Differences between main shock and aftershock ground motions derived from the Japanese KiK-net database

We considered the large ground motion Japanese database and statistically analyzed the differences between main shock and aftershock ground motions. In particular, we considered the moment magnitude (M), the rupture distance (Rrup), the fault type, and the time-averaged shear-wave velocity for the upper 30 m of soil deposits (VS30) that could have influenced ground motion characteristics. We analyzed the logarithmic ratios of the spectral accelerations (Sa) and the mean periods of the earthquake ground motions, comparing them to those of the main shock motions. In addition, main shock and aftershock ground motion records with those predicted by the ground motion prediction equation developed for Japan. We observed that aftershock ground motions with Rrup ≤ 60 km, M ≤ 5.5, and period <1 s, had smaller Sa than main shock motions; this was particularly true in the case of shallow active crustal events. Moreover, we verified that the VS30 did not influence the characteristics of aftershock and main shock ground motions. Finally, in the case of moderate nearby earthquakes, the aftershock ground motions were found to have longer mean period than the main shock ground motions.