Reconciling the conflicting extent of overriding plate deformation
before and during megathrust earthquakes in South America, Sunda, and
northeast Japan
Abstract
We aim to better understand the overriding plate deformation during the
megathrust earthquake cycle. We estimate the spatial patterns of
interseismic GNSS velocities in South America, Southeast Asia, and
northern Japan and the associated uncertainties due to data gaps and
velocity uncertainties. The interseismic velocities with respect to the
overriding plate generally decrease with distance from the trench with a
steep gradient up to a “hurdle”, beyond which the gradient is
distinctly lower and velocities are small. The hurdle is located
500–1000 km away from the trench, for the trench-perpendicular velocity
component, and either at the same distance or closer for the
trench-parallel component. Significant coseismic displacements were
observed beyond these hurdles during the 2010 Maule, 2004
Sumatra-Andaman, and 2011 Tohoku earthquakes. We hypothesize that both
the interseismic hurdle and the coseismic response result from a
mechanical contrast in the overriding plate. We test our hypothesis
using physically consistent, generic, three-dimensional finite element
models of the earthquake cycle. Our models show a response similar to
the interseismic and coseismic observations for a compliant near-trench
overriding plate and an at least 5 times stiffer overriding plate beyond
the contrast. The model results suggest that hurdles are more
prominently expressed in observations near strongly locked megathrusts.
Previous studies inferred major tectonic or geological boundaries and
seismological contrasts located close to the observed hurdles in the
studied overriding plates. The compliance contrast probably results from
thermal, compositional and thickness contrasts and might cause the
observed focusing of smaller-scale deformation like backthrusting.