A seismic shock of M 4.7 occurred at 1758 on 1 September a few kilometers ESE of the summit. The shock was felt at JMA intensity IV near the volcano, and was weakly felt at Sendai (35 km E), Fukushima (45 km S), and Onahama (135 km E). Aftershocks were numerous, but declined toward mid-September. The high seismicity was the first at the volcano since July 1990, when there were many small earthquakes below the summit. Surveys of fumarolic areas on 5 September revealed no changes in temperature from the last survey in 1990.

Geological Summary. The Zaozan (or Zaosan) volcano group, the most active of northern Honshu, consists of a complex cluster of stratovolcanoes straddling the Pacific Ocean-Japan Sea divide. The Pleistocene Ryuzan volcano forms the western group (Nishi-Zao), and Byobu and Fubo volcanoes form the southern group (Minami-Zao). The complex was constructed over granitic basement rocks as high as 1500 m and thus has a relatively small volume. The 7 km3 Zaozan volcano proper forms the central group (Chuo-Zao), a complex topped by several lava domes and the Goshikidake tuff cone, aligned along a NW-SE trend. Several episodes of edifice collapse produced debris avalanches during the Pleistocene. Goshikidake contains the active Okama crater, filled with a colorful, strongly acidic crater lake; it has been the source of most of the frequent historical eruptions, which date back to the 8th century CE.

Information Contacts: JMA.