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Eureka! Fault that triggered quake discovered in northern Bohol

October 21, 2013 11:14pm

Tags: Phivolcs
Newfound fault makes Bohol road impassable
Newfound fault makes Bohol road impassable. A remote road in Inabanga, Bohol becomes impassable after the Oct 15 earthquake thrust a portion of the road upwards by as much as 10 feet. The earthquake was triggered by a previously unknown fault stretching several kilometers through northwest Bohol. In the foreground are Anonang Bgy. Captain Felix Caray (in yellow shirt) and GMA News' Howie Severino. GMA News
(A GMA News Exclusive)

Anonang, BOHOL – Right after the October 15 earthquake that laid waste to much of Bohol, residents of this remote northern barangay noticed a long earthen wall that suddenly appeared in their community.

A team of state volcanologists confirmed Monday night that the newly formed natural structure represents a previously unknown fault that triggered the magnitude 7.2 earthquake.
“When we saw it, eureka! This is it,” said Dr. Teresito Bacolcol, the scientist who led the Phivolcs team on Monday to check out sightings of the lengthy earthen wall, which turned out to be the “face” of a fault that currently does not exist on the active fault map of the Philippines.
“We are 100 percent sure that this is the generator [of the earthquake],” said Bacolcol, citing its proximity to the epicenter near the boundary between Sagbayan and Catigbian towns.

The fault had moved six days ago, triggering the terrifying tremor that felled churches, left nearly 200 dead, and rendered scores homeless.
The wall, or the face of the fault, is the edge of a large area of land that had been thrust upwards as high as three meters. It stretched as far as the eyes can see, cutting off roads and numerous trails. The fault line's direction runs northeast to southwest through this barangay, which is located in the municipality of Inabanga.

"Nahati ang Bohol," said Leonila Divinagracia, a barangay kagawad here, observing that a wide swath of the province was now more elevated with a clear demarcation.

Still unnamed, the prominent formation here is unique in the annals of the country's recent geological history. It’s considered a “reverse fault” because of the vertical surface rupture, a sign of the enormous energy packed by last Tuesday's cataclysm. Less powerful quakes cause land to sink rather than rise.
The last great earthquake in the country, in northern Luzon in 1990, was caused by a lateral (side to side) movement of a fault, and did not leave a similarly large mark on the terrain.

Searching for fault line

Before Monday, the fault's location had been a mystery, after the East Bohol Fault near the southern coast had earlier been discounted as the source of the earthquake. "We thought the fault could be offshore or somewhere in the north of the island," said Bacolcol.
Bacolcol and his team of experts had been looking for the fault’s face for days, narrowing their search to the vast “zone of aftershocks” that stretched from the northwest part of Bohol island down to the southern coastal town of Loon, where a centuries-old church was reduced to a heap of rubble.
If they had not found the fault, or if it was a "blind fault" with no surface manifestation, the Phivolcs team would have determined the fault line only by analyzing the pattern of aftershocks, according to a member of the team, adding that “it would have taken a lot longer.”
Following a lead given by this reporter, who had accompanied residents of Inabanga town to the fault on Sunday, Bacolcol and his team spent much of Monday documenting a five-kilometer stretch of the fault, and they were far from finished. They are planning to return on Tuesday to trace the fault wherever it led, which could be the sea.

Historical earthquake data around the globe shows that an earthquake of magnitude 7.2 can create a visible fault line as long as 100 kilometers.
In the meantime, the scientists' visit to Anonang gave a measure of relief to residents who didn’t know what to make of the massive change to their land.

“We have been scared. What if there are more faults to come? Should we leave?” asked Felix Caray, barangay captain of Anonang, one of many communities across Bohol where people are sleeping outdoors out of fear.
Bacolcol’s team assured them that this will be the one and only fault here, and it may be generations before another earthquake occurs. They also advised residents not to build any houses within five meters of the fault line.
Late Monday night at the Phivolcs station in Tagbilaran, Bacolcol and his team were still busy preparing maps, data, and photos of their new discovery for the Phivolcs central office.
“This is the holy grail of active fault mappers,” said Bacolcol, who will submit their findings for a revision of the Philippine fault map. “Naiiyak ang team nang makita nila.” – DVM/YA, GMA News
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