Dynamic Rupture Simulations to Investigate the Behavior of Shallow Parts of Mega-Thrust Faults

Abstract:

The rupture process of the 2011 Off the Pacific Coast of Tohoku earthquake (Mw 9.0) showed smooth rupture and large slip on the shallow parts of the inter-plate mega-thrust fault, close to the trench. While a number of studies have been carried out to understand the mechanisms that generate such feature, some points remain to be clarified.

In order to understand the mechanism to produce the huge slip with smoother rupture on the shallow parts of faults, we have carried out dynamic rupture simulations (Tsuda et al., 2015) based on the 3D spectral element method (Galvez et al., 2014). Our model assumed very simple ingredients, such as a homogeneous elastic medium, a planar fault, slip-weakening friction, a large ‘locked area’ (100 km along strike x 75 km along dip) with high stress drop (around 8 MPa) around 15 km deep and a relatively small (100 km along strike x 25 km along dip) dynamic ‘weakening area’ with very small dynamic friction coefficients (e.g. Ujiie et al., 2013) above the locked area. This model produced large shallow slip. The resulting slip distribution as a function of depth coincided with the results by Sun et al. (2015) based on bathymetry differences before and after the earthquake rupture.

In this study, we extend these previous dynamic rupture simulations to further investigate the behavior of the shallow parts of mega-thrust faults. We studied the effect of the along-dip size of the locked area and weakening area. We also added a small area with very high stress drop (around 20 MPa) representing the strong motion generation area (SMGA) on the deeper part of the fault, where short-period seismic waves are dominantly radiated. The results of the simulations indicate that the balance of the size between locked area and weakening area plays an important role and the existence of a SMGA can enhance slip on the shallow parts of mega-thrust faults.