Effect of the Earth’s Surface Topography on Quasi-Dynamic Earthquake Cycles

ohtani, makiko ; hirahara, kazuro;

We take account of the effect of Earth’s surface topography in quasi-dynamic earthquake cycle simulations using a boundary integral equation method. While we have so far assumed a homogeneous elastic half-space medium with a flat free surface, Earth’s actual surface topography is complicated. Here, we constructed new slip response functions in half-space with an arbitrarily shaped surface topography in which we used slip response functions in full-space by introducing imaginary free surface cells in addition to embedded fault ones. By comparing analytical slip response functions in the case of a flat surface overlying half space with the new ones, we developed a computationally efficient method for setting the Earth’s surface region, which was divided into cells with the appropriate sizes depending on the fault source cell depth to maintain the computational accuracy. With these new slip response functions, we simulated simple interplate earthquake cycles in the region close to the Japan Trench, off Miyagi, Tohoku, in northeast Japan, which has the amplitude of 7 km
in depth. Compared with the case where the flat surface level was set at the trench depth, the slip response functions for the case where actual seafloor topography was used had smaller amplitudes. Hence, the actual topography produces smaller recurrence times for earthquake cycles than that for the flat surface case. These effects of the actual Earth’s surface topography mainly come from changes in the distance between the surface and the fault compared with the flat surface case. Changes in the slip response function also represent changes in the fault stiffness of the system. Considering the actual topography of the Earth’s surface to be convex upwards as opposed to the flat, the fault stiffness becomes larger compared to the case of the
flat Earth’s surface. This leads to a change in the frictional instability, and sometime leads to the change in the way of rupture.