PHIVOLCS – VDAP Responses at Pinatubo

VDAP Concept Validated within Five Years

Large eruption plume with field in forground

April 2, 1991 steam explosion

On April 2nd, a steam explosion opened a 2 km–long rift in the flank of Pinatubo. The explosion was followed by the appearance of large pressurized fumaroles and sustained minor earthquake activity. VDAP received a request shortly after April 2nd from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) to assist them evaluate unrest at Pinatubo. By the third week of April a three-person team and shipment of monitoring gear were on route. At that time, public and government officials did not think that Pinatubo looked like an active volcano. Most people did not believe that it could erupt, and if it did, that it would be dangerous.

PHIVOLCS and VDAP installed a network of seismometers around Pinatubo to locate where earthquakes were occurring and to determine if the quakes had volcanic characteristics. They also began to measure the quantity of gases being emitted by the volcano to determine if an eruption was likely. From May through early June, SO2 output increased from several hundred to several thousand tons per day. Such high emissions of sulfur dioxide generally indicate shallow magma. The PHIVOLCS-VDAP team also installed electronic tiltmeters, basically a much more sensitive version of carpenter's levels, to determine if the volcano was swelling. This particular type of instrument was developed and tested at MSH in the early 1980s.

Scientists discussing volcanic 
             deposits at Mount Pinatubo

Installing electronic tiltmeters to monitor inflation of the ground.

At the same time the monitoring network was being installed, work was being done to determine as much about the eruptive history of Pinatubo as possible. A hazards map was also being created that U.S. and Filipino authorities could use to develop a response plan. Very little was known about Pinatubo before 1991. Only one eruption had been dated to be about 600 years ago and its deposits had not been mapped. The PHIVOLCS-VDAP team quickly learned that the volcano erupted fairly frequently, and that most eruptions were big.

By the third week of May, enough of the pre-historic deposits had been mapped and several other eruptions dated, so the PHIVOLCS-VDAP team could prepare a map of the potential hazards.

Through the month of May, the amount of SO2 emitted increased several fold, earthquakes became more frequent and intense and began to focus beneath the summit. The character of the seismicity took on a more decidedly volcanic character, tiltmeters began to show inflation, and finally on June 7th a small lava dome began to grow within the area of the large fumaroles.

Given what the team was seeing from their instruments and observations, and what they knew about Pinatubo's style and frequency of past eruptions, it became increasingly clear that they were moving toward a large eruption. As activity at the volcano increased through May, the U.S. military and Philippine officials began to take the threat more seriously. On June 10, 1991, 14,500 U.S. personnel and dependents were evacuated from Clark Air Base.

Large eruption plume with field in forground

First explosive eruption on June 12, 1991

After nearly ten weeks of steadily increasing activity, Pinatubo produced its first major explosive eruption on June 12. This eruption only lasted a half an hour. But it made believers out of everyone in the area.

More explosive eruptions occurred on June 13 and June 14. The PHIVOLCS-USGS team raced to install additional equipment and keep the monitoring network functioning around the clock.

At 05:55 on June 15th, massive eruptions began. Remaining personnel on Clark Air Base were evacuated by the afternoon along with approximately 200,000 Filipinos from the area surrounding Pinatubo. The PHIVOLCS-USGS team hunkered down as the eruptions become more frequent, and they finally abandoned the base at 2 pm.

Approximately 8-10 cubic kilometers of material were erupted from Pinatubo and the eruption cloud reached 35 kilometers high. It was the second largest eruption in 20th century and it occurred on the same day Typhoon Yunya struck. Cyclonic winds and heavy rain from the typhoon compounded the hazards by increasing the area impacted by ash fall and by making the ash heavier. Large span roofs became susceptible to collapse. Heavy rains from the typhoon mobilized the new deposits into mudflows that repeatedly swept down the volcano.

U.S. military bases in the area were heavily damaged, and a conservative estimates more than $300 million of equipment had been moved out of harm's way before the eruption. Over 17 damaging encounters between aircraft and airborne volcanic ash affected air traffic in Asia. Two of the encounters caused engine shutdowns.

Large muddy river with bridge remnants on each side.

Cars and people traverse the flooded river after mudflows wiped out the
bridges. June 1991.

Nonetheless, given the enormous size of the eruption and the high population density, only about 300 people were killed, many from roof collapse.

Mudflows occurred with every monsoon into the mid-1990s. Attempts to date to control the post-eruption mudflows have failed.

The 1991 eruption of Mount Pinatubo was the second largest eruption of the 20th Century, about 10 times larger than Mount St. Helens in 1980. Using tools and techniques developed at Mount St. Helens and other volcanoes in the 1980s, a combined PHIVOLCS-USGS team was able to accurately forecast the eruption and work to move people and property out of harm's way. Five years after its founding, the VDAP concept was fully validated, representing a hugely successful response to a catastrophic eruption.

Links to additional information about Pinatubo