Precursors and eruption warnings at Mount Pinatubo, 1991

Chris Newhall

With many colleagues from PHIVOLCS and USGS

Very high stakes

• More than 1,000,000 at risk

• 2 large American military bases, and contentious ongoing renegotiation for basing rights

• Public that had almost no prior knowledge of volcanic eruptions

• Tremendous skepticism – disbelief, suspicion

Also very high uncertainties

• Minimal prior mapping and age-dating of previous eruptions of Pinatubo

• Before 1991, no prior monitoring at Pinatubo

• No modern experience, anywhere in world, with precursors to plinian eruptions

Hazard map released May 13

Throughout April, PHIVOLCS ‘ temporary net showed all in the cluster 5 km NW of volcano (dense knot). Seismicity started under summit in late May.

Alert levels and evacuations

Alert Levels

May 13 – Level 2, magmatic

June 5 – Level 3, eruption possible within 2 weeks

June 7 – Level 4, eruption possible within 24 h

June 9 – Level 5, explosive eruption in progress (actually, premature, but helpful!)

Evacuations April 7 – 10 km (temporary) May 13 – 10 km June 5 – 10 km June 7 – 20 km June 9 – 20 km June 14 – 30 km June 15 – 40 km

Goal was to get forecast just right

• Just in time … neither too soon nor too late

• Just right in size of hazard zone … neither too large nor too small.

• Acutely aware of Soufrière Guadeloupe 1976 and Nevado del Ruiz 1985

Probability tree for civil defense, military, as of 17 May 1991, an early stage of unrest

As of May 17, estimated probability of pyro flow onto Clark AB in coming months ~ 3%

How to explain volcanic hazards to those who have never experienced them?

• IAVCEI video made by Maurice Krafft and others in response to the lahar disaster in Colombia in 1985

• Helped tremendously at Pinatubo. We could hear gasps from audiences when they saw young girl trapped in lahar mud in Armero

• Irony – Maurice, Katia and others were killed at Unzen enroute to Pinatubo

Monitoring Summary, 01 May to 01 June

An unintended, helpful move

The 1st Pinatubo Volcano Observatory was near center of Clark Air Base. For safety, the team decided to move 5 km farther away from the volcano – to the far edge of Clark AB. This had the unintended effect of convincing USAF officials that the hazard was serious!

To Pinatubo summit

Monitoring Summary, 01 May to 12 June

a

5 0 0

6 / 1 6 / 3 6 / 5 6 / 7 6 / 9 6 / 1 1 6 / 1 3 6 / 1 5

M O U N T P I N A T U B O J u n e 1 9 9 1

start of dome growth

first explosive eruption

2 0 0 0

1 5 0 0

1 0 0 0 RSAM

LEVEL 3

LEVEL 4

LEVEL 5 ERUPTION IN PROGRESS

climactic eruption

LEVEL 2

May 13, 1991

Date, June 1991

Widespread skepticism because no eruption in >500 y, few knew that Pinatubo was a volcano or could imagine an eruption

Updated probability tree, 10 June 1991 … barely used

~8% chance of pf hitting Clark in near term

Sharp escalation of shallow long period seismic energy release (curve), already within explosive eruptions (spikes) from 12-15 June . Marks the start of runaway escalation toward eruption, and was the only monitoring indicator that impending eruption would be big!

NOAA-10, IR, 6/15/91

The climactic eruption, June 15, 1991

How well did hazard map predict actual pyroclastic flows?

Clark AB

At risk, killed, and saved

• ~ 1,000,000 at risk, incl. 20,000 in area devastated by eruption.

• 85,000 evacuees before climactic eruption; 250,000 after

• ~400 died in eruption; ~500 in evacuation camp (from measles!)

• Thousands of lives, and ~ USD 1 Billion, saved by scientific advice.

• Rene Solidum will describe

warnings for subsequent lahars

“You guys were just lucky” (in your forecast)

• True?

– First reaction … No!

– Second reaction … True, we were lucky that...

• the volcano proceeded smoothly to eruption from 1-15 June, and that the hazard became clearly visible just 3 days after the highest alert was declared

• the eruptive history suggested ALL eruptions were big!

• the real worst case didn’t occur!

• Civil defense and population were generally responsive

– BUT …

Luck PLUS all the necessary steps

It is not pure luck. • The PHIVOLCS-USGS team did (almost) everything that

needed to be done – eruptive history & scenarios, hazard map; new monitoring and interpretation; urgent education of officials and public, using video, teachers, etc.

• Quick, international response • Ray Punongbayan did masterful job of managing the

politics and the media, and also put his neck on the line to warn of “worst case.”

• BOTTOM LINE: To succeed, the team needs to take all the necessary steps, AND have some luck. Without the necessary steps, no amount of luck will help!

Thanks, Ray Thanks, Maurice and Katia Thanks to the rest of the team !

And thanks for your attention!