Meetings: Abstracts

The Decay Rate of Aftershock Density with Distance

FELZER, K. R. and BRODSKY, E. E., Earth and Space Sciences Department, University of California, Los Angeles, CA, 90095, kfelzer@moho.ess.ucla.edu, brodsky@moho.ess.ucla.edu

The static stress change plus rate and state friction model (Dieterich, 1994) is presently the most comprehensive and well accepted physical model for aftershock triggering. The model correctly predicts how aftershocks decay with time. We test whether the model also accurately predicts how aftershocks decay with distance. In particular, the rate and state model predicts that at short times aftershock densities should decay with distance, r, from the mainshock as e^(r^-3). We find using the relocated Southern California earthquake catalog of Shearer et al. (2003) and small mainshocks (that may be treated as point sources) a decay rate of r^-2.3. This decay rate, very different from that predicted by the rate and state model and slower even than the decay of pure static stresses (which decay as r^-3), suggests that dynamic stresses, which decay more slowly, may be the primary agent for aftershock triggering. If aftershocks are triggered by dynamic rather than static stresses we might expect to see continuous aftershock triggering from the near to very far field, again since dynamic stresses decay more slowly than static ones. Looking at very short times we find evidence that small (M 2-4) mainshocks may continuosly trigger aftershocks out to distances of 100 km, much further than previously thought. We also find, in agreement with some previous authors (Huc and Main, 2003; Davidsen and Paczuski, 2005), that the distance to which a mainshock can trigger aftershocks appears to be independent of its magnitude. Small mainshocks may appear to have a smaller aftershock zone simply because they trigger fewer aftershocks, but when the aftershocks of many small mainshocks are combined the distribution of aftershock distances is the same as for larger earthquakes.