Differences between main-shock and aftershock ground motions using the Japan’s KiK-net database

Abstract

Main-shock earthquakes are followed by numerous aftershocks which aggravate damages to structures and infrastructure. There have been many studies about forecasting frequencies of aftershocks. However, there are only few controversial studies on characteristics of aftershock ground motions. Some researchers argue that there is no difference between aftershock and main-shock ground motions given the same earthquake magnitude. Others found differences: spectra accelerations of aftershock motions at short period are smaller than those of main-shock motions. We utilize the abundant ground motion data from the Kiban-Kyoshin network (KiK-net) in Japan and statistically analyzed differences between mainshock and aftershock motions. We consider the three factors influencing characteristics of ground motions: (1) the moment magnitude (M), (2) the distance from rupture (Rrup), and (3) the time-averaged shear wave velocity in the top 30 m of soil deposits (VS30). We analyze logarithmic ratios of spectral accelerations of aftershock ground motions to those of main-shock motions (ln(SaAS/SaMS))using both surface and within-rock motions for various magnitude and distance combinations. We found that spectral accelerations of aftershock ground motions are smaller than those of main-shock motions at periods less than 1s. We also found that this difference is dominant for short rupture distances (Rrup ≤ 60 km) and moderate earthquake (M ≤ 5.5). These trends are similar for both surface and within-rock ground motions.