Plate tectonics and earthquake potential of spreading ridges and oceanic transform faults

We use the Harvard CMT catalog to separate ocean-ridge seismicity into spreading and transform sub-catalogs. We use the tapered Gutenberg-Richter distribution to estimate the total seismic moment rates of plate-boundary zones from limited catalogs of large events. We present the plate boundary model PB1999 and use it to associate marine earthquakes with particular plate boundary segments. We then combine these tools to estimate corner magnitudes ( cm), spectral slopes ( β), and coupled lithosphere thicknesses for all spreading ridges and oceanic transform faults. The distribution of spreading earthquakes is consistent with "normal " 2 3β = (although β is not well constrained) and with uniform c 5.8m =. Coupled lithosphere thickness along ridges decreases quasi-exponentially (from about 500 m to under 50 m) as spreading rate increases. Oceanic transform faults also have "normal " 2 3β ≅ , but their corner magnitudes decrease from about 7.1 to about 6.3 with increasing relative plate velocity. Oceanic transform faults also show a quasi-exponential decrease in coupled lithosphere thickness (from about 3000 m to about 300 m) as relative plate velocity increases. Perhaps this is due to formation of serpentine along slow ridges and transforms and its absence from fast ridges and transforms. Spreading ridges and oceanic transform faults both have imperfect seismic coupling because: (i) all detailed local studies of seismogenic lithosphere thickness exceed our mean values for coupled thickness, and (ii) if coupling were perfect, and seismogenic lithosphere thickness were as small as our estimated coupled thickness, it would require unreasonable stress drops or rupture shapes to explain the moments of the largest earthquakes