A Volcanic Disaster Averted in the Philippines
-- Excerpt from:
Wright and Pierson, 1992,
|
On April 2, 1991, people from the village of Patal Pinto, on the Philippine
Island of Luzon, saw small explosions followed by steaming and the smell of
rotten eggs (H2S gas) coming from the upper slopes of Mount Pinatubo, a dormant
volcano whose last known eruption was 600 years ago. Thus began the unrest that
within 10 weeks led to one of the largest 20th century eruptions.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) immediately installed portable seismometers near the mountain and began recording several hundred earthquakes a day. At the request of PHIVOLCS, made through the USAID office in Manila, U.S.Geological Survey personnel arrived on April 23. Within 2 weeks the team of Philippine and American volcanologists had installed a radio-telemetered seismic network capable of locating the increasing number of earthquakes. They began tape measurements across fractures opened during the early steam explosions, and they later installed tiltmeters to detect new ground movement. With the help of the U. S. Air Force, measurements of the SO2 content of gas in the steam plumes, now continuously visible at Pinatubo, were begun. Between May 13 and May 28, a 10-fold rise in SO2 content was measured. All signals indicated that magma was rising within the volcano. Meanwhile, Philippine and American geologists made a geological reconnaissance of the volcano and established a set of alert levels ranging form 1 (low-level unrest) to 5 (eruption underway). When the alert system was completed on May 13, the level was set at 2 meaning that magma was probably involved in the unrest. The geological reconnaissance showed the dominant lava products were dacite domes near the summit and valley-filling pyroclastic flows extending radially out from the summit to a distance of 10 to 15 kilometers. Small villages on the volcano's northwest slope and part of Clark Air Base lay within the potential range of pyroclastic flows. Numerous communities, including the city of Angeles with a population of 300,000 lay within the apron of debris flows extending well beyond the volcano. A hazard map was prepared and distributed to local officials by May 23 showing virtually all of the hazards subsequently encountered when Pinatubo erupted. Seismic activity escalated in early June, and on June 5 PHIVOLCS raised the alert level to 3 -- eruption possible within 2 weeks. A small dome extruded on the north flank on June 7 accompanied by thousands of small earthquakes triggering a level 4 alert -- explosive eruption possible within 24 hours. Continuous ash emission began, and 2 days later PHIVOLCS raised the alert level to 5 signifying an eruption had begun. Clark Air Base was evacuated on June 10. On June 12, the first of several major explosions took place, sending airborne ash to the west and pyroclastic flows down the northwest slope of the volcano. Areas potentially impacted by the pyroclastic flows had been evacuated in response to the level 3 alert. The city of Angeles was put on evacuation alert. A climactic eruption took place on June 15 during passage of a typhoon, which had already resulted in closure of the International Airport at Manila. The typhoon compounded the effects of the eruption by generating mudflows in addition to ash fall and greatly hampering evacuation efforts. By June 16 when the weather cleared, the top of the volcano was gone, replaced by a 2-kilometer-wide caldera, and pyroclastic flow deposits had largely filled preexisting valleys on all sectors of the volcano. Ash had fallen over a vast area beyond the volcano. The weight of the thick blanket of heavy, water-saturated ash combined with continued felt earthquakes resulted in the collapse of many buildings in Philippine cities and villages as well as on Clark Air Base and the more distant Subic Bay Naval Base. The ash reached thicknesses of 30 centimeters as far as 40 kilometers away from the volcano. Following the climactic eruption, continuous ash emission at a lower level continues at this writing (July 1991). Loss of life in the Pinatubo eruption was remarkably low, given the size of the eruption -- 350 people died, mostly in buildings that collapsed. The alert system put in place by PHIVOLCS combined with effective communication among the USGS, PHIVOLCS, local civil defense agencies, and the U.S.Military Command prevented a much greater human disaster. The scientific information necessary to guide the alerts was gathered by USGS and Philippine geologists working side by side, each complementing the others' expertise in order to turn potential disaster into a responsibly handled volcanic emergency. As an exercise in both eruption prediction and effective response, the 1991 Pinatubo eruption provides and important model for future situations in which dormant volcanoes come to life. |
