Geophysics and Tectonics Seminar - spring-2021

March 31, 2021
noon - 1 p.m.
zoom

Presented By:

• Kenneth Hudson - UCLA Department of Civil and Environmental Engineering

The 2019 Ridgecrest earthquake sequence produced a 4 July M 6.5 foreshock and a 5 July M 7.1 mainshock, along with 23 events with magnitudes greater than 4.5 in the 24 hr period following the mainshock. The epicenters of the two principal events were located in the Indian Wells Valley, northwest of Searles Valley near the towns of Ridgecrest, Trona, and Argus. I describe observed liquefaction manifestations including sand boils, fissures, and lateral spreading features, as well as proximate non-ground failure zones that resulted from the sequence. Expanding upon results initially presented in a report of the Geotechnical Extreme Events Reconnaissance Association, we synthesize results of field mapping, aerial imagery, and inferences of ground deformations from Synthetic Aperture Radar-based damage proxy maps (DPMs). We document incidents of liquefaction, settlement, and lateral spreading in the Naval Air Weapons Station China Lake US military base and compare locations of these observations to pre- and postevent mapping of liquefaction hazards. We describe liquefaction and ground-failure features in Trona and Argus, which produced lateral deformations and impacts on several single-story masonry and wood frame buildings. Detailed maps showing zones with and without ground failure are provided for these towns, along with mapped ground deformations along transects. Finally, I describe incidents of massive liquefaction with related ground failures and proximate areas of similar geologic origin without ground failure in the Searles Lakebed. Observations in this region are consistent with surface change predicted by the DPM. In the same region, geospatial liquefaction hazard maps are effective at identifying broad percentages of land with liquefaction-related damage. I anticipate that data presented in this talk will be useful for future liquefaction susceptibility, triggering, and consequence studies being undertaken as part of the Next Generation Liquefaction project.

April 7, 2021
noon - 1 p.m.
Zoom

Presented By:

• Kevin Shao - UCLA EPSS
• Marina Argueta - UCLA EPSS

Seminar Description coming soon.

April 14, 2021
noon - 1 p.m.
zoom

Presented By:

• Mark Panning - JPL

Seminar Description coming soon.

April 28, 2021
noon - 1 p.m.
zoom

Presented By:

• Angela Marusiak - JPL

Seminar Description coming soon.

May 5, 2021
noon - 1 p.m.
zoom

Presented By:

• Alexander Neely - UCLA EPSS

Seminar Description coming soon.

May 12, 2021
noon - 1 p.m.
zoom

Presented By:

• Julien Chaput - UTEP

Volcanoes are notoriously difficult structures to image from a seismic perspective, owing to their often pronounced topography, complex edifices composed of ash, ice, rock, air pockets, fluids, and various melt related products, and their consequential high attenuation and low scattering mean free paths. Conventional methods relying on direct arrivals, such as P-wave tomography, thus suffer from diffuse seismic envelopes due to strong multiple scattering as well as prohibitive source-station ray coverage inherent in their geometries. In recent years, seismic interferometry has presented an intriguing way to both leverage structures that are particularly complex and expand ray coverage to include any interstation path within the medium of interest. Here, we describe an approach, from basic concepts all the way to cutting edge results, that exploits pervasive distributed icequake activity to reconstruct inter-station impulse responses on Mt Erebus, Antarctica. We subsequently reconstruct a reflectivity image for the magmatic system, all the while significantly pushing back the single scattering limit inherent in classic methodologies. This approach is applicable for any strongly scattering medium in which distributed ambient seismicity exists, and will be generalized at other glaciated North American volcanoes in the future.

May 19, 2021
noon - 1 p.m.
zoom

Presented By:

• Quancheng Huang - New Mexico State University

The Earth’s mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. Mineral physics experiments predict that wadsleyite can have strong single-crystal anisotropy at the pressure and temperature conditions of the MTZ. Thus, seismic anisotropy is possible to exist in the upper MTZ where lattice preferred orientation (LPO) of wadsleyite is produced by mantle flow. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we use a body wave method, SS precursors, to study the topography change and seismic anisotropy near the MTZ discontinuities. We first investigate the sensitivity of SS precursors to azimuthal anisotropy through 3-D synthetic modeling. We then stack a global dataset of SS precursors to explore the azimuthal dependence of travel times and amplitudes of SS precursors, and constrain the azimuthal anisotropy in the MTZ. We find evidence for significant VS anisotropy (~13%) with a trench-perpendicular fast direction (Θ=95°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the lattice-preferred orientation of highly anisotropic minerals (e.g., stishovite) induced by the stress within the stagnant slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, indicating that Hawaiian mantle plume has not produced observable azimuthal anisotropy in the MTZ. Global azimuthal stacking reveals ~1% azimuthal anisotropy in the upper mantle, but negligible anisotropy (< 1%) in the MTZ. We have demonstrated that SS precursors can serve as a new method to constrain seismic anisotropy in the upper and mid-mantle, especially beneath oceanic regions where seismic stations are underpopulated for shear wave splitting measurements.

May 26, 2021
noon - 1 p.m.
zoom

Presented By:

• Valeria Jaramillo - UCLA EPSS

One of the most important factors that controls the stress state and rock failure in Earth’s crust is pore-fluid pressure. Although its role in brittle deformation in the shallow crust (< 1-3 km) has been extensively examined, and in some cases quantified by direct bore-hole measurements, how pore-fluid pressure affects crustal deformation at brittle-ductile-transition depths (~15-25 km) remains poorly constrained. This depth range is dominated by microseismicity and fluid-driven tectonic tremors such as those along the root zone of the San Andreas fault. In the field, deformation that occurred at brittle-ductile transition depths is commonly expressed by the development of semi-brittle shear zones. A semi-brittle shear zone is characterized by coeval cataclastically (frictional sliding and fracturing) and crystal-plastically (dislocation and diffusion creep) deformed rocks. The mixed deformation styles within the same shear zone require stress continuity across the contact between brittle and ductile structures. This stress-continuity condition in turn allows us to use paleopiezometry and paleobarometry to determine the stress state (i.e., the differential stress and mean stress) during semi-brittle deformation. Note that the mean stress estimated from paleobarometry differs from lithostatic stress, which represents the value of a principal stress that can be determined by the combination of the mean stress and differential stress values. Because the frictional coefficient (~0.6) and cohesive strength of crystalline rocks (<50 MPa) are well-known from laboratory experiments (i.e., Byerlee’s Law and rock-fracture experiments), we are able to use the estimated magnitudes of differential and mean stresses to determine the ratio between pore-fluid pressure and lithostatic pressure during the development of the Miocene Whipple detachment shear zone. This exhumed mid-crustal, normal-slip shear zone was formed by semi-brittle deformation, and consists of brittlely deformed amphibolite blocks and ductilely sheared quartzite. Assuming 0.6 for the friction coefficient and 50 MPa for the cohesive strength of the amphibolite blocks in the Whipple shear zone, our results require the pore-fluid pressure ratio of 0.90 to 1.10. The excessive pore-fluid pressure ratio with a value greater than 1.0 is consistent with observed tensile fractures developed during the crystal-plastic deformation of quartzite in the shear zone. Our future work is to determine the distribution of pore-fluid pressure ratios across the Whipple shear zone. This knowledge will allow us to better understand fluid-migration processes and pore-fluid-pressure evolution during semi-brittle deformation in the mid-crustal shear zone. Laboratory measurements of the friction coefficient and cohesive strength of the amphibolite blocks will provide a tighter constraint on the range of the estimated pore-fluid pressure ratios in the Whipple shear zone.

June 2, 2021
noon - 1 p.m.
zoom

Presented By:

• Jacqueline Austermann - Lamont

Seminar Description coming soon.