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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 us in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bounding 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. [ABSTRACT FROM AUTHOR]

The article discusses the March 11, 2011 Tohoku earthquake in Japan and assessments of further seismic risk in the area. Geological evidence was discovered of a larger earthquake and tsunami striking the coast of Japan in the year 869, strengthening concerns about the likelihood of a megaquake occurring in Japan. Topics include the limits of seismic forecasting, the utility of Japan's earthquake building codes, and the need for better communication between scientists and the public.