Compaction Further Enhances Inelastic Wedge Failure in Shallow Subduction Zone Earthquakes

Abstract:

Accretionary wedges in the subduction zone are typically highly porous due to continuous influx of unconsolidated sediments on the seafloor. A well-known deformation mode for porous materials is compaction. When porous materials are compacted in the undrained condition (as in the rapid loading of earthquake rupture) the pore pressure increases significantly when pore space closes while pore fluids are largely incompressible, resulting in a much larger pore pressure increase than that by the elastic compression alone. In this work, we incorporate compaction in our poroplastic model of shallow subduction zone earthquakes (Ma, 2012; Ma and Hirakawa, 2013) by using an end-cap yield criterion (e.g., Wong, 1997). We show that for a wedge on the verge of compactant failure initially (due to large porosity) up-dip rupture causes significant wedge compaction, which induces large pore pressure increase, reduces effective stress, and causes more easily shear failure. The wedge needs not be on the verge of Coulomb failure initially. Dilatancy accompanying shear failure near the end of deformation process can reduce volume loss due to initial compaction, resulting in a small net volumetric plastic strain. Compaction significantly enhances failure in the wedge, which makes our wedge-failure mechanism more plausible in explaining well-documented anomalous observations for shallow subduction zone earthquakes, such as slow rupture velocity, efficient tsunami generation, deficiency in high-frequency radiation, and low moment-scaled radiated energy. The dynamically elevated pore pressure may also explain numerous observations on the hydrologic activities in the wedge (e.g., mud volcanoes).