Dynamic Overshoot Near Trench Caused by Large Asperity Break at Depth

Eiichi Fukuyama,Sébastien Hok

doi:10.1007/s00024-013-0745-z

Abstract

In an attempt to explain the large shallow slip that occurred near the trench during the 2011 Tohoku-Oki earthquake, numerical simulations of earthquake dynamic rupture were carried out using a fault model with a subduction interface containing a bump-shaped asperity, which might result from subduction of an old submarine volcano or seamount. It was assumed that during the interseismic period, slip only occurs outside the bump area and that stress accumulates inside the bump, creating a seismogenic asperity. We roughly evaluated the amount of slip outside the bump during the interseismic period, assuming a constant long-term subduction rate. Then we could estimate the accumulated stress inside the bump. We constructed the initial stress distribution based on the stress change caused by the slip-deficit distribution. A constitutive relation was constructed based on a slip-weakening friction law and was used to compute spontaneous ruptures. The results indicate that a large slip can occur between the trench and the bump, even though a very small amount of stress is accumulated there before the rupture. This is due to an interaction between the free surface and the fault that causes slip overshoot. On the region of the fault below the bump, such overshoot cannot occur because the fault is pinned by the deeper un-slipped zone. However, on the shallower side, the edge of the fault becomes free when the rupture approaches the free surface. In this region, such a large slip can occur without releasing a large amount of stress.

Highlights

The 2011 Tohoku-Oki earthquake is one of the most important earthquakes for seismologists in many senses
Numerical simulations are carried out using a spontaneous rupture model in which a bump at seismogenic depths releases a large amount of strain, in order to study how the rupture propagates and produces large slips between the bump and the trench
A boundary integral equation method (HOK and FUKUYAMA 2011) was used for the computation of a spontaneous rupture along a subduction interface with a bump in a homogeneous half-space elastic medium. This method enables spontaneous rupture propagation to be simulated for a model of a nonplanar fault system with a free surface, which is composed of arbitrary triangle elements

Summary

Introduction

The 2011 Tohoku-Oki earthquake is one of the most important earthquakes for seismologists in many senses. At shallow depths near the trench, stress accumulation would be expected to be small because of the low lithostatic stress In such a situation, a large slip would not be expected, which contradicts the Tohoku-Oki earthquake observations. Many intraplate normal-fault aftershocks occurred, which was not the case in this region before the mainshock (ASANO et al 2011; IDE et al 2011; HASEGAWA et al 2011) These unusual phenomena are associated with the occurrence of a large slip in this region where stress accumulation was low, which led to a drastic change in the stress field. Numerical simulations are carried out using a spontaneous rupture model in which a bump at seismogenic depths releases a large amount of strain, in order to study how the rupture propagates and produces large slips between the bump and the trench

Fault Model and Initial Conditions

Computation Result

Discussion

Conclusion