(Second of a series)
The 1992 Torres report
While he was still at Phivolcs, Dr. Ronnie Torres, a foremost expert regarding pyroclastic flows who is now at the University of Hawaii, warned of volcanism and faulting at the site in a 1992 report, “The vulnerability of PNPP site to the hazards of Natib volcano” (Phivolcs Observer, Vol. 8 No. 3: 1-4).
Quoting Dr. Torres: “Natib volcano does not erupt very often but could still erupt.” As a rough rule of thumb, the longer a volcano is in repose, the more time it has to store eruptive energy, and thus, the stronger the eventual eruption.
The Sonido-Umbal 2001 Report to the Subic Bay Metropolitan Authority
Dr. Ernesto Sonido collaborated with Mr. Jesse Umbal to submit in 2000 an exhaustive, 38-page analysis for SBMA of the geology and geohazards of the Subic Bay area. Jess Umbal is one of the brightest, most competent volcanologists and geologists I know. Working with me during the Pinatubo eruption, he earned his Masters degree at the University of Illinois in 1993. Dr. Sonido is not a volcanologist, so we can assume that Umbal wrote those aspects in the report, which adjudged Natib as “potentially active.” The report documented two Natib eruptions that formed large calderas, one with a diameter more than twice as big as that of the new caldera on Mt. Pinatubo.
Sonido and Umbal also studied the system of faults exposed on land in the larger region. They estimated the recurrence period for earthquakes of Magnitude 6.4 to 7.0 at 22 years; of Magnitude 7.0 to 7.3 at 59 years; and of Magnitude 7.3 to 8.2 at 157 years.
The Cabato et al. study
In 1997, Ms. Joan Cabato, Dr. Fernando Siringan and I of the National Institute of Geological Sciences of UP Diliman, collaborating with the Mines and Geosciences Bureau and the National Power Corp., initiated a geophysical study of the marine geology of Subic Bay. The study was supported as “due diligence” hazard evaluation by then SBMA Chairman Richard J. Gordon.
From a slowly moving boat or ship, we gathered 125 kilometers of “seismic reflection” data. That method puts powerful pulses of low-frequency sound into the water. The sound passes down through the water and into the layers of sediment below the sea floor. Some of the sound is reflected back upwards from the different sediment layers, and is collected by hydrophones trailing behind the boat. Much as if we took an X-ray, electronic equipment automatically uses the returned signals to make a detailed picture of the structure underlying the sea, in our case down to a depth of about 120 meters.
After we processed the data and prepared the manuscript, it underwent rigorous scrutiny by our geological peers in the Philippines and abroad, before it was published in the international Journal of Asian Earth Sciences. I am proud to have been part of that effort, which earned a Masters degree for Joan Cabato, a very bright young woman who recently earned her doctorate from the University of Heidelberg in Germany.
Our main finding was that over the last 18,000 years, Subic Bay has experienced faulting roughly every 2,000 years. The last such episode occurred about 3,000 years ago, so the area is overdue for another.
Quite by accident, we discovered a massive deposit of sediment that can only be explained as originating as a large pyroclastic flow from the large Natib caldera, in an eruption that occurred sometime between 11,000 and 18,000 years ago. That date has wrongly been called Natib’s latest eruption. A systematic study of Natib itself could find evidence of even younger eruptions.
(To be continued)