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Rupture Directivity Effects Observed in Ground Motions From the 2022 M5.1 Alum Rock Earthquake

Description: 

We present evidence for southeastward rupture directivity during the October 25, 2022, M5.1 strike-slip Alum Rock earthquake on the Calaveras fault. The average recorded ground motions were a factor of 3 less than the expected median shaking. However, there was large variability controlled by site azimuth, with over an order of magnitude between motions recorded at the same source-to-site distance but at different azimuths. This was observed for peak ground acceleration and velocity, and at all considered oscillator periods from 0.01-5s, with peak variability at 1s. To understand if this was controlled by directivity, azimuthal path effects, or site effects, we examine ground motions from 5 smaller earthquakes (M<4.1) with epicenters within 3km of that of the M5.1 event. We assume that these events had no directivity and use them to estimate repeatable path and site effects. We adjust residuals from the M5.1 event using these path and site terms to isolate directivity effects. We find that azimuthal trends persist in the adjusted residuals, and that directivity was responsible for both a factor of 5 amplification and de-amplification. Earthquake simulations confirm this, with a point-source model fitting the observed ground motion amplitudes to the south of the epicenter, but significantly over-predicting amplitudes to the north, while a kinematic model with unilateral rupture to the south at 2.5 km/s improves the match at all azimuths. By inverting the peak ground motions, we fit a line source with southeastward directivity at the same velocity as the kinematic model (Boatwright, 2007). The same sense of directivity is observed for the 2007 M5.6 Alum Rock and 1984 M6.2 Morgan Hill earthquakes (Hartzell and Heaton, 1986), potentially indicating consistent fault properties in this section of the Calaveras. This underscores the importance of considering directivity in seismic hazard analysis, as directivity in even moderate magnitude earthquakes can lead to dramatic differences in shaking depending on site azimuth, all else being equal.

Session: High-frequency Ground Motion Measurements, Assessments and Predictions

Type: Oral

Date: 4/18/2023

Presentation Time: 09:00 AM (local time)

Presenting Author: Grace A. Parker

Student Presenter: No

Invited Presentation: 

Authors