ALBERT

Description: The new release AIUB-RL02 of monthly gravity models from GRACE GPS and K-Band range-rate data is based on reprocessed satellite orbits referring to the reference frame IGb08. The release is consistent with the IERS2010 conventions. Improvements with respect to its predecessor AIUB-RL01 include the use of reprocessed (RL02) GRACE observations, new atmosphere and ocean dealiasing products (RL05), an upgraded ocean tide model (EOT11A), and the interpolation of shallow ocean tides (admittances). The stochastic parametrization of AIUB-RL02 was adapted to include daily accelerometer scale factors, which drastically reduces spurious signal at the 161 d period in C 20 and at other low degree and order gravity field coefficients. Moreover, the correlation between the noise in the monthly gravity models and solar activity is considerably reduced in the new release. The signal and the noise content of the new AIUB-RL02 monthly gravity fields are studied and calibrated errors are derived from their non-secular and non-seasonal variability. The short-period time-variable signal over the oceans, mostly representing noise, is reduced by 50 per cent with respect to AIUB-RL01. Compared to the official GFZ-RL05a and CSR-RL05 monthly models, the AIUB-RL02 stands out by its low noise at high degrees, a fact emerging from the estimation of seasonal variations for selected river basins and of mass trends in polar regions. Two versions of the monthly AIUB-RL02 gravity models, with spherical harmonics resolution of degree and order 60 and 90, respectively, are available for the time period from March 2003 to March 2014 at the International Center for Global Earth Models or from ftp://ftp.unibe.ch/aiub/GRAVITY/GRACE (last accessed 22 March 2016).

Description: The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (~300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ~150 km large and ~100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

Description: SUMMARY We present forward and adjoint spectral-element simulations of coupled acoustic and (an)elastic seismic wave propagation on fully unstructured hexahedral meshes. Simulations benefit from recent advances in hexahedral meshing, load balancing and software optimization. Meshing may be accomplished using a mesh generation tool kit such as CUBIT, and load balancing is facilitated by graph partitioning based on the SCOTCH library. Coupling between fluid and solid regions is incorporated in a straightforward fashion using domain decomposition. Topography, bathymetry and Moho undulations may be readily included in the mesh, and physical dispersion and attenuation associated with anelasticity are accounted for using a series of standard linear solids. Finite-frequency Fréchet derivatives are calculated using adjoint methods in both fluid and solid domains. The software is benchmarked for a layercake model. We present various examples of fully unstructured meshes, snapshots of wavefields and finite-frequency kernels generated by Version 2.0 ‘Sesame’ of our widely used open source spectral-element package SPECFEM3D.

Description: SUMMARY The Central Iran plateau appears aseismic during the last few millenniums based on instrumental and historical seismic records. Nevertheless, it is sliced by several strike-slip faults that are hundreds of kilometres long. These faults display along-strike, horizontal offsets of intermittent gullies that suggest the occurrence of earthquakes in the Holocene. Establishing this is crucial for accurately assessing the regional seismic hazard. The first palaeoseismic study performed on the 200-km long, NS striking Anar fault shows that this right-lateral fault hosted three large ( M w ≈ 7) earthquakes during the Holocene or possibly Uppermost Pleistocene for the older one. These three seismic events are recorded within a sedimentary succession, which is not older than 15 ka, suggesting an average recurrence of at most 5 ka. The six optically stimulated luminescence ages available provide additional constraints and allow estimating that the three earthquakes have occurred within the following time intervals: 4.4 ± 0.8, 6.8 ± 1 and 9.8 ± 2 ka. The preferred age of the more recent event, ranging between 3600 and 5200 yr, suggests that the fault is approaching the end of its seismic cycle and the city of Anar could be under the threat of a destructive earthquake in the near future. In addition, our results confirm a previous minimum slip rate estimate of 0.8 ± 0.1 mm yr −1 for the Anar fault indicating that the westernmost prominent right-lateral faults of the Central Iran plateau are characterized by slip rates close to 1 mm yr −1 . These faults, which have repeatedly produced destructive earthquakes with large magnitudes and long recurrence interval of several thousands of years during the Holocene, show that the Central Iran plateau does not behave totally as a rigid block and that its moderate internal deformation is nonetheless responsible for a significant seismic hazard.

Description: SUMMARY A new compilation of Bouguer gravity data stemming from airborne, shipborne and terrestrial data set in the entire Dead Sea Basin (DSB) was reinterpreted by applying 3-D density modelling that incorporated independent information on other geophysical researches allowing for regional and residual filtering in the gravity field, carrying out curvature analysis and Euler deconvolution of the combined gravity field. 3-D density modelling enables us to detailed resolution of upper crustal structures from the southern to the northern subbasin below the saline Dead Sea. 3-D gravity modelling led to the identification of three salt structures, which are found beneath the Sedom area, the Lisan Peninsula and the Dead Sea. In the vicinity of the western margin of the Dead Sea, a salt diapir segment with a thickness of about 4 km has been identified at a top depth of about 2 km, which has not been recognised by any other geophysical interpretations. The thickness of the sedimentary infill overlying the basement in the DSB decreases from 14 km in the vicinity of the Lisan Peninsula to 8 km in the northern and the southern subbasins. Large negative gravity anomalies (lower than –100 × 10 −5 m s −2 ) observed in the DSB correspond with the spatial distribution of salt diapirism with an average density of 2 100 kg m −3 . The shallower microearthquakes registered in the DSB are related to the movement of salt diapir in the DSB.

Description: SUMMARY We propose a new method to analyse seismic time-series and estimate the arrival times of seismic waves. Our approach combines two ingredients: the time-series are first lifted into a high-dimensional space using time-delay embedding; the resulting phase space is then parametrized using a non-linear method based on the eigenvectors of the graph Laplacian. We validate our approach using a data set of seismic events that occurred in Idaho, Montana, Wyoming and Utah between 2005 and 2006. Our approach outperforms methods based on singular-spectrum analysis, wavelet analysis and short-term average/long-term average (STA/LTA).

Description: SUMMARY 10 Be and 36 Cl cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of offset terraces have been performed to constrain the long-term slip rate of the Dehshir fault. Analysis of cosmogenic 10 Be and 36 Cl in 73 surface cobbles and 27 near-surface amalgams collected from inset terraces demonstrates the occurrence of a low denudation rate of 1 m Ma −1 and of a significant and variable inheritance from exposure prior to the aggradation of these alluvial terraces. The significant concentrations of cosmogenic nuclides measured in the cobbles collected within the riverbeds correspond to 72 ± 20 ka of inheritance. The mean CRE age of the surface samples collected on the older terrace T3 is 469 ± 88 ka but the analysis of the distribution of 10 Be concentration in the near-surface samples discard ages older than 412 ka. The mean CRE age of the surface samples collected on terrace T2 is 175 ± 62 ka but the 10 Be depth profile discard ages older than 107 ka. For each terrace, there is a statistical outlier with a younger age of 49.9 ± 3.3 and 235.5 ± 35.4 ka on T2 and T3, respectively. The late sediments aggraded before the abandonment of T2 and inset levels, T1 b and T1a, yielded OSL ages of, respectively, 26.9 ± 1.3, 21.9 ± 1.5 and 10.0 ± 0.6 ka. For a given terrace, the OSL ages, where available, provide ages that are systematically younger than the CRE ages. These discrepancies between the CRE and OSL ages exemplify the variability of the inheritance and indicate the youngest cobble on a terrace, that minimizes the inheritance, is the most appropriate CRE age for approaching that of terrace abandonment. However, the upper bound on the age of abandonment of a terrace that is young with respect to the amount of inheritance is best estimated by the OSL dating of the terrace material. For such terraces, the CRE measurements are complementary of OSL dating and can be used to unravel the complex history of weathering and transport in the catchment of desert alluvial fans. This comprehensive set of dating is combined with morphological offsets ranging from 12 ± 2 to 380 ± 20 m to demonstrate the Dehshir fault slips at a rate in the range 0.9 mm yr −1 –1.5 mm yr −1 . The variable inheritance exemplified here may have significant implications for CRE dating in arid endorheic plateaus such as Tibet and Altiplano.

Description: Geodetic observations across most of the major strike-slip faults show an interseismic strain rate, which presents a sharp localization of elastic shear strain in the fault vicinity (20–60 km). The screw dislocation model of Savage & Burford is commonly used to fit these geodetic data and to retrieve the far field velocity and the locking depth. This model is very popular because of its inherent simplicity to derive fault slip rates, and mostly because it predicts locking depths, which are of the same order of magnitude as the base of the seismogenic zone (5–20 km). A first issue with the screw dislocation model is that localization is paradoxically introduced by imposing a step function in the velocity field at a depth where the crust is otherwise recognized to behave following a viscous rheology. A second issue with this model is that it is not consistent with the rheological model of the crust that is valid for both post-seismic (1–10 yr), interseismic (several thousand years) and long-term geodynamic (several thousand to several million years) timescales. Here we use numerical models to study how alternative and more geologically realistic boundary conditions and rheological structures can lead to the localization of elastic strain at the Earth's surface during the interseismic period. We find that simple elastic models resembling the Savage & Burford model but driven by far field plate velocity are inefficient at localizing strain unless this driving velocity is transmitted by a rigid indentor. We also find that models including a weak viscous heterogeneity beneath the fault zone are able to produce appropriate localization of the deformation near the fault. This alternative class of model is shown to be pertinent in regards to the boundary conditions and geological observations along exhumed ductile strike-slip shear zone.