Pinatubo volcano Alert Level raised after 1 722 earthquakes since January 20, Philippines

Pinatubo volcano Alert Level raised after 1 722 earthquakes since January 20, Philippines

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) has raised the Alert Level for Pinatubo volcano from 0 (Silent) to 1 (Partial Disturbance) at 23:00 UTC on March 3, 2021.

Since January 20, 2021, 1 722 weak earthquakes were detected under the volcano. 

The first cluster of earthquakes occurred from January 20 to 26 at Sacobia Lineament at depths between 15 - 28 km (9 - 17 miles) and magnitudes from 1 to 2.5.

This was followed by one cluster of earthquakes in the northwest and southeast sector at depths between 15 - 25 km (9 - 15 miles), though some were shallow and dropping at both ends of this cluster. The second cluster of earthquakes had magnitudes up to 2.8.

All recorded earthquakes so far were created by rock fractures beneath the volcano.

A total CO2 flux of 378 tonnes per day was also measured from the Pinatubo Crater lake in February.

This is not far from the average size gathered below 1 000 tonnes per day estimated background levels over the past decade.

There is a measured partial heat rise in the fumaroles or gas cylinders here but no change in other properties such as acidity (pH), PHIVOLCS said.

After repeated earthquakes, PHIVOLCS raised the alert from Alert Level 0 to Alert Level 1 on March 3. This means there is a partial disturbance that could be caused by tectonic events under the volcano.

The communities and local governments around Pinatubo are reminded to always be prepared against the dangers of an earthquake and volcanic eruptions and re-evaluate, prepare and strengthen their contingency, emergency, and other planned disaster preparedness. 

Entering the Pinatubo Crater should be taken with extreme caution and avoided as much as possible, PHIVOLCS said.

Pinatubo volcano on February 26, 2021. Credit: Copernicus EU/Sentinel-2, TW

Pinatubo is the site of the second-largest volcanic eruption of the 20th century. It started on June 15, 1991 (VEI 6) and evolved into by far the largest eruption to affect a densely populated area.

The eruption produced high-speed avalanches of hot ash and gas, giant mudflows, and a cloud of volcanic ash hundreds of miles across. The impacts of the eruption continue to this day, the USGS said.

The following is from USGS Fact Sheet 113-98 -- The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines(USGS)

On July 16, 1990, a magnitude 7.8 earthquake -- comparable in size to the great 1906 San Francisco earthquake -- struck about 100 km (60 miles) northeast of Mount Pinatubo, shaking and squeezing the Earth's crust beneath the volcano.

This earthquake caused a landslide at the volcano, some local earthquakes, and a short-lived increase in steam emissions from a preexisting geothermal area, but otherwise, the volcano seemed to be continuing its 500-year-old slumber undisturbed.

In March and April 1991, however, magma rising toward the surface from more than 32 km (20 miles) beneath Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano.

Thousands of small earthquakes occurred beneath the volcano through April, May, and early June, and many thousand tons of noxious sulfur dioxide gas were also emitted.

First magma reached the surface from June 7 to 12, 1991.

Because it had lost most of the gas contained in it on the way to the surface, the magma oozed out to form a lava dome but did not cause an explosive eruption.

However, on June 12 -- Philippine Independence Day -- millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption.

The volcano exploded in a cataclysmic eruption on June 15, ejecting more than 5 km3 (1 mile3) of material.

The ash cloud from this climactic eruption rose 35 km (22 miles) into the air. At lower altitudes, the ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, and winds at higher altitudes blew the ash southwestward. 

A blanket of volcanic ash and larger pumice lapilli blanketed the countryside. Fine ash fell as far away as the Indian Ocean, and satellites tracked the ash cloud several times around the globe.

Nearly 20 million tons of sulfur dioxide were injected into the stratosphere, and dispersal of this gas cloud around the world caused global temperatures to drop temporarily (1991 through 1993) by about 0.5 °C (1 °F). 

Geological summary

Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km (62 miles) NW of Manila with no records of historical eruptions.

The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5 km (1.5 miles) wide summit caldera whose floor is now covered by a lake.

Caldera formation lowered the height of the summit by more than 300 m (984 feet).

Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities.

Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991. (GVP)

Featured image: Pinatubo on February 26, 2021. Credit: Copernicus EU/Sentinel-2, TW

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