Repeating Earthquakes With Remarkably Repeatable Ruptures on the San Andreas Fault at Parkfield

Rachel E. Abercrombie, Xiaowei Chen, & Jiewen Zhang

Published December 7, 2020, SCEC Contribution #11741

We calculate rupture directivity and velocity for earthquakes in three well-recorded repeating sequences (2001–2016) on the San Andreas Fault at Parkfield using P waves from borehole recordings and the empirical Green's function method. The individual events in each sequence all show the same directivity; the largest magnitude sequence (M ~ 2.7, 8 events) ruptures unilaterally NW (at ~0.8Vs), the second sequence (M ~ 2.3, 9 events) ruptures unilaterally SE, and the smallest magnitude sequence (M ~ 2, 11 events) is less well resolved. The highly repetitive rupture suggests that geometry or material properties might control nucleation of small locked patches. The source spectra of the M ~ 2.7 sequence exhibit no detectable temporal variation. The smaller M sequences both exhibit a decrease in high-frequency energy following the M6 earthquake that recovers with time. This could indicate a decrease in stress drop, an increase in attenuation, or a combination of the two, followed by gradual healing.

Citation
Abercrombie, R. E., Chen, X., & Zhang, J. (2020). Repeating Earthquakes With Remarkably Repeatable Ruptures on the San Andreas Fault at Parkfield. Geophysical Research Letters, 47(23). doi: 10.1029/2020GL089820.