Derivation of near-source ground motions of the 1995 Kobe (Hyogo-ken Nanbu) earthquake from vibration records of the Akashi Kaikyo Bridge and its implications

According to the results of the previous inversion studies, the main ruptures of the 1995 Kobe earthquake occurred on two faults, namely the Nojima and Suma faults, which have a step-over about 4 km wide in the Akashi strait. However, the possibility of the rupture in the step-over itself has not been clarified yet due to the unavailability of near-epicentre seismic data. To this unavailability, free-field ground motions at the piers of a suspension bridge located in the step-over are derived from the vibration records of its towers. The ground motions are successfully derived within 0.6–1.2 Hz, where the transfer functions are stable and the influences of the cable vibrations are negligible. The preliminary analysis of the obtained ground motions shows that although the ruptures of the main faults could simulate the early parts of the ground motions, they fail to recover a major pulse afterwards. This suggests that the step-over might have had a delayed rupture.