Mohamed A. Alwahedi, Jessica Cleary Hawthorne

The magnitude of postseismic slip is useful for constraining physical models of fault slip. Here we examine the postseismic slip following intermediate-magnitude (M 4 to 5) earthquakes by systematically analyzing data from borehole strainmeters in central and northern California. We assess the noise in the data and identify 36 records from 12 earthquakes that can be interpreted. We estimate postseismic to coseismic moment ratios by comparing the coseismic strain changes with strain changes induced by afterslip in the following 1.5 days. The median estimated postseismic moment is 0.36 times the coseismic moment, with a 90% confidence interval between 0.22 and 0.54. This postseismic moment is slightly larger than typically observed following large (M > 6) earthquakes but smaller than observed following small (M 2 to 4) earthquakes. The intermediate-magnitude postseismic slip suggests a size dependence in the dynamics of earthquakes or in the properties of fault areas that surround earthquakes.