Frequency-size distributions for intraplate earthquakes

We examine the question of a possible difference in the frequency-size statistics of intraplate earthquakes, as opposed to their more numerous interplate counterparts. We use both the Harvard Centroid Moment Tensor catalogue and the data set of the National Earthquake Information Center. In the former case, we quantify earthquakes through their seismic moment and describe their population distribution through the β-value introduced by Molnar. In the latter case, we use traditional b -values computed from both body-wave magnitudes (m _b ) and surface-wave magnitudes (M _s ). We conclude that both β- and b-values for true intraplate earthquakes (i.e., not occurring in areas of broad tectonic deformation) are essentially equivalent to those of interplate earthquakes in similar ranges of moments or magnitudes. This is consistent with a fractal dimension of two for the intraplate seismogenic zones, suggesting that, like along plate boundaries, they consist of two-dimensional faults and not of volumes with greater dimensions. The distribution of earthquakes in deformed regions, principally the Mediterranean-Tethyan belt, follows that of worldwide interplate earthquakes but with a greater value for the critical moment expressing the saturation with depth of the width of the fault at the brittle-ductile transition, suggesting that the latter would take place at greater depths under large-scale orogens.