Comparison of Coulomb shear stress changes from the February 17, 1996 Biak M_w = 8.2 event and a subsequent seismic inversion.

R. Dmowska¹, M. A. J. Taylor², and J. R. Rice¹

EOS Trans. Amer. Geophys. Union., 79, no. 45, p. F632, 1998.

Coseismic changes in Coulomb shear stress have been shown to correlate well with changes in seismicity following great earthquakes for both transform faults in continental regions and, in our work, for dip-slip faults at subduction zones [Dmowska et al., 1996, Taylor et al., JGR, in press, 1998]. These studies almost invariably adopt the approach of applying the inferred slip from one or more inversions of the main event to a 3D elastic half-space dislocation model to calculate the resulting Coulomb shear stress changes. There was no such inversion available immediately after the Biak, 1996 event, and so we took the alternative approach of using the spatial distribution of post-mainshock upper-plate seismicity to infer information about the distribution of slip in the main event. Our 3D subduction models with highly heterogeneous slip along-strike reveal distinct, characteristic patterns for the distribution of stresses in the upper-plate. The shear stress on arc-parallel strike-slip faults separates into two lobes, one of increased and the other decreased coseismic stress change. The extensional stress changes resolved onto normal faults with trace inclined at moderate to large angles to the trench likewise form two lobes of increased and decreased change. Based on this pattern and the positions and mechanisms of the first two upper-plate events (Feb. 17 and 18), the area of highest seismic slip was placed in the first week after the main event and confirmed by the subsequent year of seismicity [Taylor et al., 1998].

A subsequent inversion for the event by Kikuchi [1998] using the subevent deconvolution method of Kikuchi and Kanamori [1991] reveals a two phase source-time function. The rupture propagates roughly NW with the initial phase (M_w = 7.3, 19 s) followed by a second (M_w = 8.1, 32s) in which most of the moment is released. A comparison between this inversion and the position of highest slip inferred from coseismic stress changes shows they are essentially co-incident. The remarkable correlation between these results provides grounds for confidence in our method of approximately placing the position of highest moment release in a main event.