On July 12, 1993, a large earthquake and associated tsunami caused terrible damage to the Japan Sea side of Japan, Korea, and Russia. The southwestern shore of Hokkaido and Okushiri Island were particularly hard hit. All aspects of this earthquake will undoubtedly be studied in great detail. This brief report focuses on the tectonic setting and some puzzles encountered in the preliminary seismological analysis.

At first glance, the July 12 event appears to have occurred on the “wrong side” of Japan (see Figure 1). The Pacific plate subducts to the west beneath northern Honshu and Hokkaido, and a Wadati-Benioff zone extends to 700-km depth beneath the Sea of Japan. Great underthrusting earthquakes are expected to occur on the east coast of Honshu and Hokkaido, such as the 1968 Tokachi-Oki (magnitude 8.2) and 1978 Miyagi-Oki (magnitude 7.6) earthquakes. Thus, the occurrence of large, shallow, underthrusting earthquakes on the west coast of Honshu and Hokkaido is somewhat odd. Nonetheless, there is now almost a continuous linkage of the rupture areas of such earthquakes off the western coasts of Honshu and Hokkaido, and this activity extends north to Sakalin.