Mt. Pinatubo Eruption – A forecasting success with VDAP

It is widely speculated that the largest eruptions in the near future would occur in the least monitored volcanoes.

This is primarily because our scientists and their monitoring devices are focused towards the frequently erupting volcanoes that provide more continuous monitoring data and pose the most imminent threat to life and property.

However, it is well known that larger eruptions are typically less frequent and often do not have much historical records.

As a result, we have neither identified nor monitored most of those volcanoes that could possibly show large eruptions in the foreseeable future.

One such volcano is Mt. Pinatubo in Philippines that had not erupted in centuries but its 1991 eruption went on to become the largest eruption in the last 100 years and the second largest in the 20th century.

Due to its dormant state over the last few centuries, Mt. Pinatubo was not under the watch of volcanologists around the world.

The volcano was completely unmonitored and close to 1 million people resided in its vicinity, unaware of the threat it posed.

In fact, the island’s native Aeta tribe lived in the very slopes of Mt. Pinatubo.

Things looked as good as ever until a 7.8 magnitude earthquake hit some 100 kilometres northeast of Mt. Pinatubo on July 16, 1990.

Following the earthquake, the local residents living close to the volcano felt tremors and observed the release of steam from the ground.

In August, a group of catholic nuns who resided high on the slopes of Pinatubo reported the matter to the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

When the PHIVOLCS scientists surveyed the region, they observed new gas vents and they installed the first seismometer in the area.

The seismometer showed signs of strong seismic activity over the next few days hinting at the disaster that was brewing up in the shallow depths of Pinatubo (to learn more about the activities that precede an eruption, click on “Volcanic Unrest”).

PHIVOLCS contacted the US Geological Survey for help and soon after, a VDAP team was deployed in Mt. Pinatubo.

The Volcano Disaster Assistance Program (VDAP) was created in 1986 to efficiently respond to volcanic disasters whenever they occur around the world.

Fig: Scientists from the U.S. Geological Survey and the Philippine Institute of Volcanology and Seismology unload volcano monitoring gear from a U.S. Air Force helicopter on the east side of Mount Pinatubo.

Scientists from the U.S. Geological Survey and the Philippine Institute of Volcanology and Seismology unload volcano monitoring gear

Credit: U.S. Geological Survey. The VDAP team, along with the scientists at PHIVOLCS, set up additional instruments to monitor the activities of Mt. Pinatubo.

Over the next few months, the monitoring data and a detailed study of previous eruption deposits made it clear that the volcanic mountain that had been in a slumber for centuries was preparing for the next giant eruption.

A survey of the previous eruption deposits and the extent of pyroclastic flows helped the team prepare a hazard map on the basis of which, evacuation was immediately carried out in a region of up to 40 km radius.

As expected, just a few days later, on the 12th of June, a massive explosion took place that sent ash and volcanic material spewing out of the ground.

Finally, the massive explosion occurred and Mt. Pinatubo exploded its way into our memories and history books for ages to come.

Pinatubo released so much material that it collapsed upon itself and formed a caldera.

In the area that saw maximum damage, a population of over 25,000 resided prior to the eruption.

If these people had stayed put on the day of the eruption, they would have been wiped out in all likelihood.

Many more in the affected region, beyond the ‘death zone’, would have faced serious difficulties or even death if the timely evacuation had not been carried out.

The actual death toll of the 1991 Mt. Pinatubo stands at 300-400, a number which would have been dramatically more if the nuns, the Aeta tribals, the PHIVOLCS and the USGS had not acted on time.

The successful forecasting of this eruption talks volumes about the need for international co-operation in the science community and the need for more widespread monitoring if such disasters are to be averted.

Overall, this eruption continues to remain a model for coordinated international rapid response and the integration of geologic and geophysical data to forecast eruption timing and magnitude.