On 26 September 2003, a magnitude 8.0 earthquake, Earth's most powerful in 2.5 years, struck offshore Tokachi, Hokkaido, Japan, at 0450 local time. The hypocenter was at 40 km depth along the boundary of the Pacific and North American plates (www.jma.go.jp). The focal mechanism shows a reverse fault with a northwest-southeast compressional axis (P-axis) (see http://www.bosai.go.jp).
Following the main shock, named “Heisei 15 Tokachioki-jishin,” seismic activity has continued in the region. As of 30 September, the largest aftershock was M7.1 on 26 September at 0608. Tsunamis were observed on the Pacific coast of Hokkaido and the Tohoku region, including waves of 1.2 m at Kushiro, 1.3 m at Urakawa, and 1.0 m at Hachinohe.