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

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For three decades it has been anticipated that earthquake ruptures along an interface separatingmaterials with different elastic properties will have a favored propagation direction, that being thedirection of motion of the more compliant material. However, observing this tendency on naturalfaults has been difficult, in large part because of the small number of significant earthquakes onfaults with a well-characterized velocity contrast. The PI has used spectral ratios to directly estimatedirectivity in a catalog of over 3,000 small earthquakes along a 30-km section of the San Andreasfault with a large and well-characterized velocity contrast. The spectral ratios were fitted with asimple moving point source model in which each modeled earthquake has four parameters: tworupture lengths (one to the SE and one to the NW) and their propagation velocities. Nearly 900earthquakes, mostly those larger than 70 m, appear reasonably well resolved. The inversion resultssuggest 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 theoreticalprediction. For those rupture halves that were large enough for the propagation speed to besomewhat 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 withnearby (in space and time) foreshocks tend to rupture away from those foreshocks, whether to theNW or to the SE, indicating that asymmetry of prior stressing history can exert a stronger influenceon rupture directivity than the material contrast.A major goal of the proposed work is to greatly increase the size of their database in bothspace and time. This will allow them to explore how the correlation between foreshocklocation and mainshock directivity decays with increasing spatial and temporal distance betweenforeshock and mainshock, whether the apparent lack of correlation between mainshock directivityand aftershock asymmetry we have observed stands up to a larger data sample, and how thesebehaviors 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 beginnumerical modeling of earthquake nucleation on a bimaterial interface, with the specific goals ofunderstanding 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 beenpredicted that earthquakes on such faults could have a preferred propagation direction. Establishingwhether this is actually the case is relevant to hazards mitigation because there is much strongerground shaking in the direction that the rupture propagates; it has even been proposed that buildingcodes could be altered to reflect this. This gives the earthquakes we are examining an importancewhich surpasses their small size. Their usefulness lies in their large number, so that the resultsare statistically meaningful, and in their ability to teach us about connecting numerical models ofearthquake rupture to real earthquakes generally.