Earthquakes are frequent in Japan. A survey showed that seismic disturbances, mostly of a minor nature, occurred more than three times a day. Geological research has shown that, possibly under the continuous impact of these disturbances, the W coast of the Japanese islands is settling, while the Pacific coast is rising. The E coast is subject to earthquakes affecting large areas and usually accompanied by great tidal waves; these shocks seem to begin at the bottom of the ocean near the NE coast of Honshu, where a gigantic crater is thought to exist more than 8 km (5 mi) below the surface. The most disastrous earthquake in Japanese history occurred in 1923. It was centered in Sagami Bay and damaged Tokyo and Yokohama; about 143,000 persons were killed by the earthquake and its aftermath.