Geometry of subduction and depth of the seismogenic zone in the Guerrero gap, Mexico

L'étude sismique de la zone côtière de Guerrero (Mexique) permet d'interpréter la géométrie de la subduction dans cette régio

Seismic velocities in Southern Tibet lower crust: a receiver function approach for eclogite detection

Beneath the Tibet plateau, the deficit of crustal thickening with respect to what is expected from the plate tectonic constraints is thought to be absorbed either by lateral extrusion or by vertical rock-mass transfer. To nourish the unsettled debate of the relative importance of these two processes, we propose a new approach, based on the S-to-P and the P-to-S wave conversions, enabling the precise determination of the seismic velocities. The weighted amplitudes of the direct conversion and of reverberations are stacked at their predicted arrival times for various values of layer thickness and v(P)/v(S) ratio separately for two sets of P- and S-receiver functions. For each set of receiver functions, coherent stack gives the v(P)/v(S) ratio and thickness for the considered layer (the grid search stacking method). The values of v(P)/v(S) ratio and layer thickness are functions of the velocity used for stacking the set of receiver functions, but using the P- and S-receiver functions allows us to solve this indetermination and to find the effective parameters of the layer: velocity v(S), v(P)/v(S) ratio and thickness. We use a bootstrap resampling of the receiver function data sets to estimate the parameters uncertainties. For the Southern Lhasa Block, the migrated sections of both P- and S-receiver functions (Hi-CLIMB experiment data) show a layer in the lower crust that may be related to the lower Indian crust underplated beneath Tibet. With the grid search stacking method, high shear wave velocities (v(S) similar to 4.73 km s(-1)) and low v(P)/v(S) ratios (similar to 1.69) are detected in this layer. Such values are typical for high-grade eclogites, and the low v(P)/v(S) ratio precludes the confusion with mafic granulites. There is no evidence for partial eclogitization near and south of the Yarlung-Tsangpo Suture, and the about 19 km thick eclogitic layer extends northwards only to about the middle of the Lhasa terrane

Multiple transition zone seismic discontinuities and low velocity layers below western United States

With P-to-S converted waves recorded at seismic stations of the U.S. Transportable Array, we image the fine structure of upper mantle and transition zone (TZ) beneath the western U.S. We map the topographies of seismic discontinuities by stacking data by common conversion points along profiles. Systematic depth and amplitude measurements are performed not only for the well-known “410” and “660” interfaces but also for minor seismic discontinuities identified around 350, 590, and 630 km depths. The amplitude of conversion suggests shear wave velocity (Vs) increase by 4% at the 410 and the 660. The observed 660 velocity contrast is smaller than expected from the 6% in IASP91 but consistent with a pyrolitic model of mantle composition. The Gorda plate, subducted under northern California, is tracked to the TZ where it seems to flatten and induce uplift of the 410 under northern Nevada. Maps of 410/660 amplitude/topography reveal that the TZ is anomalous beneath the geographical borders of Washington, Oregon, and Idaho, with (1) a thickened TZ, (2) a sharp change in depth of the 660, (3) a reduced 410 conversion amplitude in the North, and (4) a positive “630” discontinuity. Such anomalous structure might be inherited from the past history of plate subduction/accretion. A thinned TZ under the Yellowstone suggests higher-than-average temperatures, perhaps due to a deep thermal plume. Both the “350” and the “590” negative discontinuities extend over very large areas. They might be related either to an increased water content in the TZ, a significant amount of oceanic material accumulated through the past 100 Myr, or both.National Science Foundation (U.S.) (VICI grant NWO:VICI865.03.007

The mantle transition zone as seen by global Pds phases: No clear evidence for a thin transition zone beneath hotspots

International audienceWe present a new global study of the transition zone from Pds converted waves at the 410- and 660-km discontinuities. Our observations extend previous global Pds studies with a larger data set, especially in oceanic regions where we have been able to measure Pds travel times, sampling the mantle transition zone (MTZ) beneath 26 hotspot locations. We find significant lateral variations of the MTZ thickness. Both the maximum variations (+/- 35 - 40 km) and the long-wavelength pattern are in overall agreement with previous SS precursors studies. The MTZ is generally thick beneath subduction zones, where the observed MTZ variations are consistent with thermal anomalies ranging between -100 degrees K and -300 degrees K. In Central and North America, we observe an NW - SE pattern of thick MTZ, which can be associated with the fossil Farallon subduction. We do not find clear evidence for a thin MTZ beneath hotspots. However, the 410- km discontinuity remains generally deepened after correcting our Pds travel times for the 3D heterogeneities located above the MTZ, and its topography variations can be explained by thermal anomalies between + 100 degrees K and +300 degrees K. The depth of the 660-km discontinuity may be less temperature sensitive in hot regions of the mantle, which is consistent with the effect of a phase transition from majorite garnet to perovskite at a depth of 660 km

The 2015 Gorkha earthquake: A large event illuminating the Main Himalayan Thrust fault

International audienceThe 2015 Gorkha earthquake sequence provides an outstanding opportunity to better characterize the geometry of the Main Himalayan Thrust (MHT). To overcome limitations due to unaccounted lateral heterogeneities, we perform Centroid Moment Tensor inversions in a 3-D Earth model for the main shock and largest aftershocks. In parallel, we recompute S-toP and P-to-S receiver functions from the Hi-CLIMB data set. Inverted centroid locations fall within a low-velocity zone at 10–15 km depth and corresponding to the subhorizontal portion of the MHT that ruptured during the Gorkha earthquake. North of the main shock hypocenter, receiver functions indicate a north dipping feature that likely corresponds to the midcrustal ramp connecting the flat portion to the deep part of the MHT. Our analysis of the main shock indicates that long-period energy emanated updip of high-frequency radiation sources previously inferred. This frequency-dependent rupture process might be explained by different factors such as fault geometry and the presence of fluids

Neuronal processing of translational optic flow in the visual system of the shore crab Carcinus maenas

This paper describes a search for neurones sensitive to optic flow in the visual system of the shore crab Carcinus maenas using a procedure developed from that of Krapp and Hengstenberg. This involved determining local motion sensitivity and its directional selectivity at many points within the neurone's receptive field and plotting the results on a map. Our results showed that local preferred directions of motion are independent of velocity, stimulus shape and type of motion (circular or linear). Global response maps thus clearly represent real properties of the neurones' receptive fields. Using this method, we have discovered two families of interneurones sensitive to translational optic flow. The first family has its terminal arborisations in the lobula of the optic lobe, the second family in the medulla. The response maps of the lobula neurones (which appear to be monostratified lobular giant neurones) show a clear focus of expansion centred on or just above the horizon, but at significantly different azimuth angles. Response maps such as these, consisting of patterns of movement vectors radiating from a pole, would be expected of neurones responding to self-motion in a particular direction. They would be stimulated when the crab moves towards the pole of the neurone's receptive field. The response maps of the medulla neurones show a focus of contraction, approximately centred on the horizon, but at significantly different azimuth angles. Such neurones would be stimulated when the crab walked away from the pole of the neurone's receptive field. We hypothesise that both the lobula and the medulla interneurones are representatives of arrays of cells, each of which would be optimally activated by self-motion in a different direction. The lobula neurones would be stimulated by the approaching scene and the medulla neurones by the receding scene. Neurones tuned to translational optic flow provide information on the three-dimensional layout of the environment and are thought to play a role in the judgment of heading