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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 variations in network
density and observation uncertainties. 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, 3-D 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 five 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.
