The surface fractures nearby the Ridgecrest earthquakes

shijie001 Seismic Source
Time 24 Nov. 10:00 AM.
Speaker: Xiaohua Xu
Abstract:
What can we learn from the surface fractures nearby the Ridgecrest earthquakes?
Contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detected mm-scale deformation over broad regions of active faulting. Satellite radar observations revealed hundreds of previously unmapped linear strain concentrations (or fractures) surrounding the 2019 Ridgecrest earthquake sequence. We documented and analyzed displacements and widths of 169 of these fractures. While most fractures are displaced in the direction of the prevailing tectonic stress (prograde), a large number of them are displaced in the opposite (retrograde) direction. We constructed fault slip model and computed static Coulomb stress change for a suite of receiver fault orientations consistent with those exhibiting surface fractures. Comparing static stress change models to the phase gradient maps, we were able to determine that these surface fractures can largely be explained with static stress change from the earthquake sequence, with most antithetic motions are associated with compliant fault deformation, where most prograding ones being shallow frictional slip. We further explore a conceptual model that can explain these observations. One important implication from the model is that much of the “off-fault” strain in the Mojave shear zone is due to permanent inelastic deformation on many small faults. This is in support of the hypothesis that compensation of shallow slip deficit occurs on distributed shallow structures.
Reference:
Xu X, Sandwell D T, Ward L A, et al. Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence[J]. Science, 2020, 370(6516): 605-608.  

 

Three-dimensional thermochemical model and melt distribution

shijie001 Seismic modeling
Time: Thursday, 4 pm, 12.11.2020.
Title: Three-dimensional thermochemical model and melt distribution beneath Northeast China.
By: Anqi Zhang.
Anqi is a Postdoc at Department of Ocean Science and Engineering, Southern University of Science and Technology. her research is mainly focused on using multiple geophysical data to infer the subsurface temperature/composition etc., via a multi-observable probabilistic inversion method. 
 
Abstract:

Intraplate volcanisms spanning from the Late Cretaceous to present are widespread in Northeast China (NEC). Numerous geophysical and geochemical studies have been carried out during the past few decades to explore the origin and mantle dynamic of these intraplate volcanisms. However, the hypotheses put forward for the geodynamic evolution of the region are diverse and often controversial, such as mantle plume, mantle upwelling originates from the transition zone, small-scale convection, and the influx of fluid released from the stagnant slab of the Pacific plate in the mantle transition zone. Here we use a probabilistic inverse method to jointly invert Rayleigh wave dispersion data, surface heat flow measurements, geoid height, and absolute elevation data. The output is a 3D model of temperature, bulk density, seismic velocity, and compositional structure of the lithosphere beneath NEC. Overall, our results reveal high temperature anomalies beneath all active volcanoes as expected. The resulting lithosphere-asthenosphere boundary (LAB) depth shows significant correlations with independently collected geochemical parameters of young basalts (< 5 Ma) in NEC. This observation manifests lithosphere thickness effect on compositional variations of continental basalts (the lid effect) and emphasizes the consistency between the geophysical and geochemical data. We also estimate distribution of melt in the uppermost mantle beneath the NEC from joint inversion, which is in good agreement with the surface locations of young basalts. Our predicted melt distribution indicates that Changbaishan (CBS) basalts are produced by partial melting of asthenospheric mantle underlies relatively thin lithosphere. The mantle upwelling beneath the CBS volcano may originate from deep mantle sources, which causes large degree of partial melting at 60 km depth. In contrast, basalts in the Xingmeng belt at the west of Songliao basin, such as the Halaha and Abaga volcano, may be derived from relatively low-degree partial melting caused by localized asthenospheric upwelling at shallow depths. 

Please note that this talk will be given in Chinese (Mandarin), but the slices will be shown in English.