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Report on Unzendake (Japan) — December 1993


Bulletin of the Global Volcanism Network, vol. 18, no. 12 (December 1993)
Managing Editor: Edward Venzke.

Unzendake (Japan) Continued ground deformation and seismicity; fewer pyroclastic flows

Please cite this report as:

Global Volcanism Program, 1993. Report on Unzendake (Japan) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:12. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199312-282100



32.761°N, 130.299°E; summit elev. 1483 m

All times are local (unless otherwise noted)

Strong endogenous growth started in mid-December in the SW part of the lava dome (figure 65). The N half of lobe 10, which uplifted during November, subsided about 30 m in early December at the same time that swelling on the SW part of the dome began. Faults, cracks, and pressure ridges were observed on the WSW floor around the dome during strong endogenous growth, similar to those observed during lava-dome growth at Mount St. Helens. These deformation phenomena were all a result of pressure from the dome. This type of surface deformation was previously observed in May 1991 when lava first appeared, and in March and April 1993 during partly endogenous growth. Lobe 11, covering the top and E slope of the dome, moved little during December and early January, implying that there was no magma-supply into the lobe.

Figure (see Caption) Figure 65. Sketch map of the lava dome at Unzen, early January 1994. Arrows indicate the main direction of rockfalls, "f" indicates fumarole locations, and the hachured line indicates a depression. The remains of lobe 10 consist of lava fragments and blocks just W of lobe 11. Courtesy of S. Nakada.

The volume of lava added from beneath during mid-December to early January was estimated to be ~9 x 106 m3, based on results of theodolite and laser-distance measurements. The endogenous eruption rate reached ~0.3 x 106 m3/day in mid-December, and remained at that level until early January when it decreased to ~0.1 x 106 m3/day. Earthquakes as large as M 3 occurred when the eruption rate was high. The SW talus slope of the dome rapidly advanced outward by about 150 m. Talus has now covered the western Jigokuato crater.

Rockfalls frequently traveled W and SW from the growing part of the dome (figure 65). The talus and pyroclastic-flow deposits covered much of the SW floor, and pyroclastic flows threatened to cascade down the S slope of Mt. Fugen. A small shrine situated on the rim of western Jigokuato crater, near a steep cliff of the prehistoric Byobu-iwa lava dome, was deeply buried under large lava blocks. The shrine had already been broken and half-buried by bombs and ash in 1991. Few pyroclastic flows were generated by collapse from the lava dome in December. Large earthquakes caused collapses on the SW side of the dome, generating a few small pyroclastic flows to the SW. No collapses took place from the E side of the dome, the dominant area of collapse until October.

Seismicity beneath the lava dome began on 19 November. A significant increase in the number of microearthquakes occurred in December, exceeding 1,000/day by mid-month. The magnitude of the earthquakes also gradually increased after 10 December. After 15 December, some of the larger earthquakes (M 2-3) were felt at the UWS, 4 km SW. The total number of earthquakes in December was 25,340, including 170 felt events. This is almost double the previous high monthly total during this eruption (12,946 events in August 1993), and represents almost half of the earthquakes recorded in all of 1993 (54,988). As of 5 January, there had been 27 felt earthquakes in 1994.

The ground deformation that began in early November continued through December. EDM measurements SW of the dome taken by the GSJ revealed 50 m of horizontal displacement to the SW between early November and late December, at a rate ~1 m/day. Upward displacement during this period was 17 m at a point near the dome. Theodolite measurements indicated that the lava dome gradually grew >100 m to the SW. Ground deformation and larger earthquakes had both ceased by 5 January.

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; S. Nakada, Kyushu Univ.