Azimuth Dependence of Basin Response in Southern California

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

We consider the source-to-site azimuth dependence of site response in Southern California basin structures. Site response is traditionally parameterized to explicitly capture linear amplification that scales with shallow shear wave velocity, resonance effects conditioned on site period, nonlinear damping effects that scale with the strength of shaking and basin effects conditioned on depth to bedrock. However, assumptions are made in models developed for forward use, including: (1) independence between site response and earthquake source properties, such as location and frequency content; and (2) that shear waves are the primary drivers of site response. Simulation-based work has shown that there is additional complexity in site response for locations within basins due to the trapping and reflection of surface waves and that these effects vary on an event-to-event basis. For example, Olsen et al. (2006) show amplification in the Los Angeles Basin during a south-to-north San Andreas rupture scenario due to waveguide effects through sedimentary basins and Wirth et al. (2019) show variation in Seattle Basin response from earthquakes depending on the location of those events. However, models that harness these types of findings are limited. We compile earthquake records from M3+ earthquakes in Southern California within 200 km of the Los Angeles Basin, including over 100 events from the Ridgecrest sequence. We group earthquakes by path azimuth, including those within basin structures. A mixed-effects analysis is performed on each group to estimate site response. We find earthquakes with east-west source-to-site azimuths roughly perpendicular to the strike of regional structures, like the Ridgecrest events and earthquakes offshore of the Channel Islands, excite basin response different from earthquakes with source-to-site azimuths roughly parallel to regional structures. This is especially the case in the San Bernardino Basin, which does not exhibit the strong amplification modeled in the simulated Shakeout San Andreas scenario for earthquakes with east-west path azimuths.

Presenting Author: Grace A. Parker

Authors