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

AbstractWe calculate rupture directivity and velocity for earthquakes in three wellrecorded repeating sequences (20012016) 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 highfrequency 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.

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