A Dynamic Explanation for the Ruptures of Repeating Earthquakes on the San Andreas Fault at Parkfield

Analyzing the relative source time functions (STFs) of repeating earthquakes (REs) at Parkfield and their spectra indicates that there are two groups of REs. One group of REs (G1) displays simple relative STFs and smooth spectra, whereas the other group (G2) displays complex relative STFs and spectra with obvious holes. We use the boundary integral equation method with different configurations of parameters to simulate the spontaneous dynamic ruptures and fit the observed characteristics. Finally, we find that the differences in the observed characteristics are due to the different configurations of the dynamic parameters. Earthquakes in G1 are self-arresting ruptures, and earthquakes in G2 are runaway ruptures. Moreover, holes in the spectra of G2 are caused by the dynamic rupture propagation, and the frequency of the holes is a function of the radius of the circular fault, the angle between the station azimuth and the rupture propagation, and the rupture velocity. Plain Language Summary The spontaneous dynamic rupture model can simultaneously explain the observed characteristics of two groups (G1 and G2) of repeating earthquakes at Parkfield. The earthquakes in G1 are self-arresting ruptures, whereas the earthquakes in G2 are runaway ruptures. The holes in the spectra of G2 may be caused by the dynamic rupture propagation.