12:00 PM - 1:00 PM
We address the question as to why earthquakes occur in some places and not others? The depth extent of seismogenic zones ranges from about 20 km on the continents, in strike-slip or rift zones, to depths greater than 600 km in subduction zones. In ocean plates earthquakes are rarely deeper than 60 km. We show how this range is described by a thermo-mechanical model in which the transition from brittle to ductile behavior occurs when the tectonic strain-rate equals the dislocation creep-rate for material flow. The former depends on the tectonics while the latter depends on homologous temperature, i.e., the ratio of temperature to effective-melting temperature. The model describes depth of earthquakes in California, in ocean plates, subduction zones, the variable widths of double Wadati-Benioff zones worldwide, and the deeper than average events beneath outer rises and Hawaii.