On April 2, 1991, a hydrothermal explosion at Pinatubo’s crater interrupted its 450 years of slumber. Rumbling sounds were heard as steam clouds and a small amount of ash were ejected from several active vents, shooting them to heights ranging from 500 to 800 meters.

Quakes (or Seismicity)
Three days later, in a hastily installed temporary seismic station at Sitio Yamut, scientists recorded 223 high frequency volcanic quakes (HFVQ) during its first 24 hours of operation. Then, for two months from April 6 to June 6, the seismograph’s daily count varied from a minimum of 26 to a maximum of 178 incidents of HFVQs. The occurrence of HFVQs indicate fracturing of rocks and movement of fault structures due to the pressure exerted by intruding magma and escaping steam.

Then on June 7, daily incidents of HFVQs leaped to a daily average of 1,500 to 2,000. At the same time, low frequency volcanic quakes started to manifest indicating that magma was nearing the surface. From then on, the earthquakes became shallower and for the first time on June 9, seismographs record harmonic tremors indicating intensified seismic activities. On June 12, more harmonic tremors were recorded which continued during the volcano’s eruption.

Ash Ejections
It was 7:30 in the morning of June 3, 1991, when Pinatubo Volcano first started ejecting ash which lasted for about 30 minutes. The following day, more incidents of ash ejections, although at short intervals, were observed. After a few days of quiescence, Pinatubo’s seismic activities intensified as scientists recorded an explosion type earthquake at around 3:25 pm on June 8.

Dome Formation
An aerial survey conducted by geologists around the volcano established that magma intrusion was very near the surface as evidenced by a dome forming near its summit. The dome was formed by sticky lava that reached the surface.

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