Laboratory measurements quantifying elastic properties of accretionary wedge sediments: Implications for slip to the trench during the 2011 Mw 9.0 Tohoku-Oki earthquake

Abstract

The 11 March 2011 (Mw 9.0) Tohoku-Oki earthquake ruptured through the shallowest part of the subduction zone plate-boundary fault, producing tens of meters of displacement at the seafloor and a devastating tsunami. Because elastic and mechanical properties of faults and wall rocks are controlling factors in earthquake generation and propagation, information on these properties is essential to understanding and accurately modeling earthquake rupture and tsunami generation. Laboratory ultrasonic velocity measurements of samples of rock surrounding the Tohoku earthquake principal fault zone recovered by drilling during IODP Expedition 343 (JFAST) show that, under in situ conditions, materials in the frontal accretionary prism are slow and compliant, with P-wave velocities of 2.0-2.4 km/s, S-wave velocities of 0.8-1.0 km/s, and shear modulus of 1.0-2.3 GPa. In order to better understand the elastic properties of shallow subduction zone sediments, we compare our measurements from the Japan Trench to values from analogous shallow subduction zone drill core samples from other locations. We find that shallow subduction zone accretionary prism sediments in general have shear modulus values an order of magnitude lower than what is commonly used in earthquake rupture simulation models. We present a model of the shear modulus of the Japan Trench outer forearc based on laboratory and field measurements that can be adapted for use at other subduction zones and used in dynamic rupture models to improve seismic and tsunami hazard estimates.