Evidence on lateral velocity variation in lower crust – upper mantle beneath Californian margin and short note on modeling small temporal variations of seismic velocities at the Geysers Geothermal Field, California
About the Speaker:
Voonhui Lai (goes by Voon) is currently a third year Graduate student at Caltech. Her interest is in structural seismology, where she uses waveform modeling to study deep lithospheric structure. She is also interested in understanding how stress causes small perturbations in velocity structures. Previously she completed her bachelor degree in Geophysics at University of California, Berkeley in 2014.
About the Event:
The talk will be split into two parts, the first, on the evidence on lateral velocity variation in lower crust – upper mantle beneath Californian margin:
Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30 – 50 seconds) SH waves arrive up to 15 seconds earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust − mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong lid buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and is consistent with the observed asymmetric strain rate across the boundary.
The second part of the talk focuses on the short note on modeling small temporal variations of seismic velocities at the Geysers Geothermal Field, California:
Perturbations in subsurface elastic parameters induce changes in seismic velocity. To understand the stress perturbations due to geothermal operation, we apply seismic noise interferometry to examine the temporal variations of seismic velocity (dv/v) at the Geysers Geothermal Field, California. Our observations show a strong positive correlation between dv/v and net production (steam production minus fluid injection). A number of hypotheses have been suggested to cause stress perturbations in the field, including poroelastic-induced stresses, direct elastic loading and thermoelastic-induced stresses. Here, we show the framework on the modeling the observed dv/v by considering the dv/v induced by each stress.