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Empirical Map-Based Models of Nonergodic Site Response in the Greater Los Angeles Area

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

Type: Oral

Date: 4/20/2021

Presentation Time: 09:45 AM Pacific

Description: 

We develop empirical estimates of nonergodic site response factors at seismic stations in the greater Los Angeles area using a dataset of recorded ground motions from 473 M 3-7.3 earthquakes in Southern California. The data are a combination of those from the NGA-West2 project (Ancheta et al. 2014), the 2019 Ridgecrest earthquake sequence (Rekoske et al. 2020), and about 10,000 newly-processed records. We estimate site response using an iterative mixed-effects approach that takes into account azimuthal variations in anelastic attenuation and potential bias due to large spatial clusters of co-located earthquake sources. This process yields site response factors for peak ground acceleration (PGA), peak ground velocity, and pseudo-spectral acceleration (PSA) relative to a 760m/s reference condition as defined by the Boore et al. (2014; BSSA14) ground motion model. We employ regression kriging to interpolate the nonergodic site response factors to a densely-spaced grid between stations in the greater Los Angeles area. We find our interpolated model resolution ranges from about 20 arcsec for PGA to about 38 arcsec for 10s PSA. We validate the model using site response calculated at densely-spaced stations in the Community Seismic Network (CSN) that were not considered in model development. We find good agreement between the interpolated model and CSN data, especially for long periods (R²= 0.81 for 5.0 s PSA). We demonstrate that using the model in the USGS ShakeAlert Earthquake Early Warning System for the 1994 M6.7 Northridge earthquake improves the accuracy and timeliness of alerts for Modified Mercalli Intensity 5 shaking. Incorporating the model into nonergodic probabilistic seismic hazard analyses allows for a reduction of aleatory variability to the single-station sigma value and for the use of spatially varying epistemic uncertainty, thus improving hazard estimates especially at long return periods.

Presenting Author: Grace A. Parker

Student Presenter: No

Authors