scientific discoveries

[OPINION] Beneath the waters of Subic Bay an old pyroclastic-flow deposit, and many faults

Kelvin S. Rodolfo

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[OPINION] Beneath the waters of Subic Bay an old pyroclastic-flow deposit, and many faults

Graphic by Alejandro Edoria

'[Tsunamis] and seiches are genuine threats to the [Bataan Nuclear Power Plant]'

The following is the fourth in a series of excerpts from Kelvin Rodolfo’s ongoing book project “Tilting at the Monster of Morong: Forays Against the Bataan Nuclear Power Plant and Global Nuclear Energy.

My research path has taken some strange karmic turns. It led our tropa of Phivolcs geologists to study the small lahars of Mt. Mayon in the 1980s, almost as if we were being prepared for our life-saving work in the 1990s on the catastrophic lahars of Pinatubo. Afterwards, it involved my colleague Fernando Siringan, his students, and me in geological research to either side of Mt. Natib, first in Subic Bay to the west, then in the coastal plains of Pampanga and Bulacan east of Natib. 

This was years before the attempts to activate the BNPP in 2009, before we realized that our work had serious hazard implications for the plant. I will describe the Subic Bay research first, the work in Pampanga in a later Foray.

In 1997, Joan Cabato studied the submarine geology of Subic Bay for her Masters thesis. Her co-advisors were Ando Siringan and I. Chairman Richard J. Gordon, who was Chairman of the Subic Bay Metropolitan Authority at the time, supported our work as “due diligence” evaluation of the natural hazards that threaten the SBMA. Joan is a bright and very capable scientist who went on to earn her doctorate from the University of Heidelberg in Germany. This was the work that Mark Cojuangco cited in his congressional bill meant to activate the BNPP.   

We borrowed “seismic reflection” equipment from the Mines and Geosciences Bureau and the National Power Corporation and crisscrossed Subic Bay to gather 125 kilometers of underwater geologic data.  

Far too often, science is “mumbo jumbo” for non-scientists.  Let’s describe seismic reflection in understandable terms. 

Seismic-reflection profiling

The method is like taking x-rays of what lies beneath the sea floor. But instead of x-rays, we use strong sound waves from an explosion-maker trailing behind the boat. We use low-frequency sound because it penetrates better. Think of the last time that a party several apartments away kept you awake: it was the drums and bass notes that came through all those walls, not the high notes.  

Slowly moving along, the survey boat makes large bubbles behind it once or twice a minute with either a compressed-air gun, or an electrical “sparker” that vaporizes seawater. Each bubble claps shut explosively, making powerful, low-frequency sound. The sound goes down through the water to the sea floor, down through the mud and into the underlying rock layers. Some of the sound bounces off the sea floor, some bounces off the bottom of the mud, and some bounces off each of several underlying rock layers. All these small echoes pass back up through the water to a string of waterproof microphones – “hydrophones” – trailing behind the boat. Every tiny echo is captured electronically and printed on slowly unrolling paper, continually extending a vertical profile of the rocks beneath the sea floor as the boat moves along.

It takes time for each bit of sound to travel down to a rock layer and bounce back up to the hydrophones. The deeper it penetrates, the more time this takes. It is this ”two-way travel time” that measures and defines the depths of all the features in the recorded profile. We used explosions that were strong enough to penetrate about 120 meters into the sea floor. 

The old pyroclastic-flow deposit under Subic Bay 

In this illustration the red lines are underwater faults under Subic Bay  revealed by the profiling. The “U” and “D” at each fault indicate which side moved up or down relative to each other.

One of our seismic-reflection lines in southeastern Subic Bay, colored orange in the right panel, crossed what can only be the deposit of a large, ancient pyroclastic flow from Mt. Natib, shown by the large orange arrow. 

Its valley is two kilometers wide and more than 40 meters thick. Our ability to estimate the volume of this mass is limited; we would have to do more seismic profiling to be more precise.  

But a conservative estimate, judging from its thickness and width and that it extends offshore at least 2.5 kilometers, would be at least 200 million cubic meters – about a fifth of a cubic kilometer.

The Subic Bay faults

Cojuangco also ignored our most significant findings: the faults underlying Subic Bay, which are colored green in the profile below. In the past, faults broke the bay floor roughly every 2,000 years, the last time about 3,000 years ago. Faulting does not occur like clockwork, so we really can’t say we are overdue for the next one. But there is certainly no reason to assume that the faulting is over. 

When one of these faults was active, the rocks on one side of it moved vertically up or down as much as 5 meters relative to the rocks on the other side. The faulting may have also involved horizontal movements, but our data cannot tell us if this happened, and if so, how much. 

Neither can we say whether or not the faulting was rapid enough to generate tsunamis or bay “seiches,” what we call standing waves that slop back and forth several times between the sides of the bay during an earthquake, but tsunamis and seiches are genuine threats to the BNPP.

What’s next?

We turn next to the active Lubao Fault that runs right through Napot Point. It should, of course, be the kiss of death for BNPP. But in our country the wishes of the wealthy and well-connected drown out honest science, and needlessly imperil our people. Our next foray will examine the propaganda. It was the most painful one to write. –

Previous pieces from Tilting at the Monster of Morong:

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