My major research interests lie in understanding tectonic, magmatic and near surface geological processes that help shape the Earth in its current form. I employ seismic imaging, monitoring and numerical modeling tools to address questions related to these topics. With recent explosion of “big” seismic data, availability of low-cost and convenient seismic sensors and growing high-performance computing capabilities, there is tremendous opportunity to probe the Earth’s internal structure and capture its response to dynamic perturbations in high spatial-temporal resolution. I strive to be part of this effort by pioneering new approaches to transform the large amounts of seismic data in different types (e.g., dense vs. sparse array, temporary vs. permanent, broadband vs. narrow band) to 1) a fundamental understanding of the underlying mechanism driving the plate tectonics via seismic tomography, 2) an improved understanding of geological and hydrological processes through a seismic imaging and monitoring framework and 3) more accurate ground shaking estimation in metropolitan cities for potential future earthquakes.

1. Developing HPC tools for ambient noise seismology

2. Seismic monitoring of volcanic and hydrological processes

To be updated.

3. High-resolution upper mantle imaging through joint inversion of Rayleigh and S wave data

Rayleigh and S wave tomography constraints on subduction termination and lithospheric foundering in central California

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault.

More details on this research topic can be found in Jiang et al., 2018EPSL, doi: 10.1016/j.epsl.2018.02.009. The resulted 3D model from Surface wave data only can be found here.

4. Probing anisotropic structures of continental lithosphere using Surface wave and Body wave data

Seismically anisotropic magma reservoirs underlying silicic calderas

Seismic anisotropy can illuminate structural fabrics or layering with length scales too fine to be resolved as distinct features in most seismic tomography. Radial anisotropy, which detects differences between horizontally (VSH) and vertically (VSV ) polarized shear wave velocities, was investigated beneath Yellowstone caldera (Wyoming, United States) and Long Valley caldera (California). Significant positive radial anisotropy indicating VSH> VSV and low isotropic velocities, were found beneath both calderas at ~5–18 km depths. The positive radial anisotropy (>8%) volumes beneath the calderas are anomalously strong compared to the surrounding areas. We propose that the anisotropic volumes represent sill complexes of compositionally evolved magma, and the magma’s seismic contrast with the crust would largely fade upon crystallization.

Links for 3D radially anisotropic Vs model at Yellowstone and Long Valley.

Crustal deformation in southern California constrained by radial anisotropy from ambient noise adjoint tomography

We build a new radially anisotropic shear-wave velocity model of southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of the Pacific-North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust to the east of the San Andreas Fault, attributed to sub-horizontal alignment of mica/amphibole’s foliation planes resulting from significant extension. Substantial negative radial anisotropy (6%) is revealed in the mid/lower crust to the west of the San Andreas Fault, which has been not observed before. The negative anisotropy is observed beneath geological units with high shear-wave speeds. We interpret it as the result of crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed sub-horizontal foliation. This study highlights the potentially complex CPO patterns resulted from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.

Models can be either downloaded from the IRIS EMC website here or Dr. Kai Wang's website (who is the leading developer).

A middle crustal channel of positive radial anisotropy beneath the northeastern Basin and Range

The northeastern Basin and Range is an area of Earth’s crust that has been dramatically stretched and thinned by tectonic forces. Seismic anisotropy, or wave speed dependence on direction, can provide useful insights into the way in which such deformation organizes crustal structure over long periods of time. We used surface waves to identify discrepancies between horizontally and vertically polarized wave speeds. Anisotropy focused in the middle crust at ~8‐20 km is found to best resolve the observed discrepancies. The results suggest that development and preservation of anisotropy is more effective in the middle crust compared to the lowermost crust. The transition with depth may be explained by increasingly high temperature in the lowermost crust that reduces the abundance of highly anisotropic mica minerals and promotes ductile flow that is distributed across larger volumes rather than localized shear zones. Additionally, we find that areas of exceptionally localized extension called metamorphic core complexes have middle‐to‐lower crustal seismic structure that is similar to the surrounding region despite their distinctive upper crustal structure. These structures formed early in the development of the Basin and Range, consequently we suggest that subsequent ductile deformation in the middle‐to‐lower crust largely over‐printed their structural legacies.