Seismic moment-frequency relation for shallow earthquakes: Regional comparison

We determine the parameter values for the seismic moment-frequency relation using Flinn-Engdahl's regionalization of global seismicity and the Harvard centroid-moment tensor data. The earthquake size distribution is approximated by the gamma law: a version of the Gutenberg-Richter distribution with an exponential taper at the maximum moment. There is no statistically significant variation of the beta value (the analog of the b value) for all seismic regions except for the midocean ridge systems. For the latter regions beta = 0.93, whereas for all other zones beta = 0.63. The maximum moment M(xg) can be statistically evaluated only for subduction zones treated as a whole, M(xg) = 10(21) - 2 x 10(22) Newton m, which corresponds to the worldwide M(xg) value. For other regions, as well as for single subduction zones, M(xg) is determined by comparing the number of events in each zone with the seismic moment rate calculated on the basis of the NUVEL-1 model of plate motion. For subduction zones, we obtain the estimate of M(xg) which agrees with the statistical value, giving evidence that most tectonic deformation is released by earthquakes. We test the hypothesis that no statistically significant variation in M(xg) occurs in subduction and continent collision zones; the hypothesis cannot be rejected with available data. For the midocean ridges, the M(xg) estimate cannot be unambiguously determined; it is quite possible that the estimate is significantly biased because only a small part of tectonic deformation at the ridges is released by earthquakes. These results have importance in evaluating seismic risk and may also lead to developing a physical theory for earthquake generation.