Additional Report for Parraguirre

Estero de Parraguirre

Stream near Tupungatito Volcano, Chile

On 29 November 1987, an avalanche in the Estero del Parraguirre, a stream in a small valley 20 km NW of Tupungatito, generated a mudflow that killed 41 people and caused major damage along the valley of the Río Colorado, which pases through Chile's capital, Santiago. Initial reports suggested a possible linkage with increased activity at Tupungatito, a linkage that was disputed by other geologists. The material below primarily discusses the debris flow; information about Tupungatito can be found under that volcano.


Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

12/1987 (SEAN 12:12) 50-km-long lahar kills 41 people

04/1988 (SEAN 13:04) Fatal 29 November debris flow not related to volcanism

06/1988 (SEAN 13:06) Volcanic seismicity may have triggered November debris flow

10/1988 (SEAN 13:10) Volcano-seismic trigger for 1987 debris flow disputed


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

All times are local (= UTC - 3 hours)

12/1987 (SEAN 12:12) 50-km-long lahar kills 41 people

"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.

04/1988 (SEAN 13:04) 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.

06/1988 (SEAN 13:06) 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.

10/1988 (SEAN 13:10) 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 mt corresponds to an estimation of mb from the total duration of the PEL station record.

    Date    Time  Latitude  Longitude  Depth (km)  Magnitude (mt)

    27 Nov  1038   33°31'S    72°14'W      50           4.3
            1130   33°07'S    71°39'W      18           4.1
    28 Nov  0916   33°44'S    72°32'W      10           4.1
            2153   33°18'S    70°30'W      75           3.7
    29 Nov  0214   34°12'S    70°14'W      00           3.7
            0332   33°53'S    72°02'W      20           4.5
            0807   33°06'S    71°53'W      27           4.4
    30 Nov  0010   32°43'S    71°24'W       5           4.0
            0637   33°18'S    71°34'W      23           4.2
            1055   34°53'S    70°46'W     109           4.3
            1325   32°00'S    71°30'W      28           4.1
            1510   32°12'S    71°42'W      28           4.1

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