Level-set imaging of the Los Angeles Basin using the Community Seismic Network

Abstract:

The Los Angeles Metropolitan Area is the second most populous urban zone in the USA and the third largest city economy in the world with an annual gross domestic product in excess of 1 trillion US dollars. It is also situated at a major transpressional bend of the San Andreas Fault and is principally built on top of several deep sedimentary basins. As such, the risk posed by earthquake damage to Los Angeles by both local thrust faults and regional major strike slip faults is high. Ground motions of the Mw 7.1 July 5 2019 Ridgecrest Earthquake in the 4-10s period band, relevant to high-rise buildings in the basin, exhibited sustained higher amplifications than predicted by reference community velocity models. Particle motion analysis indicates that the strongest component in this period band is a large coherent Love wave. In an effort to better map earthquake hazard and improve velocity models within the Los Angeles Basin, we utilize the high resolution data from the Community Seismic Network (CSN), a large 400-station permanent urban deployment, to invert Love wave dispersion, derived from eikonal tomography of two-station cross-correlation travel-time delays, and relative amplification data from the Mw 7.1 July 5 2019 Ridgecrest Earthquake. To fit the data, we employ a geologically informed level-set parametrization that seeks to define the basin edge, which we invert using a recently developed Tikhonov Ensemble Kalman Inversion scheme, a highly efficient derivative-free optimizer. We find that the Ridgecrest Earthquake data is best explained by a deepening of the LA Basin (compared to the CVMS-4.26 reference model) along its Northwest-Southeast axis relative to its deepest point, just south of downtown LA. Additionally, the deeper basin edge extends further to the East of downtown LA towards East Los Angeles. This result offers new progress towards the parsimonious incorporation of detailed local basin models within regional reference models utilizing an objective inverse-problem framework, and highlights the importance of accurate basin geometry models when accounting for the potentially significant amplification of surface waves from regional earthquakes in the high-rise building band.