Seismic moment distribution revisited: II. Moment conservation principle

Abstract

We compare the tectonic rate with earthquake data using several theoretical distributions to approximate the seismic moment-frequency relation. We derive formulae to estimate parameters of the seismic moment distributions by comparing the earthquake occurrence rate with tectonic strain. We analyse the tectonic moment rate and the earthquake moment distribution for several types of tectonic environments, including subduction zones, plate bounding transform faults and deforming continental regions. In this analysis we use finer subdivisions of tectonic regions to infer whether earthquake size distribution properties established previously for large provinces (Flinn-Engdahl seismic regions) could be applied to smaller zones extending over a few hundred kilometres. Although the shortness of the available earthquake catalogues makes our results less reliable, several conclusions can be drawn from the analysis. The universality of the β-parameter (the slope of the moment-frequency relation) is reasonably well confirmed for all of these tectonic regions and their subdivisions. In each region, we estimate the strain rate or relative plate motion from a plate tectonic model, active fault data or geodetic data. According to the moment-frequency relation, the ratio of the tectonic moment rate to the earthquake rate depends on the β value, the corner magnitude (mc) and the effective seismogenic thickness. We find that this ratio does not vary systematically with the relative plate velocity, the ratio of parallel to perpendicular plate motion, time since the last major earthquake and the length of fault zone segments established by previous earthquake history. We obtain similar results for the western United States and southeast Asia; the corner magnitude is approximately equal to that for the circum-Pacific Rim and there is no systematic dependence of the mc parameter on a geographic region or strain rate. Assuming the commonly accepted values for seismogenic thickness, elastic modulus and 100 per cent seismic coupling, the corner magnitude values for plate boundary zones and continental areas are 8.3-8.8, i.c. similar to those values obtained by statistical analysis.