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Science 31 October 2008:
Vol. 322. no. 5902, pp. 727 - 730
DOI: 10.1126/science.1163113
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Reports

Trampoline Effect in Extreme Ground Motion

Shin Aoi,* Takashi Kunugi, Hiroyuki Fujiwara

In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction as in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments.

National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki 305-0006, Japan.

* To whom correspondence should be addressed. E-mail: aoi{at}bosai.go.jp

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
The Slapdown Phase in High-acceleration Records of Large Earthquakes.
M. Yamada, J. Mori, and T. Heaton (2009)
Seismological Research Letters 80, 559-564
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Science. ISSN 0036-8075 (print), 1095-9203 (online)