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Rupture directivity of small earthquakes at Parkfield

Deborah L. Kane, Peter M. Shearer, & Bettina P. Goertz-Allmann

Published 2013, SCEC Contribution #1725

Theoretical modeling of strike-slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction. We test this model with 450 small earthquakes (2 < M < 5) from Parkfield, California, to look for evidence of consistent rupture directivity along the San Andreas Fault. We analyze azimuthal variations in earthquake source spectra after applying an iterative correction for wave propagation effects. Our approach avoids directly modeling source spectra because these models generally assume symmetric rupture; instead, we look for azimuthal variations in the amplitudes of the source spectra over specified frequency bands. Our overall results show similar proportions of events exhibiting characteristics of rupture directivity toward either the southeast or northwest. However, the proportion of events with southeast rupture directivity increases as we limit the data set to larger magnitudes, with 70% of the 46 events M > 3 exhibiting southeast rupture characteristics. Some spatial and temporal variability in rupture directivity is also apparent. We observe a higher proportion of northwest directivity ruptures following the 2004 M 6 Parkfield earthquake, which ruptured toward the northwest. Our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors.

Kane, D. L., Shearer, P. M., & Goertz-Allmann, B. P. (2013). Rupture directivity of small earthquakes at Parkfield. Journal of Geophysical Research: Solid Earth, 118(1), 212-221. doi: 10.1029/2012JB009675.