Mechanics of Slip-to-the-Trench and Frontal Prism Deformation for the 2011 Tohoku-oki Earthquake

Abstract:

The slip magnitude of the 2011 Mw 9.0 Tohoku-oki earthquake rupture was as much as ~50 m below the ~20-km-wide frontal prism of accreted sediments at the trench. Mechanical explanations for prism deformation and shallow slip consider the rate-dependence and dynamic weakening of friction along the basal thrust, dynamic unloading of the thrust from wave propagation into a compliant wedge with a free upper surface, and large magnitude stress release at depth that propagates slip to shallow depths. Borehole data and core samples from IODP expedition 343/343T are used to understand frontal prism behavior over the seismic cycle. Wedge taper, experimental determination of Coulomb failure strength of the prism, and measurements of pore pressure and sediment density are used to determine apparent friction of the basal thrust and stress in the prism for compressively critical and extensionally critical conditions assuming an elastic – perfectly Coulomb plastic wedge. Model results are compared to independent measures of in situ stress in the prism from borehole deformation, sliding friction of basal thrust material at quasi-static and seismic slip-rates in experiments, and the average coseismic shear strength of the thrust from borehole temperature-profiles. These data define the pre-seismic, co-seismic, and post-seismic stress states and suggest the prism remains in a stable, elastic state over the seismic cycle with a dynamic stress drop of approximately 1 MPa in the vicinity of the borehole. Results likely describe the state of the frontal prism ~15-20 km landward from the borehole. Trenchward, however, the prism has a much smaller taper and undergoes coseismic shortening under a compressively critical state and diminishing basal slip. Variations in shallow slip magnitude along the trench could partly reflect changes in prism geometry associated with roughness of the subducting slab and sediment input, and variations in frictional properties of the basal thrust.