Factors affecting seismic moment release rates in subduction zones

Abstract

The amount of shallow seismic activity in subduction zones varies greatly from region to region. We quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones. To calculate the moment release, we sum the seismic moment for all interplate thrust‐type events with surface wave magnitude Ms≥7.0, occurring from 1904 to 1980. We present a time history plot of the seismic moment release for each subduction zone; these exhibit the differences in the seismic release patterns. For subduction zones where the time window of our study is not representative, the total moment release is corrected using information on repeat times. The moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling. These parameters include the age of the subducting lithosphere, absolute velocities of the upper and subducting plates, convergence velocity, and length, maximum depth, and dip of the Wadati‐Benioff zone. The moment release rate decreases as the age of the subducting lithosphere increases, when the zones belonging to a single subducting plate are considered. This age versus moment release relation is consistent for the zones in which the Pacific, Cocos, Philippine Sea, and Indian plates are subducting. The moment release rates for the subduction zones in which the Pacific plate is subducting are much higher than for zones of other plates with similar age. The age versus moment release relation holds among the zones which belong to one subducting plate; however, zones with similar ages but belonging to different plates do not have the same moment release rates. This suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget. Zones with retreating upper plates tend to have lower moment release rates. The moment release rate does not increase with convergence velocity; no simple relationship was found between these two parameters. The moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate. These are the two independent subduction zone parameters among the variables considered. The other variables depend on these two parameters.