Long Shaking Durations within the Los Angeles Basin from Shallow Earthquakes

Voon Hui Lai, Zhongwen Zhan, Robert W. Graves, & Donald V. Helmberger

Published August 13, 2018, SCEC Contribution #8473, 2018 SCEC Annual Meeting Poster #001

Shaking duration is a critical factor in assessing seismic hazards and building collapse risk, potentially as important as peak ground motion in large earthquake scenarios. The depths of earthquakes are often overlooked in ground motion studies, yet we observed that that earthquake depth strongly modulates waveform amplitude and ground motion duration in the Los Angeles basin. Shallow earthquakes excite particularly long durations of shaking within the Los Angeles basin, rising concerns over future large earthquakes with extensive surface ruptures. Current SCEC community velocity models (CVMs) can predict well the waveforms up to 2s period, but not the late coda waves, which contribute to the long shaking durations. 3D simulations of the two similar magnitude Fontana events of different depths using the latest CVM-H15.1.1 and CVM-S4.26-M01 models reveals several promising results to accurately model ground motion at higher frequency (up to 0.5 Hz). Overall, the CVM-S4.26-M01 model fits the waveforms better for both events, including the first several tens of seconds at period up to 2 seconds and can also capture well the unusually large ground motions at Whittier Narrows region. For the shallow event, which is not used in the inversion for the velocity model, the model captures most of the initial ground motions well but do not capture the long duration even at the period of 5 seconds. Given the small earthquakes can be modeled as point source at our targeted frequency, the discrepancy between the model and the synthetics suggests that the shallow crustal layer in the CVMs may not be fully described. In addition, beam-forming analysis of a shallow earthquake event recorded by temporary dense arrays shows several packets of coherent energy with different slowness, and in particular at 80 – 110 seconds arriving from off-great circle azimuths, indicative of strong scattering behavior within basin. A better understanding of the shallow structures in basin can help explain the long shaking duration and overall stronger ground motions observed for shallow earthquakes.

Lai, V., Zhan, Z., Graves, R. W., & Helmberger, D. V. (2018, 08). Long Shaking Durations within the Los Angeles Basin from Shallow Earthquakes. Poster Presentation at 2018 SCEC Annual Meeting.

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