It is a long-standing question whether the focal mechanisms of small earthquakes can be used to provide information about tectonic deformation on a regional scale. Here we address this question by using a 28-year record of seismicity in the San Francisco Bay area to compare the strain released by small earthquakes with geological, geodetic and plate-tectonic measurements of deformation in this region. We show that on a small spatial scale, the strain released by small earthquakes is closely related to specific geological features. But when averaged over a regional scale, strain release more closely follows the regional pattern of tectonic deformation : this relationship holds for all but the largest earthquakes, indicating that the earthquake strain is self-similar(1,2) over a broad range of earthquake magnitudes. The lack of selfsimilarity observed for the largest earthquakes suggests that the time interval studied is not large enough to sample a complete set of events-the fault with the highest probability(3) for hosting one such missing event is the Hayward fault.