Overview of the 1990–1995 eruption at Unzen Volcano

Abstract

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.

Introduction

Unzen Volcano's dacite lava dome eruption began in May 1991 without preceding explosive eruptions such as outburst of a plinian column (Table 1; Ohta et al., 1992; Nakada and Fujii, 1993; Ohta, 1997). Precursors to the beginning of eruption were clearly detected. The dome growth continued for about 4 years (Fig. 1) with repeated partial collapses which frequently generated pyroclastic flows (e.g., Yamamoto et al., 1993). Phenomena suggesting the finale of the dome growth were also evident. The activity of the eruption was monitored by Shimabara Earthquake and Volcano Observatory of Kyushu University (SEVO), Unzendake Weather Station of Japan Meteorological Agency (JMA), Geographical Survey Institute (GSI), Geological Survey of Japan (GSJ), and Joint University Research Groups (JURG). An overview of the eruption and its characteristics is presented here.

Unzen Volcano is located about 70 km behind the volcanic front of the Southwest Japan arc, and not above but behind the Wadati–Benioff zone related to the subduction of the Philippine Sea plate (e.g., Ishihara and Yoshida, 1992). Upwelling of mantle in the back-arc region may be responsible for magma genesis in the Unzen area. Unzen Volcano is a composite volcano with the volume of at least 35 km³, and consists of multiple volcanic cones of dacite to silicic andesite (Watanabe et al., 1995; Hoshizumi et al., 1999-this volume). It has grown within an active volcanic graben since ca. 500 ka (Fig. 2). The volcano consists of thick lava flows (or domes) and their collapsed debris (dome collapse pyroclastic flow, debris flow, and debris avalanche deposits). Its geology does not include any extensive pumiceous deposits (flow or fall deposits). Few, if any, large-scale explosive eruptions occurred throughout the history of this volcano. A large-scale collapse of the volcanic cone occurred about 20 ka, resulting in the formation of Myoken Caldera (Fig. 2; Hoshizumi et al., 1999-this volume). Eruptions that form lava domes within the Myoken Caldera, including the latest eruption, have occurred every 4–5 thousand years. The latest eruption was different in scale from the much smaller historically documented eruptions in 1663 and 1792.

Jigokutao, Kujukushima, and Fugen-ike (pond) craters existed east of the Mt. Fugen summit before the latest eruption. The Kujukushima Crater smoked in 1663 as andesite lava emerged at the northern edge of the Myoken Caldera and flowed down to the northern slope. The Jigokuato Crater smoked in 1792, and dacite lava appeared on the northern slope of Mt. Fugen. A swarm of volcano-tectonic earthquakes with a maximum magnitude (M) of 5.7 occurred near the western coast of the Shimabara Peninsula in 1984.