Stress drops of repeating earthquakes on the San Andreas Fault at Parkfield

I calculate well-resolved corner frequencies and stress drops for 25 earthquakes (1989-2006) in the three repeating sequences targeted by the San Andreas Fault Observatory at Depth, using borehole data and multiple, highly correlated empirical Green's functions (EGFs). The earthquakes in the largest magnitude (M 2.1) cluster exhibit source spectra well-fit by a circular source model. The corner frequencies correlate with those from the regional study by Allmann and Shearer (), suggesting that the interevent variability is resolvable. The earthquakes have stress drops between 25 and 65MPa, with a gradual increase before the 2004M6 earthquake, followed by an immediate decrease, then a rapid return to previous levels. The spectra of the cluster of M 1.9 earthquakes include high-frequency energy not fit by simple source models and so stress drops are unreliable, and probably underestimated (1-20MPa). There is no correlation with previous studies, and interevent variation is not resolvable. The earthquakes in the smallest magnitude cluster (M 1.8) have the highest corner frequencies, but similar stress drops (4-120MPa). The stress drops exhibit the same temporal variation as the first cluster, but there is poor correlation with Allmann and Shearer (), probably because their frequency bandwidth is too limited.