A (mostly) Observational Study of Microearthquakes on a Bimaterial Interface

Princeton University, Princeton NJ

Investigators

Abstract

For three decades it has been anticipated that earthquake ruptures along an interface separating materials with different elastic properties will have a favored propagation direction, that being the direction of motion of the more compliant material. However, observing this tendency on natural faults has been difficult, in large part because of the small number of significant earthquakes on faults with a well-characterized velocity contrast. The PI has used spectral ratios to directly estimate directivity in a catalog of over 3,000 small earthquakes along a 30-km section of the San Andreas fault with a large and well-characterized velocity contrast. The spectral ratios were fitted with a simple moving point source model in which each modeled earthquake has four parameters: two rupture lengths (one to the SE and one to the NW) and their propagation velocities. Nearly 900 earthquakes, mostly those larger than 70 m, appear reasonably well resolved. The inversion results suggest that 40% of the well-resolved events are roughly bilateral, although more than 80% 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% for those halves propagating to the SE, qualitatively consistent with numerical and laboratory experiments. They find that events with nearby (in space and time) foreshocks 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. A major goal of the proposed work is to greatly increase the size of their database in both space and time. This will allow them to explore how the correlation between foreshock location and mainshock directivity decays with increasing spatial and temporal distance between foreshock and mainshock, whether the apparent lack of correlation between mainshock directivity and aftershock asymmetry we have observed stands up to a larger data sample, and how these behaviors correlate with the local across-fault velocity contrast. The investigators will also undertake a systematic search for asymmetry in the location of sub-events in compound earthquakes, and begin numerical modeling of earthquake nucleation on a bimaterial interface, with the specific goals of understanding the aforementioned decay of the influence of foreshocks on mainshock directivity, and the influence of the bimaterial contrast on earthquake nucleation generally. Large faults that are capable of producing damaging earthquakes have also slipped large distances, and so they often juxtaposes rocks with different mechanical properties. It has long been predicted that earthquakes on such faults could have a preferred propagation direction. Establishing whether this is actually the case is relevant to hazards mitigation because there is much stronger ground shaking in the direction that the rupture propagates; it has even been proposed that building codes could be altered to reflect this. This gives the earthquakes we are examining an importance which surpasses their small size. Their usefulness lies in their large number, so that the results are statistically meaningful, and in their ability to teach us about connecting numerical models of earthquake rupture to real earthquakes generally.