LARGE-SCALE TECTONIC DEFORMATION INFERRED FROM SMALL EARTHQUAKES

It is a long-standing question whether the focal mechanisms of small e arthquakes can be used to provide information about tectonic deformati on on a regional scale. Here we address this question by using a 28-ye ar record of seismicity in the San Francisco Bay area to compare the s train released by small earthquakes with geological, geodetic and plat e-tectonic measurements of deformation in this region. We show that on a small spatial scale, the strain released by small earthquakes is cl osely related to specific geological features. But when averaged over a regional scale, strain release more closely follows the regional pat tern of tectonic deformation: this relationship holds for all but the largest earthquakes, indicating that the earthquake strain is self-sim ilar(1,2) over a broad range of earthquake magnitudes. The lack of sel fsimilarity observed for the largest earthquakes suggests that the tim e interval studied is not large enough to sample a complete set of eve nts-the fault with the highest probability(3) for hosting one such mis sing event is the Hayward fault.