Abstract

During the 2008 Iwate–Miyagi Nairiku, Japan, earthquake (Mw 6.9), an unprecedented vertical surface acceleration of nearly four times gravity, was measured at the KiK-net, IWTH25 station located 3 km southwest of the epicenter. The station is equipped with three-component accelerometers, installed at both the free surface and the bottom of a 260-m borehole. The waveform of the vertical acceleration shows a clearly asymmetric form with large amplitude in the upward direction. Aoi et al. (2008) reported and qualitatively explained the mechanism of this phenomenon by the analogy of bouncing a piece of matter on a trampoline; thus, they called it the trampoline effect. To study this recently discovered nonlinear behavior of the surface ground motion, numerical analysis with a two-dimensional finite-element method has been conducted with parameters derived from the borehole data at the station. The analysis successfully simulates the asymmetric vertical motion. Results indicate that the asymmetric motion may be characterized by the existence of a lower bound of negative acceleration, which in most cases corresponds to the acceleration of gravity and high positive pulses caused by the compression stress of the disturbed surface ground material.

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