Upper Plate Stressing and Back Arc Seismicity in the Subduction Earthquake Cycle

Investigation:

Relations between stress changes associated with subduction and seismicity in the upper plate.

Results:

The Andreanof Islands earthquake of May 7, 1986 (M_w = 8.0) was followed in the first 1.5 months by a series of shallow upper-plate earthquakes, the five largest of which range in magnitude between M_w = 5.3 and M_w = 6.5 and are consistent with right-lateral motion on arc-parallel transform faults (Ekström and Engdahl, JGR, 1989, Fig. 1).  The February 4, 1965 Rat Islands earthquake (M_w = 8.7) was followed on July 4, 1966 by a shallow mb = 6.2 upper-plate strike-slip event, which can be interpreted as a right-lateral earthquake in a relative position and time very similar to the Andreanof Islands earthquakes, here associated with slip on the eastern, strongest asperity of the 1965 main event, (Fig. 3). Finally, the Biak, Irian Jaya, Indonesia earthquake of Feb. 17, 1996 (M_w = 8.2, Fig. 5) was followed in the first two days by two upper-plate events, one extensional (M_w = 6.5, Feb. 17) and the other left-lateral strike-slip (M_w = 6.4, Feb. 18), and later by many more shallow upper plate events, mainly extensional in the western part and strike-slip in the eastern part of the area (Fig. 5).

Subduction in the Aleutians is such that it principally produces coseismic increases in extensional normal and left-lateral shear stress on back-arc transforms.  For a net increase in right-lateral Coulomb shear stress, consistent with the strike-slip events in the Andreanof and Rat Islands, the increase in extensional stress multiplied by f must be greater than the increase in left-lateral shear stress.  Positions in the back arc where this occurs depend strongly on distance perpendicular from the trench and the choice of f.  For a 2D model with homogeneous slip along strike during the main event at angles of obliquity from the trench normal typical of those found in the Aleutians, increases in right lateral Coulomb shear stress in the back arc in positions coincident with the observed seismicity are achieved for values of f greater than about 0.2.  A 2D model also predicts Coulomb stress increases, both extensional and left-lateral, consistent with the back arc activity in Indonesia.

However, if there is a significant slip heterogeneity during these events, as known to be the case for the Aleutians, then stress changes may be better estimated from a simple 3D model (Dmowska et al., JGR, 1996) of a localized asperity sustaining strong stress drop in the center of an interface rupture zone with free slip around it.  Such modeling for oblique subduction reveals that Coulomb stress changes, for a range of friction coefficients, favor back arc strike-slip events only to one side of the asperity and extensional normal events primarily to the other.  When the coseismic change in right-lateral Coulomb shear stress in the upper plate resolved onto arc-parallel strike-slip faults, , is calculated for angles of dip and oblique slip on the thrust interface that reflect the situation in the Andreanof Islands (Fig. 2), a zone of increased shear stress emerges, back and to the right of the asperity, consistent with the position of the observed back-arc seismicity relative to the asperity in the inversion of Das and Kostrov (JGR, 1990).  A very similar picture of  emerges for the more oblique slip observed in the Rat Islands, and is consistent with the July 4, 1966 strike-slip event (Stauder, JGR, 1968) if the arc-parallel fault plane is assumed.  Stauder interprets the event as occurring on an arc-perpendicular NS striking fault plane.  A plot of the coseismic change in left-lateral Coulomb shear stress resolved onto arc-perpendicular faults, , for typical parameters for the Rat Islands region (Fig. 4), shows that such a mechanism is also consistent with the stress changes at that position in the back-arc, relative to the main asperity (Fig. 3).  For Biak, Irian Jaya, the modeling suggests, that if there is a similar slip asperity on the interplate interface, then in order for it to be consistent with the partitioning of subsequent seismicity into separate regions of strike-slip and extensional activity (Fig. 5), it must be located along strike between the 17 Feb. extensional and 18 Feb. left-lateral strike-slip back-arc events, indicated by the hatched region in Figure 5.  Figure 6 shows the change in left-lateral Coulomb shear stress for angles of dip and oblique slip consistent with subduction in Irian Jaya, and Figure 7 shows the extensional normal stress changes as resolved onto a plane 30° from the trench (as for the M_w 6.5, Feb. 17 back-arc event).  These figures indicate the main regions of stress increase to be south and, respectively, east and west of the asperity.

Related Reports:

Dmowska, R., G. Zheng, and J. R. Rice, "Seismicity and deformation at convergent margins due to heterogeneous coupling", J. Geophys. Res., 101, 3015-3029, 1996.

Geist, E., and R. Dmowska, Mechanics of dip-slip faulting related to the generation of local tsunamis (abstract), EOS Trans. Amer. Geophys. Union, vol. 77, No. 46, Fall Meeting Supplement, p. F510, 1996.

Rice, J. R., and Y. Ben-Zion, "Slip complexity in earthquake fault models", Proceedings of the National Academy of Sciences USA, 93, pp. 3811-3818, 1996.

Taylor, M. A. J., R. Dmowska, and J. R. Rice, Upper plate stressing and back arc seismicity in the subduction earthquake cycle (abstract), EOS Trans. Amer. Geophys. Union, vol. 77, No. 46, Fall Meeting Supplement, p. F687, 1996.

Taylor, M. A. J., G. Zheng, J. R. Rice, W. D. Stuart, and R. Dmowska, "Cyclic stressing and seismicity at strongly coupled subduction zones", J. Geophys. Res., 101, 8363-8381, 1996.

Zheng, G., R. Dmowska, and J. R. Rice, "Modeling earthquake cycles in the Shumagins subduction segment, Alaska, with seismic and geodetic constraints", J. Geophys. Res., 101, 8383-8392, 1996.

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Upper Plate Stressing and Back Arc Seismicity in the Subduction Earthquake Cycle (non-technical summary):

Page established by:  Mark Taylor, Division of Engineering and Applied Sciences, Harvard University, Feb. 20, 1997.