COSEISMIC SLIP AND DEEP TREMOR IN SUBDUCTION ZONES AND ITS RELATION TO CRUSTAL STRUCTURE REVEALED BY GRAVITY ANOMALIES

Large co-seismic slip and seismic moment release on shallow subduction zone megathrusts have been shown to correlate with satellite free-air gravity lows along the forearc (Wells and others, JGR, 2003; Song and Simons, Science, 2003). We hypothesized that asperities, or areas of large co-seismic slip, are persistent features reflecting long-term fore arc structure, and that future slip is more likely to occur in the fore arc gravity lows. In a further test of the hypothesis, Simons and others (2004) examined the 2003 Mw8.3 Tokachi-Oki, Japan, 2003 Mw 7.8 Rat Islands, and 1995 Mw8.1 Antofagasta, Chile events, all of which nucleated in high-gravity regions and released most of their moment in low gravity regions. Using our approach of mapping maximum gravity gradients, we get similar results, and we show that large co-seismic slip from preliminary seismic inversions of the 2004 Mw9.0 Aceh, Sumatra earthquake also occurred in a prominent gravity low. Some slip inversions place large slip on the maximum gravity gradient forming the boundary between the basins and the coastal gravity high.

We extend our analysis onshore to examine how recently discovered episodic tremor and slip (ETS) in subduction zones may relate to crustal structure. Preliminary results indicate that the locus of ETS at depth in both northern Cascadia and SW Japan lie inboard of the free-air gravity low, mostly within the Coast Range gravity high and along its inboard edge. In western Washington, some tremor bursts appear spatially associated with gradients characteristic of crustal faults in the basaltic Crescent Fm. basement. The apparent correlation with block boundaries might make sense if some tremor occurs in the upper plate, as Kao and his colleagues have suggested. We speculate that tremor could be generated by migration of fluids within the deep roots of crustal faults. Dragert et al. (in press) suggest that ETS may also mark the ultimate down-dip limit of co-seismic slip along the Nankai megathrust. If this is true for Cascadia, the Coast Range gravity high may mark the transition from large co-seismic slip along its western gradient to ETS along its eastern gradient.