Volcano-seismic trigger for 1987 debris flow disputed

The following is from Alfredo Eisenberg and M. Pardo. "González-Ferrán (13:6) attributed the 29 November 1987 debris flow to an increase in local shallow volcano-seismic activity that he said can be observed on our seismograms of 28, 29, and 30 November. The seismic network in central Chile did not record any activity of this kind during those days, nor the previous ones. It is also important to note that only one station is located within 40 km of Tupungatito volcano's main crater (figure 3 and table 1). Stations on the Argentine side of the Andes did not register any volcano-seismic activity either. We did record the debris fall, however, at 1033 on 29 November, with an equivalent local magnitude of 4.5. This event, as far as we can see from the records, was not preceded by a triggering earthquake."

Figure 3. Seismicity in central Chile, 27-30 November 1987. Circles mark epicenters of located earthquakes (table 1), squares the position of seismological stations. The source of the debris flow is marked by a circled cross, and Tupungatito volcano by a star.

Table 1. Seismicity recorded in central Chile, 27-30 November 1987, by Univ of Chile seismograph stations. Magnitude m_t corresponds to an estimation of m_b from the total duration of the PEL station record.

Information Contacts: A. Eisenberg and M. Pardo, Univ of Chile.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

February 1980 (SEAN 05:01) Cite this Report

Ash eruption and seismicity

A series of subterranean noises at 0623 on 10 January was followed by an explosion that ejected a 1,500-m-high cloud of gas and ash from Tupungatito's SW crater. The next day, Oscar González-Ferrán and Sergio Barrientos flew over the volcano, observing that ash covered the snow NE of the vent and that the eruption was continuing, but with decreased intensity.

The Seismologic Service of the Geophysics Department, University of Chile, recorded considerable seismic activity near Tupungatito. The principal earthquake, at 1851 on 14 January, was a shallow event with an epicenter calculated at 33.2°S, 69°W, 78 km NE of the volcano. It was felt at intensity 3 in Santiago, about 150 km from the calculated epicenter. In the next 2 hours, 17 similar events were recorded, of which three were located with the same epicenter. Between 2100 on 14 January and 0100 on 16 January, 13 more local events were recorded, one of which was fairly large. As of 18 January, both seismic and eruptive activity had diminished. Tupungatito's last eruption, in 1964, consisted of explosions from the central crater.

Information Contacts: O. González-Ferrán, Univ. de Chile, Santiago.

December 1987 (SEAN 12:12) Cite this Report

Increased summit thermal activity, small ash eruption in January 1986; 50-km-long lahar kills 41 people

"Reactivation of one of the NW craters with a weak emission of black ash occurred 20 January 1986 at 1030 (figure 1). The activity was observed by the pilot of a Chilean Air Force C-130 and reported directly by radio-telephone to the author. The volcano had been under observation since the last pyroclastic eruption occurred 10 January 1980. After the M 7.8 earthquake that affected the Santiago region on 3 March 1985, vertical aerial photographic coverage was carried out at a scale of 1:20,000 on 25 March 1985. No increased activity was observed within the caldera, which has a diameter of ~5 km and is covered with ice and snow. Glaciers descend from it toward the W. The group of cones and craters with historic activity, in the extreme NW of the caldera, have only a thin covering of snow. Observations on 24 November 1987 show ice partially covered by 20 January 1986 ash, and intense solfataric activity in the same crater (figure 1).

Figure 1. Sketch map by O. González-Ferrán of the summit area of Tupungatito, labeling craters active in recent eruptions and the location of vigorous solfataric activity on 24 November 1987.

"On 4 December, new aerial photographic coverage of the Tupungatito volcanic region was taken by the Air Force Aerophotogrammetric Service at the request of the author and the National Emergency Office.

Analysis of the airphotos revealed a notable increase in snowmelt inside Tupungatito's active craters and an important increase in fumarolic activity without showing a direct relation with the 29 November lahar.

"The volcano remains under observation. If it produced an eruptive reactivation of some magnitude it could: affect the glacier covering the caldera; cause lahars larger than the one that occurred 29 November; and seriously damage the S sector of Santiago, Chile and Mendoza, Argentina."

Event in the Estero del Parraguirre. "On 29 November 1987, because of a regional (summer) thaw and the instability of mountains affected by the 1985 earthquake, an avalanche was generated in a small valley (Quebrada de Parraguirre) in the volcanic area. The avalanche caused a mudflow that descended the Río Colorado, killing 41 persons, destroying roads, machinery, and the Los Maitenes hydroelectric plant along some 50 km until its confluence with the Río Maipó, which passes through the city of Santiago. Also affected were the city's drinking water installations, and some 500,000 persons were left without water for 48 hours."

Information Contacts: O. González-Ferrán, Univ de Chile.

April 1988 (BGVN 13:04) Cite this Report

Fatal 29 November debris flow not related to volcanism

The event started as a rockfall of ~3-4 x 10⁶ m³ of lower Cretaceous sedimentary rocks and gypsum on the eastern slope of the Estero (stream) del Parraguirre, 20 km NW of Tupungatito and separated from it by at least four high ridges of Mesozoic rocks (figure 1). The rockfall mass developed into a mudflow after reaching the bottom of the stream valley and moved into the Río Colorado, causing casualties and destroying roads, bridges, camps, and the Los Maitenes hydroelectric power plant. Geologists noted that the rockfall appeared to have been triggered by a combination of geologic and hydrologic factors: steep walls, highly fractured rocks, water influx, and the presence of gypsum and other salts.

Figure 1. Sketch map by Hugo Moreno R., Arturo Hauser Y., and Lucio Velasco V., based on ERTS satellite data. The source of the rockfall in the headwaters of the Estero del Parraguirre is indicated by an arrow and its path down that stream and the Río Colorado is shown by a bold line. Positions of ridges between the source of the rockfall and Tupungatito Volcano are marked by heavy dash-dot lines; their approximate elevations are designated.

Information Contacts: L.Velasco Villegas, Compañía Chilena de Generación Eléctrica, Santiago; A. Yung, SERNAGEOMIN, Santiago; H. Moreno, Univ de Chile.

June 1988 (BGVN 13:06) Cite this Report

Volcanic seismicity may have triggered November debris flow

O. González-Ferrán presented additional information about the cause and dynamics of the 29 November 1987 debris flow.

"Tupungatito volcano has been increasing its thermal activity since January 1986 and on 28, 29, and 30 November 1987 registered an increase in local shallow seismic activity. Seismographs of the Chilean and Argentine nets registered some B-type shocks. The seismicity caused 15 rockfalls of different magnitudes within 5-20 km of the historically active craters . . . (N and NW of the ice-filled summit caldera) during that period (figure 2).

Figure 2. Sketch map illustrating the 15 rockfalls documented near Tupungatito in late November (heavy arrows), and tabulating the casualties and damage caused by the 29 November debris flow. Courtesy of O. González-Ferrán.

"One of the shocks occurred at 103340 on the 29th, causing one of the rockfalls, which reached a volume of 17.25 x 10⁶ m³ in the headwaters of the Estero Parraguirre, 17 km from the active crater and the epicentral zone of the seismic activity.

"A mass of sedimentary rocks from the headwaters of the Estero Parraguirre free-fell 1100 m onto the terminal front of a glacier, impacting it with a velocity of ~300 km/hour. Incorporated in the impact was 1.2 x 10⁶ m³ of ice, as well as 11 x 10⁶ m³ of snow along the Estero.

"This generated the avalanche debris flow that discharged into the Río Colorado, temporarily obstructing the flow of the Colorado's waters. Finally, the debris flow reached the Central Hidroeléctrica de Maitenes at 1114 with a velocity of 50-60 km/hour, causing the death of 41 persons and millions of dollars in losses to buildings and machinery along its path. In addition, by damaging the Las Vizcachas plant, it affected the supply of drinking water to nine communes of the city of Santiago (population 4.5 million) for 24 hours.

"The high instability and strong fracturing of the nearly vertical strata of Mesozoic sedimentary rocks in this mountainous region of the Tupungatito area, along with the supersaturation and water pressure generated by snowmelt and the abnormal seasonal temperature increase, facilitated rockfalls and avalanches as a consequence of the local volcano-seismic activity of Tupungatito."

Information Contacts: O. Gonzalez-Ferrán, Univ de Chile; J. Castrano, Instituto Nacional de Prevención Sísmica, San Juan, Argentina; S. Kunstmann, Empresa Nacional de Electricidad, Santiago; G. Ugarte, Pontificia Univ Católica de Chile, Santiago.

October 1988 (BGVN 13:10) Cite this Report

Volcano-seismic trigger for 1987 debris flow disputed

The following is from Alfredo Eisenberg and M. Pardo. "González-Ferrán (13:6) attributed the 29 November 1987 debris flow to an increase in local shallow volcano-seismic activity that he said can be observed on our seismograms of 28, 29, and 30 November. The seismic network in central Chile did not record any activity of this kind during those days, nor the previous ones. It is also important to note that only one station is located within 40 km of Tupungatito volcano's main crater (figure 3 and table 1). Stations on the Argentine side of the Andes did not register any volcano-seismic activity either. We did record the debris fall, however, at 1033 on 29 November, with an equivalent local magnitude of 4.5. This event, as far as we can see from the records, was not preceded by a triggering earthquake."

Figure 3. Seismicity in central Chile, 27-30 November 1987. Circles mark epicenters of located earthquakes (table 1), squares the position of seismological stations. The source of the debris flow is marked by a circled cross, and Tupungatito volcano by a star.

Table 1. Seismicity recorded in central Chile, 27-30 November 1987, by Univ of Chile seismograph stations. Magnitude m_t corresponds to an estimation of m_b from the total duration of the PEL station record.

Information Contacts: A. Eisenberg and M. Pardo, Univ of Chile.