Seismicity declined during September (248 earthquakes, from 345 in August), but remained at high levels (figure 5). Two earthquakes were felt [3.9] km SW of the volcano (at UWS) and 49 tremor episodes were recorded (a decrease from 56 in August). Tremor amplitude also decreased slightly, from 0.2-0.9 µm in August to 0.1-0.6 µm in September.

Figure 5. Daily number of earthquakes at Unzen, March 1989-early October 1990. Courtesy of JMA.

Geological Summary. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA.