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 0.610.66. 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 & Sykes [1992]). Combining both earthquake statistics and the plate-tectonic constraint on moment rate, corner magnitudes include: CRB 7.64(+0.76 -0.26), CTF 8.01(+0.47 -0.21), CCB 8.46(+0.21 -0.39), OCB 8.04(+0.52 -0.22), and SUB 9.58(+0.48 -0.46). Coupled lithosphere thicknesses are found to be: CRB 3.0(+7.0 -1.4)km; CTF 8.6(+11 -4) km; CCB 18(+? -11) km; OSR 0.13(+0.13 -0.09) km for normal-faulting and 0.40(+? -0.21)km for strike-slip; OTF 13(+? -7.3), 1.8(01.1 -0.5), and 1.6(+1.1 -0.5) km at low, medium, and high velocities; OCB 3.8(+13.7 -2.3) km, and SUB 18.0(+? -10.8) km. Generally high coupling of subduction and continental plate boundaries suggests that here all seismic gaps are dangerous unless proven to be creeping. Generally low coupling within oceanic lithosphere suggests a different model of isolated seismic fault patches surrounded by large seismic gaps which may be permanent.