资源环境科技发展态势分析平台

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.