Rupture directivity of microearthquakes on the San Andreas Fault from spectral ratio inversion

Earthquakes often occur on faults separating materials with different elastic properties. On theoretical grounds, it is expected that earthquakes on such bimaterial interfaces might have a preferred rupture propagation direction, that being the direction of motion of the more compliant material. The goal of this paper is to determine whether a large sample of natural earthquakes on a bimaterial interface exhibits this tendency. Since the creeping section of San Andreas Fault has a large across-fault velocity contrast and has produced thousands of microearthquakes over the last few decades, the rupture directions of similar to 3100 magnitude 0.5-3.0 earthquakes were studied using spectral ratio analysis. The spectral ratios of all earthquake pairs in spatially defined clusters were fitted with synthetic spectral ratios at qualified stations. The synthetics were computed from a simple moving point source model in which each modelled earthquake has four parameters: two rupture lengths (one to the SE and one to the NW) and their propagation velocities. The resolution of rupture directivity increases with event size, such that nearly 900 events, mostly those larger than similar to 70 m, appear reasonably well resolved. The inversion results suggest that similar to 40 per cent of the well-resolved events are roughly bilateral, although more than similar to 80 per cent of the 144 events classified as strongly unilateral rupture to the SE, consistent with the theoretical prediction. For those rupture halves that were large enough for the propagation speed to be somewhat resolved, that speed was greater by roughly 10 per cent for those halves propagating to the SE, qualitatively consistent with numerical and laboratory experiments. We also found that events with nearby foreshocks within several hours tend to rupture away from those foreshocks, whether to the NW or to the SE, indicating that asymmetry of prior stressing history can exert a stronger influence on rupture directivity than the material contrast.