal strength to total lithospheric strength varies considerably laterally. In the region between lowland and plateau and inside the plateau the depth-integrated crustal strength is approximately equal to the depth-integrated strength of the mantle lithosphere. Simple analytical formulae predicting the lateral variation of depth-integrated stresses agree with numerically calculated stress fields, which show both the accuracy of the numerical results and the applicability of simple, rheology-independent, analytical predictions to highly variable, rheology-dependent, stress fields. Our results indicate that (1) crustal strength can be locally equal to mantle lithosphere strength and that (2) crustal stresses must be at least one order of magnitude larger than median stress drops in order to support the plateau relief over a duration of ca. 10 Ma.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Scanning magnetometers are increasingly used to characterize the magnetization of mineral grains in rock samples. Up-scaling this measurement technique to large numbers of individual particles is hampered by the intrinsic non-uniqueness of potential-field inversion. Here it is shown that this problem can be circumvented by adding tomographic information that determines the location of the possible field sources. Standard potential theory is used to prove a uniqueness theorem which completely characterizes the mathematical background of the corresponding source-localized inversion. It exactly resolves under which conditions a potential field measurement on a surface can be uniquely decomposed into signals from the different source regions. The intrinsic non-uniqueness of potential field inversion prevents that the source distribution inside the tomographically outlined regions can be recovered, but the potential field of each region is uniquely defined. For scanning magnetometers in rock magnetism, this result implies that magnetic dipole vectors of large numbers of individual magnetic particles can be reliably reconstructed from surface scans of the magnetic field, if the particle positions are independently determined. This provides an incentive to improve scanning methods for future paleomagnetic applications.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The most common earthquake forecasting models assume that the magnitude of the next earthquake is independent from the past. This feature severely limits the capability to forecast large earthquakes with high probabilities. Here we investigate empirically on the magnitude-independence assumption, exploring if: i) background and triggered earthquakes have the same frequency-magnitude distribution, ii) variations of seismicity in the space-time-magnitude domain encode some information on the future earthquakes size. For this purpose, and to verify the stability of the findings, we consider seismic catalogues covering different space-time-magnitude windows, such as the Alto Tiberina Near Fault Observatory (TABOO), the California and Japanese seismic catalogues. Our approach is inspired by the nearest-neighbour method proposed by Baiesi & Paczuski (2004) and elaborated by Zaliapin et al. (2008) to distinguish between triggered and background earthquakes. Here we implement the same metric-based correlation to identify the precursory seismicity of any triggered earthquake; this allows us to analyse, for each triggered earthquake, the space-time-magnitude distribution of the seismicity that likely contributed to its occurrence. Our results show that the magnitude-independence assumption holds reasonably well in all catalogues, with a remarkable exception that is consistent with a previous independent study; this departure from the magnitude-independence assumption shows that larger events tend to nucleate at a higher distance from the ongoing sequence. We also notice that the reliability of this assumption may depend on the spatial scale considered; it holds for seismic catalogues of large areas, but we identify possible departures in small areas, reflecting different ways to release locally seismic energy. Finally, we come across an important issue that may lead to misleading results in similar studies, i.e., if a seismic catalogue appears overall complete above a fixed magnitude threshold, it may still yield spurious signals into the analysis. Specifically, we show that some significant departures from the magnitude-independence assumption do not survive when considering spatiotemporal variations of the magnitude of completeness.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Vp/Vs models provide important complementary information to Vp and Vs models, relevant to lithology, rock damage, partial melting, water saturation, etc. However, seismic tomography using body-wave traveltime data from local or regional earthquakes does not constrain Vp/Vs well due to the different resolution of Vp and Vs models, with the Vp models usually better constrained than Vs. Since surface-wave data are most sensitive to Vs, which leads to complementary strengths with respect to body-wave data, we jointly invert body- and surface-wave data to better resolve the Vp/Vs models. In order to show the robustness of our joint inversion method, we compare the results to other approaches, including dividing Vp by Vs models and Vp/Vs parameterization with body-wave or both body- and surface-wave data, using synthetic data and real data from the southern California plate boundary region. We confirm that Vp/Vs models from body-wave inversion obtained by division tend to include artifacts, even when both Vp and Vs models seem reasonable. The joint inversion with Vp/Vs parameterization is found to improve the Vp/Vs ratio model significantly and the resultant Vp/Vs model shows more geologically consistent features, such as high Vp/Vs along fault traces at shallow depths likely indicating fault-related damage. The Vp/Vs model also exhibits contrasts at intermediate depths along the Peninsular Range compositional boundary, and high Vp/Vs in the lower crust near the Salton Sea region correlated with high heat flow and may indicate partial melting. The improved Vp/Vs as well as individual Vp and Vs models are useful for earthquake relocation, high-resolution Moho depth imaging, and interpretation of other data and tectonic evolution in the region.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Interpretation of surface fault scarps and palaeoseismic trenches is a key component of estimating fault slip rates, earthquake recurrence rates and maximum magnitudes for hazard assessments. Often these analyses rely on the assumption that successive earthquakes all breached the surface and that the ruptures are recorded topographically, or by the deposits exposed in a trench. The M〈sub〉〈span〉w〈/span〉〈/sub〉7.2 1992 Suusamyr earthquake, Kyrgyzstan, is an apparently problematic case for such analyses because its ruptures show significant displacement but are only mapped as having broken the surface along small, disparate portions of the fault. Here we present the results of surveys conducted along the Suusamyr Fault to establish whether that is the case. Two sets of ruptures were identified following the earthquake. They are unusually short for their displacement and are separated by a 25 km gap. Using satellite imagery, high-resolution digital elevation models and palaeoseismic trenching we first reassess the distribution of the 1992 ruptures and then reconstruct the Holocene earthquake record to establish the extent to which the 1992 earthquake is representative of the rupture behaviour of this fault. We find evidence for at least two prehistoric surface rupturing earthquakes in the Holocene: one ∼3 ka and one 〉8 ka that, along with the modern event, gives recurrence intervals of ∼3 kyr and ∼5 kyr. Within spatial gaps between segments of the 1992 ruptures there are clear prehistoric surface ruptures and the ruptures in each prehistoric earthquake were discontinuous. We conclude that there is significant variability in the surface rupture pattern of successive earthquakes on the Suusamyr Fault, with implications for the completeness of palaeoseismic records obtained from thrust scarps.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉We provide a six-component (6-C) polarization model for 〈span〉P〈/span〉-, 〈span〉SV〈/span〉-, 〈span〉SH〈/span〉-, Rayleigh-, and Love-waves both inside an elastic medium as well as at the free surface. It is shown that single-station 6-C data comprised of three components of rotational motion and three components of translational motion provide the opportunity to unambiguously identify the wave type, propagation direction, and local 〈span〉P〈/span〉- and 〈span〉S〈/span〉-wave velocities at the receiver location by use of polarization analysis. To extract such information by conventional processing of three-component (3-C) translational data would require large and dense receiver arrays. The additional rotational components allow the extension of the rank of the coherency matrix used for polarization analysis. This enables us to accurately determine the wave type and wave parameters (propagation direction and velocity) of seismic phases, even if more than one wave is present in the analysis time window. This is not possible with standard, pure-translational 3-C recordings. In order to identify modes of vibration and to extract the accompanying wave parameters, we adapt the multiple signal classification algorithm (MUSIC). Due to the strong nonlinearity of the MUSIC estimator function, it can be used to detect the presence of specific wave types within the analysis time window at very high resolution. We show how the extracted wavefield properties can be used, in a fully automated way, to separate the wavefield into its different wave modes using only a single 6-C recording station. As an example, we apply the method to remove surface wave energy while preserving the underlying reflection signal and to suppress energy originating from undesired directions, such as side-scattered waves.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The Sentinel-1 mission comprises two synthetic aperture radar satellites, each with a 12 day orbital repeat, orbiting 6 days apart within a narrow tube. The mission design promises the ability to respond quickly to earthquakes with InSAR, and to facilitate production of interferograms with good interferometric correlation globally. We report on our efforts to study global seismicity using Sentinel-1 Interferometric Wide-Swath data between April 2015 and December 2016. We select 35 potentially detectable terrestrial earthquakes in the range 5.5 ≤ 〈span〉Mw〈/span〉 ≤ 7.8 on the basis of their locations, depths and magnitudes, and process the first post-event interferogram with the shortest possible time-span for each using the ISCE software. We evaluate each interferogram for earthquake deformation signals by visual inspection. We can identify deformation signals attributable to earthquakes in 18 of these interferograms (51%); a further six interferograms (17%) have ambiguous interferometric phase affected by tropospheric noise. 11 events (31%) could not be identified from their interferograms. The majority of these failed detections were due to interferogram decorrelation, particularly apparent for earthquakes that occurred between 15°N and 15°S, where climate conditions promote dense vegetation. The majority of the ambiguous interferograms are affected by tropospheric noise, suggesting that techniques to mitigate such noise could improve detection performance. The largest event we do not detect with Sentinel-1 data is a 〈span〉Mw〈/span〉7.0 earthquake that occurred in Vanuatu in April 2016; we also fail to detect the 2016 〈span〉Mw〈/span〉6.2 Kurayoshi earthquake in one out of two possible 24-day interferograms. We propose these as upper and lower estimates on the magnitude of completeness for earthquakes studied with Sentinel-1 data; to raise the magnitude of completeness we suggest that more frequent (e.g. six day) recurrence may be necessary in low latitude areas.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉The correct estimation of site-specific attenuation is crucial for the assessment of seismic hazard. Downhole instruments provide in this context valuable information to constrain attenuation directly from data. In this study, we apply an interferometric approach to this problem by deconvolving seismic motions recorded at depth with those recorded at the surface. In doing so, incident and surface-reflected waves can be separated. We apply this technique not only to earthquake data but also to recordings of ambient vibrations. We compute the transfer function between incident and surface-reflected waves in order to infer frequency dependent quality factors for S-waves. The method is applied to a 87 m deep borehole sensor and a co-located surface instrument situated at a hard-rock site in West Bohemia/Vogtland, Germany. We show that the described method provides comparable attenuation estimates using either earthquake data or ambient noise for frequencies between 5-15 Hz. Moreover, a single hour of noise recordings seems to be sufficient to yield stable deconvolution traces and quality factors, thus, offering a fast and easy way to derive attenuation estimates from borehole recordings even in low to mid seismicity regions.〈/span〉