Modeling earthquake cycles in the Shumagin subduction segment, Alaska, with seismic and geodetic constraints

Deformation associated with the earthquake cycle in the Shumagin Islands segment of the Alaska‐Aleutian subduction zone is analyzed with the use of a two‐dimensional finite element model. The model consists of an oceanic plate dipping under an upper plate, both of which respond elastically to stress fluctuations in the earthquake cycle, and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically. It is tailored to the geometry of the Shumagin Islands region, by using seismicity to define the position of the interplate interface and (partially) coupled region along it. The model is preconditioned by forcing this interface to undergo periodically repeated slips up to (and including) the time of the May 31, 1917, event ( M s = 7.4) in that region, with each chosen to be consistent with the moment and estimated rupture area of that event. We investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle, including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface. In models with significant relaxation in the wedge or downdip thrust zone, results show that as the intraseismic stage matures, there is a region of diminished compressional strain rates, and even of locally extensional rates, on the Earth's surface above the downdip end of the seismically coupled zone. Based on the seismic estimates of the location of the coupled zone, this region is in the area of the Shumagin Islands. We find that if approximately 20% of the convergence takes place seismically (compatible with the previous seismic history), and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and/or along the downdip interface, then deformations predicted by the model can be made consistent with the measured strain data from the Shumagin Islands geodetic network [ Lisowski et al. , 1988; Larson and Lisowski , 1994], as well as uplift and tilt data [ Savage and Plafker , 1991; Beavan , 1992]. Our model simulations here suggest that the Shumagin segment is capable of large earthquakes. The hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints.