Abstract

Small-scale faulting at seismogenic depths in the crust appears to be more homogeneous than previously thought. I study three new high-quality focal- mechanism datasets of small (M < ∼3) earthquakes in southern California, the east San Francisco Bay, and the aftershock sequence of the 1989 Loma Prieta earthquake. I quantify the degree of mechanism variability on a range of length scales by comparing the hypocentral distance between every pair of events and the angular difference between their focal mechanisms. Closely spaced earthquakes (interhypocentral distance <∼2 km) tend to have very similar focal mechanisms, often identical to within the 1-sigma uncertainty of ∼25°. This observed similarity implies that in small volumes of crust, while faults of many orientations may or may not be present, only similarly oriented fault planes produce earthquakes contemporaneously. On these short length scales, the crustal stress orientation and fault strength (coefficient of friction) are inferred to be homogeneous as well, to produce such similar earthquakes. Over larger length scales (∼2–50 km), focal mechanisms become more diverse with increasing interhypocentral distance (differing on average by 40–70°). Mechanism variability on ∼2- to 50-km length scales can be explained by relatively small variations (∼30%) in stress or fault strength. It is possible that most of this small apparent heterogeneity in stress or strength comes from measurement error in the focal mechanisms, as negligible variation in stress or fault strength (<10%) is needed if each earthquake is assigned the optimally oriented focal mechanism within the 1- sigma confidence region. This local homogeneity in stress orientation and fault strength is encouraging, implying it may be possible to measure these parameters with enough precision to be useful in studying and modeling large earthquakes.

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