A Case Study Exploring Azimuthal Dependence of Peak Ground Acceleration: Motivated by Observations from the 29 May 2013 Isla Vista M4.8 Earthquake

Lilah M. Guerra, Alan Yong, Tara Nye, Andrea Di Martino, Jose R. Gomez, Jamison H. Steidl, Antony Martin, Koichi Hayashi, Alessandro Amorosi, & Ali Shafiee

Submitted September 11, 2022, SCEC Contribution #12298, 2022 SCEC Annual Meeting Poster #043

During the M4.8 2013 Isla Vista Event, seismic stations CE.25932 and CI.USB recorded anomalous peak ground acceleration (PGA) values. We consider the role of path effects; specifically, the azimuthal dependence of PGA for this region to see if this explains the anomalous recording. The University of California Santa Barbara (UCSB) stations, CE.25091, CE.25392, and CI.USB, recorded peak ground accelerations of 0.26g, 0.08g, and 0.26g, respectively. Measured VS30, or time-averaged shear wave velocity of the upper 30 m from the surface, is 172 m/s for CE.25091, 323 m/s for CE.25392, and 300 m/s for CI.USB. PGA from stations CE.25392 and CI.USB differ by a factor of three, an anomalous value when considering that these two stations are separated by only 350m, their azimuths vary (w.r.t. epicenter) by approximately 2°, and they are positioned above the same marine terrace formation. In order to investigate the possible difference in site conditions between the two stations, we perform multiple microtremor-based horizontal-to-vertical spectral ratio (mHVSR) measurements around CE.25392 and CI.USB. We find spectra for these mHVSR locations to share similar characteristics, which lack clear resonance frequency peaks. Thus, the lack of significant impedance contrast, as implied by the indistinguishable amplitudes across different mHVSR measurements, suggests there are no appreciable differences in resonance properties and that these observations are insufficient to explain differences in observed PGAs. Though our original assumptions are that both source and path effects are essentially the same for both stations, we proceed to focus our investigation on possible differences in path effects, particularly, azimuthal dependence on PGA. We investigate the origin of this anomaly by examining whether the contrasting PGA values are a consistent observation across various events recorded by the three UCSB stations or whether this 2013 event is an isolated incident. We also investigate the effects of directionality on the same three stations by analyzing PGA with varying azimuth. Other studies may be necessary to understand the underlying mechanisms that produced this anomaly.

Key Words
Isla Vista, peak ground acceleration, azimuth, mHVSR, Vs30

Guerra, L. M., Yong, A., Nye, T., Di Martino, A., Gomez, J. R., Steidl, J. H., Martin, A., Hayashi, K., Amorosi, A., & Shafiee, A. (2022, 09). A Case Study Exploring Azimuthal Dependence of Peak Ground Acceleration: Motivated by Observations from the 29 May 2013 Isla Vista M4.8 Earthquake. Poster Presentation at 2022 SCEC Annual Meeting.

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