Report on Tungurahua (Ecuador) — January 2000
Bulletin of the Global Volcanism Network, vol. 25, no. 1 (January 2000)
Managing Editor: Richard Wunderman.
Tungurahua (Ecuador) More than two-fold increase in long-period earthquakes during December
Please cite this report as:
Global Volcanism Program, 2000. Report on Tungurahua (Ecuador) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 25:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200001-352080.
1.467°S, 78.442°W; summit elev. 5023 m
All times are local (unless otherwise noted)
This report describes activity during December 1999 based on daily reports and other resources from the Geophysical Institute's website.
In comparison to other months, long-period earthquakes grew substantially during December (figure 6). The previous monthly high during the crisis, consisting of 883 events, took place in November 1999. December's count of LP earthquakes reached 1,995 earthquakes, more than double the November high. In addition, the count for December LP earthquakes stood higher than the sum for these events during all the previous months of the crisis.
Intervals of both low-frequency tremor and repeated explosions (often 10 to 25/day) prevailed during the reporting interval. Scientists explained the frequent, ash-bearing explosions in terms of a magmatic system that maintained an open conduit to the surface.
The Instituto's daily reports stated that on the morning of 1 December an ash column from Tungurahua rose to unstated height and moved NW. Ash fell on the city of Ambato and its vicinity. The next day ash blown SW fell on the town of Riobamba.
Modest ash columns, typically under 2 km, were seen almost daily during December. Cannon-like blasts were also widely heard. For example, despite bad weather on 4 December, observers heard explosions and later saw incandescent blocks descending 1 km below the summit. A blast on 5 December was audible 8 km from the crater. An explosion on 10 December discharged a plume that rose to 3 km altitude; that same day drivers discovered small mud flows cutting across three segments of the Baños-Penipe highway. Plumes from explosions on 11-12 December sent ash 4 to 5 km in altitude; in the case of the former, winds carried the plume 20 km laterally.
At 0840 on 16 December members of TAME flight 172 reported an ash plume above Ambato at an altitude of ~8 km. It drifted N at ~30 km/hour. These events correlated with seismic and ground-based visual observations. A second example of an upper-altitude ash-bearing plume took place on 28 December when a TAME flight crew reported a 6.2-km-altitude plume directed SW. A third such report came on 31 December when a TAME crew noted an ash plume at 7.5 km altitude extending SW.
On 26 December, a COSPEC instrument was used to estimate the SO2 flux at 1,700 tons/day. Land surveys with an EDM instrument detected an absence of measurable geodetic changes. That same day, however, it was announced that in the past few days there had been some extended periods of tremor.
By mid-December 1999 the people of Baños became dissatisfied with the military occupation of their town. Some sources claimed that evacuated residents had seen looting of homes on television news. On 5 January several thousand residents of Baños bearing sticks and machetes forcibly drove the military off, re-entered the town, and proclaimed that they would be responsible for their own safety in the event of a threatening eruption. By some estimates, 25% of the town's ~ 16,000-20,000 residents had returned. Tourists, too, have come to enjoy the town's ambiance, hot springs, and resort atmosphere. Baños is located on the N-flank of the volcano, where it is vulnerable to various volcanic hazards.
Correction: In BGVN 24:11 the plot showing monthly earthquakes at Tungurahua presented values that have since been revised. The chief difference lies in the number of long-period earthquakes during November 1999 (now reported at 883 events). Figure 2 shows the corrected values.
Reference. Hall, M., Robin, C., Beate, B., Mothes, P., Monzier, M., 1999. Tungurahua Volcano, Ecuador: structure, eruptive history and hazards: Journal of Volcanology and Geothermal Research, v. 91, p. 1-21.
Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II itself collapsed about 3000 years ago and produced a large debris-avalanche deposit and a horseshoe-shaped caldera open to the west, inside which the modern glacier-capped stratovolcano (Tungurahua III) was constructed. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.
Information Contacts: Instituto Geofísico, Escuela Politécnica Nacional, Apartado 17-01-2759, Quito, Ecuador; Embassy of Ecuador, 2535 15th Street NW, Washington, DC 20009 USA (URL: http//www.ecuador.org/); United Nations Office for the Coordination of Humanitarian Affairs (OCHA), Palais des Nations, 1211 Geneva 10, Switzerland; El Universo, Quito, Ecuador (URL: http://www.eluniverso.com/); Associated Press, International Headquarters, 50 Rockefeller Plaza, New York, NY 10020 USA (URL: http://www.ap.org/); ABC News (URL: http://abcnews.go.com/).