Patterns of co-seismic strain computed from southern California focal mechanisms

S U M M A R Y Geometrical properties of an earthquake population can be described by summation of seismic potency tensors that provide a strain-based description of earthquake focal mechanisms. We apply this method to∼170 000 potency tensors for 0<ML ≤ 5 southern California earthquakes recorded between January 1984 and June 2003. We compare summed tensors for populations defined by faulting region and earthquake magnitude to investigate the relation between earthquake characteristics, tectonic domains and fault-related length scales. We investigate spatial scales ranging from ∼1–700 km and use the results to identify systematic differences between seismic behaviour for different faults and different regions. Our results show features that are indicative of both scale-invariant and scale-dependent processes. On the largest scale the overall potency tensor summation for southern California 0 < ML ≤ 5 earthquakes over ∼20 yr corresponds closely to a double-couple (DC) mechanism with slip direction parallel to relative plate motion. The summed tensors and derived quantities for the different regions show clear persistent variations that are related to the dominant tectonic regime of each region. Significant differences between the non-DC components of the summed tensors, which we relate to fault heterogeneity, indicate systematic differences in deformation associated with earthquake populations from different fault zones or different magnitude ranges. We find an increase of heterogeneity for populations of smaller earthquakes and for regions where faulting deviates strongly from the overall sense of deformation, even when corrected for quality. The results imply an overall organization of earthquake characteristics into domains that are controlled to first order by geometrical properties of the largest faults and the plate motion. Smaller scale characteristics are related to local variations in the orientation, complexity and size of faults.