Plate-Tectonic Analysis of Shallow Seismicity: Apparent Boundary Width, Beta, Corner Magnitude, Coupled Lithosphere Thickness, and Coupling in Seven Tectonic Settings

Peter Bird, & Yan Y. Kagan

Published December 2004, SCEC Contribution #749

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 ß 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 plate-tectonic constraint on moment rate, corner magnitudes include the following: CRB, {03107-eq001}; CTF, {03107-eq002}; CCB, {03107-eq003}; OCB, {03107-eq004}; and SUB, {03107-eq005}. Coupled lithosphere thicknesses are found to be the following: CRB, {03107-eq006}; CTF, {03107-eq007}; CCB, {03107-eq008}; OSR, {03107-eq009} for normal faulting and {03107-eq010} for strike slip; OTF, {03107-eq011}, and {03107-eq012} at low, medium, and high velocities; OCB, {03107-eq013}; and SUB, {03107-eq014}. 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.

Citation
Bird, P., & Kagan, Y. Y. (2004). Plate-Tectonic Analysis of Shallow Seismicity: Apparent Boundary Width, Beta, Corner Magnitude, Coupled Lithosphere Thickness, and Coupling in Seven Tectonic Settings. Bulletin of the Seismological Society of America, 94(6), 2380-2399. doi: 10.1785/0120030107.