SCEC Award Number 18128 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Understanding long shaking durations within the Los Angeles Basin from shallow earthquakes
Investigator(s)
Name Organization
Zhongwen Zhan California Institute of Technology
Other Participants Voon Hui Lai
SCEC Priorities 4a, 4c, 4d SCEC Groups Seismology, GM, CXM
Report Due Date 03/15/2019 Date Report Submitted 03/11/2019
Project Abstract
 The depths of earthquakes are often overlooked in ground motion studies, yet we observed that earthquake depth strongly modulates waveform amplitude and ground motion duration in the Los Angeles basin. Shallow earthquakes have been observed to excite particularly long durations of shaking within the Los Angeles basin, rising concerns over future large earthquakes with extensive surface ruptures. Using beamforming technique and 3D simulation runs based on the community velocity models, we show evidence that the generation of long shaking duration is mostly due to shallow structures along great circle path. Structures like microbasins within the main basin structure can effectively sustain energy and generate long duration observed in shallow earthquakes.
Intellectual Merit Our findings show the distinct effects of shallow earthquakes on ground motion, particularly shaking duration in basin region. Understanding the origin of long shaking duration will help improve our current community velocity models, which are currently inadequate in describing the observed shaking behavior, and contribute to better estimation of ground motion for seismic hazard assessments.
Broader Impacts This project supported training of graduate student Voon Hui Lai in modeling strong seismic ground motion. We have presented preliminary findings from this project at the 2018 SCEC Annual Meeting as a poster. In addition, we plan to submit a paper detailing our findings to the Journal of Geophysical Research in summer 2019.
Exemplary Figure Figure 4: FK analysis of shaking across a dense array. (a) The back azimuth and slowness of P, S and Surface wave arrivals for both data and 3D synthesis recorded by the Santa Fe Springs array located 35.5 km away from the hypocenter. Earthquake epicenter is at a back azimuth around 300 degree. The dashed circle highlights the missing arrival of energy in synthetics which is observed in data as extended shaking. (b) A polar plot showing coherence measured from data and are stacked over the total duration. Warmer color indicates higher coherence. Most coherent energy are confined to the great circle path. There is some coherent energy observed arriving from an off-great circle azimuth.