Detecting the Eruption

 

Beginning on March 15, 1991 a series of small earthquakes were felt near Mt. Pinatubo. On April 2 a series of small, phreatic explosions occurred on the volcano creating several small new craters and depositing rock debris and ash. Scientists from the Philippine Institute of Volcanology and Seismology (PHILVOLCS) installed several portable seismographs on the northwest part of the volcano. They began recording many small earthquakes per day. In late April PHILVOLCS was joined by scientists from the United States Geological Survey (USGS) and the team installed a network of seven seismometers around the volcano.

http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/images.html

The group also began analyzing the volcano’s past eruptive history to determine the potential threat. Analysis of the area surrounding the volcano showed that much of it had been covered by lahars and pyroclastic flows. Radiocarbon dating on charcoal from these areas showed that the volcano had erupted in the past, likely 500, 3,000 and 5,000 years ago.

Measurements of the sulfur dioxide emission from the volcano provided further evidence that an eruption might occur. The first measurement of sulfur dioxide was made on May 13 showing an average of 500 tons/day, which by May 28 had increased to 5,000 tons/day.

Convinced there was a serious threat from Pinatubo, scientists from PHILVOLCS and the USGS used an analysis of the volcano’s activity and a hazard map to warn civil defense authorities.

 

Mt. Pinatubo Hazard Map

http://vulcan.wr.usgs.gov/Vdap/Responses/Pinatubo91/pina_yrbk-1991.html

 

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