ALBERT

Description: The latitudinal motion of the Tibetan Himalaya - the northernmost continental unit of the Indian plate - is a key component in testing paleogeographic reconstructions of the Indian plate before the India-Asia collision. Paleomagnetic studies of sedimentary rocks (mostly carbonate rocks) from the Tibetan Himalaya are complicated by potentially pervasive yet cryptic remagnetization. Although traditional paleomagnetic field tests reveal some of this remagnetization, secondary remanence acquired prior to folding or tilting easily escapes detection. Here, we describe comprehensive rock magnetic and petrographic investigations of Jurassic to Paleocene carbonate and volcaniclastic rocks from Tibetan Himalayan strata (Tingri and Gamba areas). These units have been the focus of several key paleomagnetic studies for Greater Indian paleogeography. Our results reveal that while the dominant magnetic carrier in both carbonate and volcaniclastic rocks is magnetite, their magnetic and petrographic characteristics are distinctly different. Carbonate rocks have ‘wasp-waisted’ hysteresis loops, suppressed Verwey transitions, extremely fine grain sizes (superparamagnetic), and strong frequency-dependent magnetic susceptibility. Volcaniclastic rocks exhibit ‘pot-bellied’ hysteresis loops and distinct Verwey transitions. Electron microscopy reveals that magnetite grains in carbonate rocks are pseudomorphs of early diagenetic pyrite, whereas detrital magnetite is abundant and pyrite is rarely oxidized in the volcaniclastic rocks. We suggest that the volcaniclastic rocks retain a primary remanence, but oxidation of early diagenetic iron sulfide to fine-grained magnetite has likely caused widespread chemical remagnetization of the carbonate units. We recommend that thorough rock magnetic and petrographic investigations are prerequisites for paleomagnetic studies throughout southern Tibet and everywhere in general.

Description: Both the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault; MMF) are poorly understood, in part because of the fault's undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double-difference relocation and 3-D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) Both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km, (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin, and (3) the magnitude and direction of dip of the main fault changes under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.

Description: Lawsonite CaAl 2 Si 2 O 7 (OH) 2 ·H 2 O is an important water carrier in subducting oceanic crusts, and the primary hydrous phase in basalt at depths greater than ~80 km. We have conducted high-pressure synchrotron single-crystal x-ray diffraction experiments on natural lawsonite at room temperature up to ~10.0 GPa to study its high-pressure polymorphism. We find that lawsonite remains orthorhombic with Cmcm symmetry up to ~9.3 GPa, and shows nearly isotropic compression. Above ~9.3 GPa, lawsonite becomes monoclinic with P 2 1 / m symmetry. Across the phase transition, the Ca polyhedron becomes markedly distorted, and the average positions of the H 2 O molecules and hydroxyls change. The changes observed in the H-atom positions under compression are different than the low temperature changes in this material. We resolve for the first time the H-bonding configuration of the high-pressure monoclinic phase of lawsonite. A bond valence approach is deployed to determine that the phase transition from orthorhombic to monoclinic is primarily driven by the Si 2 O 7 groups, and in particular their bridging oxygen atoms (O1). The changes in the structure strongly indicate that entropy increases across the symmetry-lowering transition, and hence that the slope of the phase transition is negative. Monoclinic lawsonite is thus stable under the pressure and temperature conditions that exist in the Earth, and is likely to be a major water carrier in colder, deep subducted slabs. Monoclinic lawsonite also likely has enhanced electrical conductivity along its c -axis due to its dynamically disordered hydrogen atoms.

Description: Receiver functions (RF) have been used for several decades to study structures beneath seismic stations. Although most available stations are deployed on-shore, the number of ocean bottom station (OBS) experiments has increased in recent years. Almost all OBSs have to deal with higher noise levels and a limited deployment time (∼1 year), resulting in a small number of usable records of teleseismic earthquakes. Here, we use OBSs deployed as mid-aperture array in the deep ocean (4.5-5.5 km water depth) of the eastern mid-Atlantic. We use evaluation criteria for OBS data and beam forming to enhance the quality of the RFs. Although some stations show reverberations caused by sedimentary cover, we are able to identify the Moho signal, indicating a normal thickness (5-8 km) of oceanic crust. Observations at single stations with thin sediments (300-400 m) indicate that a probable sharp lithosphere-asthenosphere boundary (LAB) might exist at a depth of ∼70-80 km which is in line with LAB depth estimates for similar lithospheric ages in the Pacific. The mantle discontinuities at ∼410 km and ∼660 km are clearly identifiable. Their delay times are in agreement with PREM. Overall the usage of beam formed earthquake recordings for OBS RF analysis is an excellent way to increase the signal quality and the number of usable events.

Description: Since the discovery of slow slip events, many methods have been successfully applied to model obvious transient events in geodetic time series, such as the widely used network strain filter. Independent seismological observations of tremors or low frequency earthquakes and repeating earthquakes provide evidence of low amplitude slow deformation but do not always coincide with clear occurrences of transient signals in geodetic time series. Here, we aim to extract the signal corresponding to slow slips hidden in the noise of GPS time series, without using information from independent datasets. We first build a library of synthetic slow slip event templates by assembling a source function with Green's functions for a discretized fault. We then correlate the templates with post-processed GPS time series. Once the events have been detected in time, we estimate their duration T and magnitude M w by modelling a weighted stack of GPS time series. An analysis of synthetic time series shows that this method is able to resolve the correct timing, location, T and M w of events larger than M w 6 in the context of the Mexico subduction zone. Applied on a real data set of 29 GPS time series in the Guerrero area from 2005 to 2014, this technique allows us to detect 28 transient events from M w 6.3 to 7.2 with durations that range from 3 to 39 days. These events have a dominant recurrence time of 40 days and are mainly located at the down dip edges of the M w 〉 7.5 SSEs.

Description: Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one-dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K -means, fuzzy K -means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high-temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Description: We investigate the dynamics of the inner core wobble (ICW), the Euler-Liouville wobbling motion of the Earth's solid inner core, under the mantle-inner core gravitational (MICG) torques within the Earth. Chao [2016] derived the full 3-D equation of motion for the MICG dynamics in terms of the spherical-harmonic multipoles of mass density, and focused on the axial component for inner-core's torsional libration. Here, aiming for the ICW, we deduce the 2-D equatorial component of the MICG torque owing to the oblateness of the mantle and the inner core. The period of the free Eulerian wobble of a hypothetical isolated rigid inner core would be a prograde +414 days. The action of the added MICG equatorial torque is found to be (negatively) strong enough to render the wobbling motion to become retrograde (with a negative frequency), which is further but slightly modified by the elastic yielding feedback of the inner core. Imposing yet further the passive effect of the hydrostatic pressure due to the fluid outer core greatly lengthens the natural period to become decadal. The final estimate of the ICW natural period in accordance with the (seismological) PREM Earth model is a retrograde P ICW ≈ −15.6 years, in contrast to a prograde +6.6 years supposed in the literature. The corresponding spring constant (per radian of wobble) is 5.5 × 10 22 N m. Our results instigate likely identification of the ICW with decadal wobbles observed in the Earth's polar motion.

Description: To understand the processes involved in rheological weakening due to phase mixing in olivine + orthopyroxene aggregates, we conducted high-strain torsion experiments on two-phase samples at a temperature of 1200°C and a confining pressure of 300 MPa. Samples composed of iron-rich olivine plus 26% orthopyroxene were deformed to outer radius shear strains of up to γ ≈ 26. Values for the stress exponent, n , and grain size exponent, p , were determined based on least-squares fits of the strain rate, stress, and grain size data to a power-law creep equation both at smaller strains (γ ≤ 3) and at larger strains (γ ≥ 24). Microstructural observations demonstrate that, with increasing shear strain, grain size decreased and mixtures of small, equant grains of olivine and pyroxene developed. The values of n and p combined with associated changes in microstructure demonstrated that our samples deformed (i) by dislocation-accommodated grain-boundary sliding with subgrains present at lower strains and (ii) by dislocation-accommodated grain-boundary sliding with subgrains absent at higher strains. The evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere.

Description: We analyze three-dimensional GPS coordinate time series from continuously operating stations in Nepal and South Tibet and calculate the initial one-year postseismic displacements. We first investigate models of poroelastic rebound, afterslip and viscoelastic relaxation individually and then attempt to resolve the tradeoffs between their contributions by evaluating the misfit between observed and simulated displacements. We compare kinematic inversions for distributed afterslip with stress-driven afterslip models. The modeling results show that no single mechanism satisfactorily explains near- and far-field postseismic deformation following the Gorkha earthquake. When considering contributions from all three mechanisms, we favor a combination of viscoelastic relaxation and afterslip alone, as poroelastic rebound always worsens the misfit. The combined model does not improve the data misfit significantly, but the inverted afterslip distribution is more physically plausible. The inverted afterslip favors slip within the brittle-ductile transition zone down dip of the coseismic rupture and fills the small gap between the mainshock and largest aftershock slip zone, releasing only 7% of the coseismic moment. Our preferred model also illuminates the laterally heterogeneous rheological structure between India and the south Tibet. The transient and steady-state viscosities of the upper mantle beneath Tibet are constrained to be greater than 10 18 Pa s and 10 19 Pa s, whereas the Indian upper mantle has a high viscosity ≥ 10 20 Pa s. The viscosity in the lower crust of southern Tibet shows a clear tradeoff with its southward extent and thickness, suggesting an upper bound value of ~8×10 19 Pa s for its steady-state viscosity.

Description: The transition from sub-Rayleigh to supershear propagation of mode II cracks is a fundamental problem of fracture mechanics. It has extensively been studied in homogeneous uniform setups. When the applied shear load exceeds a critical value, transition occurs through the Burridge-Andrews mechanism at a well-defined crack length. However, velocity structures in geophysical conditions can be complex and affect the transition. Damage induced by previous earthquakes causes low-velocity zones surrounding mature faults and inclusions with contrasting material properties can be present at seismogenic depth. We relax the assumption of homogeneous media and investigate dynamic shear fracture in heterogeneous media using two-dimensional finite-element simulations and a linear slip-weakening law. We analyze the role of heterogeneities in the elastic media, while keeping the frictional interface properties uniform. We show that supershear transition is possible due to the sole presence of favorable off-fault heterogeneities. Sub-critical shear loads, for which propagation would remain permanently sub-Rayleigh in an equivalent homogeneous setup, will transition to supershear as a result of reflected waves. P-wave reflected as S-waves, followed by further reflections, affect the amplitude of the shear stress peak in front of the propagating crack, leading to supershear transition. A wave reflection model allows to uniquely describe the effect of off-fault inclusions on the shear stress peak. A competing mechanism of modified released potential energy affects transition and becomes predominant with decreasing distance between fault and inclusions. For inclusions at far distances, the wave reflection is the predominant mechanism.