Fault zone heterogeneity and earthquake occurrence in subduction zones.

Heterogeneity in the rupture process of subduction zone earthquakes is examined and interpreted in terms of inhomogeneity in the stress distribution on the plate interface. The largest earthquakes in subduction zones dominate seismic energy release; knowledge of the spatial pattern of their coseismic displacement provides information on the first-order material properties of the fault plane or asperity locations. For the great 1971 Solomon Islands doublet events, occurring at the junction of the Solomon Islands-New Britain Arc, and for the 1968 Tokachi-Oki event, occurring near the junction of the Japan-Kurile Islands Arc, the regions of enhanced displacement (asperities) correspond to geometric peculiarities in the subducting plates. The association between highly stressed portions of the fault plane and contortions in the subducted lithosphere, provides a physical mechanism in which to interpret the distribution of stress heterogeneity on these fault planes. A study of the four largest earthquakes occurring in the souther Kurile Islands Arc, has allowed the construction of a map of the dominant asperity distribution. Hypocenters of an additional 80 earthquakes with m$\sb{\rm b}\geq$ 5.3 and known underthrusting mechanisms were recomputed using the method of joint hypocenter determination in order to assess the influence that the strength distribution exerts on the interplate seismicity patterns. The distribution of earthquake hypocenters with respect to the mainshock asperities reveals that nearly all of the earthquakes locate outside of the mainshock asperities. This observation is consistent with complete release of accumulated strain energy in asperity regions during the mainshocks and a redistribution of stress in adjacent areas following their rupture. Also the mainshock asperities and most of the M$\sb{\rm s}\geq$ 7 earthquakes occur in the down-dip portion of the mainshock aftershock zones, producing a sharp peak in accumulated moment release at distances between 100 to 120 km from the trench axis. The peak in moment release at the down-dip half of the coupled plate interface reflects an increase in mechanical strength with depth. This suggests that models of the rheological properties of continental fault zones, which predict a maximum in shear stress near the base of the seismogenic zone, may also be appropriate for subduction zone environments.PhDEarth SciencesGeophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128237/2/8821649.pd