Time-varying Upper-plate Deformation During the Megathrust Subduction Earthquake Cycle

Abstract:

Our abilities to observe and image the deformational behavior of the upper plate in subduction zones has dramatically improved, concurrently with a series of great (Mw 8.0+) earthquakes. Inferences from these observations include patterns of apparent lateral variations in locking along subduction zones, which differs from interseismic to coseismic periods; the significant magnitude and longevity of post-earthquake deformation; and incompatibilities between rates of slip deficit accumulation, recurrence intervals, and resulting earthquake co-seismic slip. In sync with these improved observations, modeling capabilities have evolved from fitting simple elastic accumulation/rebound curves to sparse data to having spatially dense continuous time series that allow us to infer details of plate boundary coupling, rheology-driven transient deformation, and partitioning among inter-earthquake and co-seismic displacements. Preliminary results from such modeling lead to the following observations:

1. Co-seismic stress transfer from the unloading elastic layer (shallow) into a transiently-elastically loaded layer (deeper visco-elastic) - extends inboard of locked zone - affecting both coseismic and post-seismic surface displacements.

2. Post-seismic response of upper plate involves seaward motion for initial ~ 2 Maxwell times after EQ. This occurs in spite of there being no slip on locked plate boundary - (i.e. not plate boundary after-slip, but a consequence of visco-elastic stress relaxation).

3 Later in the cycle (~ 8+ Maxwell times) the system generates a simple linear displacement pattern - the expected behavior for a shortening elastic beam..

These results indicate that care is needed in interpreting observed surface displacement fields during the post- and inter-seismic period - much of the observed deformation that is inferred to be a consequence of after-slip on the megathrust interface may rather be associated with viscous relaxation, masking a period of significant slip-deficit accumulation on the megathrust.