An Observational Study of Microearthquakes on a Bimaterial Interface

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The PI has amassed a large catalog of precise earthquake locations using waveform cross-correlation. Previous work with this catalog has shown that aftershocks of microearthquakes along the San Andreas fault near San Juan Bautista are distributed very asymmetrically. Within an estimated 1-2 radii of the margin of the mainshock, there are nearly 3 times as many aftershocks to the northwest as to the southeast. In contrast, there is no discernible asymmetry in the number of aftershocks above or below the mainshock, or along the Calaveras fault, which has no significant across-fault velocity contrast. These observations are consistent with the asymmetry being related to the contrast in seismic velocities across the San Andreas, which in this region may reach 35 or even 50%. Numerical studies of elastodynamic ruptures on a bimaterial interface, carried out under a previous NSF grant, show that ruptures governed by slip-weakening friction grow as slightly asymmetric cracks. Significantly, there is a large reduction in normal stress immediately behind the rupture front moving (in the context of the San Andreas) to the southeast. When this rupture front is slowed by a stress barrier the tensile stress continues onward, and for a wide range of parameters carries a dying slip pulse with it. After slip ceases, the southeast edge of the rupture is left far below the failure threshold because the tensile stress that carried the slip pulse there is gone. In contrast, the northwest rupture front slows gradually and stops much as it would in a homogeneous medium, with the rupture front remaining very nearly at the failure threshold after slip has stopped. This provides an appealing explanation for the aftershock asymmetry. The current project extends this work in two important ways. First, because the numerical models are suggestive of a pronounced asymmetry among subevents in compound earthquakes, and because tantalizing evidence for this was uncovered in older work (but recognized only retrospectively), the PI is systematically searching for such asymmetry throughout this region. Second, because the aftershock asymmetry is thought to be a manifestation of how the material contrast across the fault affects the dynamics of the mainshock, the PI is searching for directivity of the microearthquakes themselves, as this has also been proposed to be a byproduct of the material contrast across the fault. Quasi-static and elastodynamic models are being applied to interpret these observations.The main goal of this work is to further characterize a rather remarkable observation, and to place it in the larger context of rupture along a bimaterial interface. There are theoretical reasons to suppose that rupture on a bimaterial interface leads to preferential propagation in a particular direction, but the geologic evidence for this is mixed. Because of the implications of such directivity for seismic hazards, there is considerable interest in this question. Although these observations concern only microearthquakes, they have the advantage of statistical significance, and characterizing the effect of the material contrast on microearthquakes will contribute to our overall understanding of this phenomenon.