THE EVOLUTION OF EXTENSIONAL DEFORMATION THROUGHOUT SUBDUCTION ZONE EARTHQUAKE CYCLES

The deformation at subduction zones is usually dominated by compression. Global geophysical and geological observations of normal faulting aftershocks, mapped normal faults and tension cracks, and extensional strain inferred from geodetic measurements indicate that this state of broad compression can become abruptly transformed to extension by large megathrust earthquakes. In addition, the total offsets on normal faults and tension cracks at the surface are too large to have occurred in a single event, suggesting that permanent extension accumulates over multiple earthquake cycles. To understand how extensional features form at a convergent subduction plate boundary, we develop a 3D finite element model of many subduction earthquake cycles. The model has an appropriate curved plate interface geometry (comparable to most subduction zones globally) and internally consistent pre-stresses.

The model results provide a direct estimate of the dominantly compressional inter-seismic stress field that accumulates and eventually drives the subsequent earthquake slip. The immediate co-seismic effect of the modeled earthquake is to cause approximately trench-normal extension of the upper plate and slab relative to the pre-earthquake stress field, but the distribution of stress changes varies spatially. The largest magnitude extension occurs in parts of the upper plate above the landward edge of the rupture zone and smaller magnitude extension occurs in the subducting plate. The transformation from compression to extension in some locations indicates the potential for faults in these regions to host both reverse and normal slip, depending on the stage of the earthquake cycle. Afterslip on the subduction interface broadens the footprint of extension but reduces its magnitude. Viscous relaxation in the mantle wedge does not have a significant effect on the crustal stress field. The pre-earthquake state of broadly compressional stress is re-established by the re-locking of the plate interface and continued plate convergence. The spatial extent and duration of post-seismic extension increase with the rupture dimensions and slip magnitude, and these also vary with the rupture depth, which agrees well with global observations of extension at subduction zones.