Monday, June 8, 2015

Kapampangans need not worry about the West Valley Fault

Kapampangans need not worry about the West Valley Fault in faraway Marikina—they have enough local faults to worry about!
Photo 1: A pre-1991 satellite photo showing the network of local faults radiating from Pinatubo, including the Sacobia Lineament and the Maraunot Fault, both of which have become the path of similarly named rivers.

There’s the Sacobia Lineament, a fault line that has become the Sacobia river valley, which intersects with the Maraunot Fault exactly where Pinatubo stands. The Maraunot Fault is a fracture on the earth’s crust which has become the Maraunot river valley on the Zambales side. (Hundred thousand years ago, a huge volcano, ten times larger than the present Pinatubo, erupted and formed a caldera lake on the exact same spot where the present Pinatubo crater lake is. That ancient crater collapsed and drained the lake towards China Sea—a doomsday scenario that nearly happened again in 2002).

And then there’s the Lubao Lineament, a straight linear fault line cutting across Lubao town and pointing directly towards Mount Natib in Bataan, a volcano with a history of Plinian eruptions, similar to Pinatubo’s explosive type of eruptions (in fact, it has a caldera type of crater similar to Pinatubo’s). The discovery of crisscrossing fault lines in the area is one of the reasons the Bataan Nuclear Power Plant (BNPP) project was scrapped (in fact, Napot Point, the exact spot where the nuclear plant stands, is an old pyroclastic flow deposit from an eruption of Mount Natib 27,000 years ago).

Photo 2: A satellite photo showing the straight linear fault line called the Lubao Lineament connected to the sleeping volcano Mount Natib, on whose foothills the BNPP was built.
Pinatubo itself is surrounded by a network of local faults and sub-faults, e.g. the Tayawan Fault which intersects with the Dangey Fault on the volcano’s northern slope. The Great Luzon Quake on July 16, 1990 at 4:26 P.M. rattled these local faults so that two hours later, at 6:10 P.M., an earthquake with a magnitude of 4.8 struck the Pinatubo area, and one year later, on June 15, 1991, Mount Pinatubo erupted.

Written by Robby Tantingco
Photos courtesy of Phivolcs and USGS

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