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High-Resolution Site Response Study of the Los Angeles Basin From the 2019 Ridgecrest Earthquake Sequence

Session: How Well Can We Assess Site Effects So Far? IV

Type: Oral

Date: 4/21/2021

Presentation Time: 05:15 PM Pacific

Description: 

We study site response in urban Los Angeles during the two largest events of the 2019 Ridgecrest earthquake sequence, using recordings from multiple regional seismic networks as well as a subset of 350 stations from the much denser Community Seismic Network (www.csn.caltech.edu). We calculate response spectral amplitudes for a selection of periods of engineering significance (1,3,6, and 8 s) and compute site amplification factors relative to three bedrock sites. The site effects present in urban Los Angeles are significant and reproducible between the M7.1 and M6.4 events. Coherent longer-period amplification patterns are present in the Los Angeles basin with the maximum amplifications found in the central and western Los Angeles basin regions for both events (amplification factors of up to 7, for 6-s period). We observe the largest amplification factors for the shorter periods in the south-eastern Los Angeles basin region (amplification factors of up to 6 for 1-s period); however the amplifications are less spatially coherent. The high network density provides an increase in the spatial resolution of the observations and an opportunity to capture these smaller scale variations. We observe high amplifications in the south-eastern quadrant of the San Fernando Valley basin at all periods up to factors of 9 at 6-s period. We examine possible correlations of the site amplifications with geophysical parameters such as the depth-to-basement and the uppermost 30-m average shear-wave velocity (VS30). We found no significant correlation for the 1-s period, while a weak correlation appears for the 3-s period and gets stronger for the longer periods, which likely reflects the correlation of Vs30 with sedimentary basins.

Presenting Author: Filippos Filippitzis

Student Presenter: No

Authors