Fig. 1
(174K,JPG)
The June 12-15, 1991, eruption of Mount Pinatubo volcano, located 100 kilometers northwest of Manila in the Philippines , was the largest eruption in the past five decades and led to the largest known evacuation of people due to a volcanic threat. Quick deployment of monitoring instruments and preparation of a volcanic hazards map by scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the USGS helped scientists to better understand the precursors of volcanic activity and provided the basis for accurate warnings of impending eruptions. These warnings were widely disseminated and led to the evacuation of more than 58,000 residents near the volcano, including 14,500 US military personnel from Clark Air Base prior to June 12. Before the climactic eruption of June 15, more than 100,000 people had left the area.
USGS monitoring instrumentation used at Pinatubo was drawn principally from a permanent supply of specialized equipment kept ready for volcano crises under the auspices of the USGS Volcano Hazards Program and the joint USGS-Office of Foreign Disaster Assistance (OFDA) Volcano Disaster Assistance Program. OFDA is an office of the Agency for International Development (AID) and is responsible for administering the AID overseas disaster assistance program. By the end of fiscal year 1991, more than 23 USGS geologists, seismologists, hydrologists, and electronics and computer specialists had each spent between 3 and 8 weeks at Pinatubo and helped PHIVOLCS advise community and national leaders and those at risk.
April 2,1991
The eruptive cycle began, with a series of small phreatic explosions of steam and mud from an east- northeast-trending, 1.5-kilometer-long chain of vents on the north side of the volcano. Vigorous steam emissions continued from vents on the north side of the summit dome through the end of May.
April 5
PHIVOLCS installed 4 non-telemetered seismographs on the northwest side of Pinatubo, 10 to 15 km from the summit. During the next several weeks these stations recorded between 40 and 140 seismic events per day, most of which were less than magnitude 1.
April 22.
In response to a request from the Government of the Philippines and the U.S. Air Force at Clark Air Base, an initial 3-person USGS team was dispatched to Pinatubo with telemetered seismic instrumentation, a computer-based seismic- data collection and analysis system, telemetered tiltmeters and a COSPEC (correlation spectrometer) instrument for measuring the emission of sulfur dioxide (SO2) gas. A 7-station telemetered seismic net, installed by a USGS-PHIVOLCS team, was fully operational by mid-May; the central recording site was located on Clark Air Base.
A 5-level alert system (see table) was developed by PHIVOLCS and the USGS to provide communication about the status of Mount Pinatubo to public officials and to provide a framework against which they could make contingency plans.
May 13
The alert system (1 is low alert and 5 is high alert) was adopted and distributed to civil defense and local officials beginning when the volcano was in level 2.
Only limited geologic information was available before April 2, 1991. The volcano was known to be a 3-kilometer-diameter dacite dome complex surrounded by voluminous fans of geologically young (600-8000 years before the present) ashflow deposits. No historic eruptions had occurred at Mount Pinatubo, but the volcano was known to be thermally active and had been explored as a potential geothermal energy source by the Philippine National Oil Company. Although there was no volcanic hazards map of the volcano, one was quickly compiled by the USGS-PHIVOLCS team to show areas most susceptible to ashflows, mudflows and ashfall. This map was based on the maximum known extent of each type of deposit from past eruptions and was intended to show a worst-case scenario.
May 23
The map was first distributed to Philippine civil defense officials, local governments and the US military. The map proved to forecast closely the areas that were impacted on June 15. Figure 2 is a redrafted version of this map.
Fig. 2
(24 Kb GIF)
Late May
The number of seismic events fluctuated from day to day. Trends in rate and character of seismicity, earthquake hypocenter locations, or other measured parameters were insufficiently conclusive to allow forecasting of an eruption.
End May to beginning June
Subtle changes in seismicactivity (a single long-period earthquake, periods of tremor, and hypocenters began to concentrate beneath the steam vents) heralded a trend of accelerating precursory activity. SO2 gas emissions, which had increased tenfold from May 13 to May 28, suddenly decreased as if magma had sealed its conduit so that gas could not escape.
June 6-12
A swarm of progressively shallower volcano- tectonic earthquakes accompanied by inflationary tilt on the upper east flank culminated in the extrusion of a small lava dome, near the most vigorous steam vent, and continuous low-level ash emission.
An Alert Level 4 was announced on June 7, and residents of Zambales, Tarlac and Pampanga Provinces within 20 kilometers of the volcano were evacuated. Early on June 10, in the face of a growing dome, increasing ash emission, and worrisome seismicity, 14,500 nonessential personnel and dependents were evacuated by road from Clark Air Base to Subic Bay Naval Station. Almost all aircraft had already been removed from Clark when this action was taken.
From June 7 to 12, periods of tremor became more frequent and had lower frequency, and SO2 emissions dropped to low levels. Although magma continued to move beneath the volcano, the tremor fluctuations and SO2 emissions suggested that the vent had been sealed by a cap of degassed magma.
June 12-14
Fig. 3
(72 Kb JPG)
At 0851 on June 12, a powerful explosion sent an ash column 19,000 meters above sea level (m asl), as measured by weather radar at Clark Air Base. Additional explosions occurred on the night of June 12 and in the morning of June 13. Explosions destroyed part of the dome and formed a small crater adjacent to the dome. Seismic activity during this period became intense. Recognizable seismic build-up, as much as several hours prior to the explosions from June 12 to 14, permitted short-term notification of impending eruptions to Philippine civil authorities and US military authorities.
June 14-15
On June 14 an eight-hour episode of vigorous seismic activity, bigger than any observed before at Pinatubo, lasted until 13:09 when the first explosion of what became the climactic eruptive phase began. Explosive eruptions became more frequent into the night and following morning when, at 05:55, a lateral blast spread southwest, north, northwest, and west from the volcano and sent a broad column of ash to 12,000 m asl. Six additional eruptive pulses followed the blast and by early afternoon the eruption was essentially continuous, lasting approximately through June 15.
At this time Typhoon Yunya was nearing Pinatubo. The extreme vigor of the eruption, loss of telemetered seismic stations between the summit and the observatory, and uncertainty about the effect of Yunya on immediate mudflow hazards precipitated the evacuation of the remaining Air Force, USGS and PHIVOLCS personnel from Clark Air Base at 14:30 June 15. Personnel returned to Clark the following morning. Seismicity began to decline late on June 15, but the volcano continued to erupt a 10,000-meter-high column of ash for the next several weeks. The various explosions of June 12 to 15 destroyed 6 of the 7 seismic field stations and both telemetered tiltmeters. By June 18 the USGS had dispatched replacement equipment and additional personnel to reinstall the seismic net and to replace the USGS group on site.
The effects of the June 15 climactic eruption were impressive. A crater 2 km in diameter was formed on the north side of the summit dome, and ashflow deposits covered approximately 100 square kilometers of the area around the volcano. These voluminous ashflow deposits filled in the deeply dissected landscape on the north, west, and south flanks of the volcano to such a degree that these areas are now broad featureless plains. Preliminary estimates of the bulk volume of ejecta range from 7 to 11 cubic kilometers.
Fig. 4
(66 Kb JPG)
Airfall and mudflow hazards accompanying the eruption were exacerbated by the passage near Pinatubo of typhoon Yunya during the peak eruptive activity. Cyclonic winds spread tephra over a circular area more than 200 kilometers in diameter, covering at least 20,000 square kilometers with enough ash to damage crops, infrastructure, and buildings. Most of the deaths (more than 330 people) and injuries from this eruption were from the June 15 collapse of roofs under wet heavy ash; many of these roof failures would not have occurred if there had been no rainfall. Heavy rain from the typhoon remobilized ashflow deposits into mudflows on all sides of the volcano. These flows destroyed homes, bridges, and irrigation-canal dikes, and buried crop land beyond the volcano.
Fig. 5
(16 Kb GIF)
The onset of the wet season in July brought continued hazards as the ashflow and airfall deposits were remobilized into secondary-mudflows. Damage to bridges, irrigation-canal systems, roads, cropland, and urban areas occurred in the wake of each significant rainfall. In early July, a system of telemetered rain gages and a demonstration system of flow monitors was installed to measure precipitation rates and track mudflows in specific drainages. These data allowed the USGS-PHIVOLCS group to provide Philippine civil and U.S. military authorities with real-time mudflow-hazard information.
By August of this year over 100,000 people were still displaced by the eruption and continuing mudflow hazard. The main transportation corridors connecting Manila to Angeles City and Olongapo were subject to disruption with each heavy rain. Areas near the volcano will undoubtedly be subject to inundation by mudflows for several more years.
Effects of the eruption on global weather patterns remain to be seen. The mass of SO2 erupted on June 15 to 16 has been estimated by the National Aeronautics and Space Administration at approximately 18 million tons~about double what was erupted by the 1982 eruptions of El Chichon in Mexico. On the basis of data from other large eruptions in the historical past, some climatic cooling can be anticipated. Plans are that the USGS will continue to support to PHIVOLCS in fiscal year 1992 and will assist, as requested, in the continued monitoring of Pinatubo.
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Alert level Criteria Interpretation
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0 Background; quiet............. No eruption in
foreseeable future.
1 Low-level seismic, fumarolic, Magmatic, tectonic, or
other unrest. hydrothermal
disturbance;
no eruption imminent.
2 Moderate level of seismic, Probable magmatic
other unrest, with positive intrusion; could
evidence for involvement of eventually lead to an
magma. eruption.
3 Relatively high and increasing If trend of increasing
unrest including numerous continues, eruption
shallow, volcanic earthquakes; possible within 2 weeks.
accelerating ground deformation;
increased vigor of fumaroles,
gas emission.
4 Intense unrest, including Eruption possible within
harmonic tremor and (or) many 24 hours.
long-period (low-frequency)
earthquakes.
5 Eruption in progress .......... Eruption in Progress
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Stand-down procedures
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In order to protect against lull-before-the-storm phenomena,
alert levels will be maintained for the following periods after
activity decreases to the next lower level:
From level 4 to level 3: Wait 1 week.
From level 3 to level 2: Wait 72 hours.
