Coulomb shear stress shanges along subduction segments and seismicity in the upper plate

M. A. J. Taylor, R. Dmowska and J. R. Rice

Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, U.S.A.

EOS Trans. Amer. Geophys. Union, 78, No. 46, F643, 1997.

The correlation of seismicity following a major event with static coseismic changes in Coulomb shear stress has found much recent success, particularly in continental regions, e.g. along the San Andreas fault [Jaume and Sykes, 1992, Stein et al., 1992, Simpson and Reasenberg, 1994, King et al., 1994, Harris et al., 1995, Harris and Simpson, 1996]. Here we apply the same concept to back-arc seismicity following great subduction earthquakes in an attempt to understand the position and timing, and to resolve the fault-mechanisms, of such events.

The distribution of slip along strike in large subduction earthquakes is invariably very heterogeneous, with the largest slip localized in "asperity" regions [Lay and Kanamori, 1981]. We use a 3D elastic finite-element model [Dmowska et al., 1996] to investigate the co-seismic stress changes along the surface of the over-riding plate due to heterogeneous slip of the interplate interface in a zone of oblique convergence. The distribution of shear and extensional stresses on arc-parallel faults in the back-arc form distinct, characteristic patterns. The former separates into two lobes, one of increased and the other decreased coseismic stress change. The normal stress changes resolved onto faults with trace inclined at moderate to large angles to the trench likewise form two lobes of increased and decreased change. This distribution is exemplified by the back-arc seismicity following the Feb. 17, 1996 Biak (Indonesia) earthquake (M_w = 8.2) where 13 subsequent back arc events separate along strike into regions of right-lateral strike slip and of extensional seismicity. These allow us to suggest the position of highest moment release in the main event.

A lower level of back-arc seismicity follows the May 7, 1986 (M_w = 8.0) Andreanof Islands and the Feb. 4, 1965 (M_w = 8.7) Rat Islands earthquakes. However, using known inversions for slip distributions, we calculate the coseismic Coulomb shear stress changes on the possible fault planes for the events. Right-lateral strike-slip seismicity on arc-parallel transform faults consistent with the deficit between slip directions in the main event and plate convergence is favored back and to the east of the highest slip in the Aleutians, as is left-lateral slip on arc-perpendicular faults in the same region. The coseismic Coulomb shear stress distributions in the region of back-arc seismicity are fairly insensitive to friction coefficient and details of the slip distribution. However, post-mainshock change in friction coefficient (perhaps due to draining of pore fluids [Simpson and Reasenberg, 1994]) may have provided a mechanism for triggering of seismicity west of the Rat Islands rupture later in the earthquake cycle.

return to Publications List