DEPTH DISTRIBUTION OF MOMENT RELEASE IN UNDERTHRUSTING EARTHQUAKES AT SUBDUCTION ZONES

We present a global analysis of the depth distribution of moment release along the thrust contact at subduction zones using data from Harvard's centroid moment tensor catalogue. Substantial regional variations in the depth distribution of moment release are observed. A significant correlation exists between the depth of maximum moment release and the convergence rate of subducting lithosphere and the sediment thickness at the trench. Subduction zones with fast convergence rates and thick sediment accumulations concentrate moment release at greater depths than subduction zones with slow convergence rates and little sediment accumulation. The correlation between the convergence rate of oceanic lithosphere and the depth distribution of moment release may be best explained by differences in the thermal structure of the plate interface. Subduction zones with fast convergence velocities transport cold material to greater depths than subduction zones with slow convergence velocities. The colder temperature of the deep portions of the thrust contact at subduction zones with fast convergence velocities allows seismic failure to occur at greater depth than possible in subduction zones with slow convergence velocities. As has been demonstrated in intraplate environments, our observations in interplate thrust settings support the idea that temperature is the most important factor controlling the maximum depth of seismic failure. In subduction zones with thick sediment accumulations, the stable sliding properties of unconsolidated sediments may be responsible for the low levels of shallow moment release.