Ground Amplification Factors Considering Topographic Effect and Incidence Angle

Abstract

It is well known that the ground motions can be altered due to surface topographic irregularities which is referred to as topographic effects. Previous studies revealed that the topographic ground motions amplifications were affected by various factors such slope angle, relative elevation, and curvature. In addition to the topographic factors, incidence angles of ground motions can affect the ground motion amplifications. Ground motions of the five aftershocks of the 2017 local magnitude 5.4 Pohang, South Korea, earthquake had been recorded at the temporary seismic stations located on the small-size hill. The five earthquakes had similar hypocenters with focal depths of approximately 5.6 – 8 km, and the stations had epicentral distances of approximately 4.0 – 4.2 km. The two stations were located on the slope facing towards the epicenters, and the other one was located on the slope facing against the epicenters. It turned out that the ground motions recorded on the slope facing towards the epicenters were larger than those in the opposite directions.

This study developed a numerical model of the slope using the SPECFEM2D software which is based on the finite element method. The absorbing boundary conditions were applied to the two sides of the model to eliminate the wave reflections. To mimic the real ground motion in the simulation, we considered Ricker wavelets with nine dominant frequencies (i.e., 0.1, 0.5, 1, 2, 5, 10, 20, 35, 50 Hz). Spectral accelerations of the ground motions computed for the slope facing towards the epicenters were divided by those against the epicenters. These ratios were comparable with the ratios from the ground motions recorded during the five earthquakes.

We performed a series of simulations using the validated slope model. We computed ground motion amplification factors at various periods as maximum acceleration at the topographic ground model divided by the maximum accelerations of the free-field surface model. It turned out that motions were amplified on the slopes with large relative elevations. In addition, we also tested the effect of incidence angles of input motions. The results demonstrate that ground motions were more amplified as the incident angle decreased.