Trampoline Effect in Extreme Ground Motion

• Published in: Science (American Association for the Advancement of Science) Vol. 322; no. 5902; pp. 727 - 730
• Main Authors: Aoi, Shin, Kunugi, Takashi, Fujiwara, Hiroyuki
• Format: Journal Article
• Language: English
• Published: Washington, DC American Association for the Advancement of Science 31-10-2008; The American Association for the Advancement of Science
• Subjects: Amplitude; Earth sciences; Earth, ocean, space; Earthquakes; Earthquakes, seismology; Elasticity; Exact sciences and technology; Geophysics; Internal geophysics; Kinetics; Mass; Ratios; Risk assessment; Seismographs; Seismology; Sensors; Trampolines; Waveforms; Far East; Asia; Japan; models; asymmetry; ground motion; earthquakes; acceleration; seismic response; amplitude; horizontal movements; prediction; magnitude; seismic moment; waveforms; direction; seismic risk
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1163113

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.