Plate-tectonic analysis of shallow seismicity: Apparent boundary width, beta, corner magnitude, coupled lithosphere thickness, and coupling in seven tectonic settings

A new plate model is used to analyze the mean seismicities of seven types of plate boundary (CRB, continental rift boundary; CTF, continental transform fault; CCB, continental convergent boundary: OSR, oceanic spreading ridge; OTF, oceanic transform fault; OCB, oceanic convergent boundary; SUB, subduction zone). We compare the platelike (nonorogen) regions of model PB2002 (Bird, 2003) with the centroid moment tensor (CMT) catalog to select apparent boundary half-widths and then assign 95% of shallow earthquakes to one of these settings. A tapered Gutenberg-Richter model of the frequency/moment relation is fit to the subcatalog for each setting by maximum likelihood. Best-fitting beta values range from 0.53 to 0.92, but all 95% confidence ranges are consistent with a common value of 0.61-0.66. To better determine some corner magnitudes we expand the subcatalogs by (1) inclusion of orogens and (2) inclusion of years 1900-1975 from the catalog of Pacheco and Sykes (1992). Combining both earthquake statistics and the platetectonic constraint on moment rate, corner magnitudes include the following: CRB, 7.64(-0.26)(+0.76); CTF, 8.01(-0.21)(+0.47); CCB, 8.46(-0.39)(+0.21); OCB, 8.04(-0.22)(+0.52); SUB, 9.58(-0.46)(+0.48); Coupled lithosphere thicknesses are found to be the following: CRB, 3.0(-1.4)(+7.0); CTF, 8.6(-4)(+11); CCB, 18(-11)(+?); OSR, 0.13(-0.09)(+0.13) for normal faulting and 0.40(-0.21)(+?) for strike slip; OTF, 13(-7.3)(+?), 1.8(-0.5)(+1.1), and 1.6(-0.5)(+1.1) at low, medium, and high velocities; OCB, 3.8(-2.3)(+13.7); and SUB, 18.0(-10.8)(+?). In general, high coupling of subduction and continental plate boundaries suggests that here all seismic gaps are dangerous unless proven to be creeping. In general, low coupling within oceanic lithosphere suggests a different model of isolated seismic fault patches Surrounded by large seismic gaps that may be permanent.