Philippines has hidden quake faults

Metro Manila quake from hidden fault

MANILA, Philippines — The Philippine Institute of Volcanology and Seismology (Phivolcs) warned yesterday there are still many undetected active faults in the country that could trigger deadly earthquakes like the one that hit Central Luzon last week and killed 16 people.

Science Undersecretary and Phivolcs director Renato Solidum Jr. issued the warning as he reiterated the agency’s call for Filipinos to always be prepared for earthquakes.

Phivolcs geologists, he said, are using state-of-the-art technology to search for hidden faults on land and offshore.

“Our active faults mapping is a continuing activity. We are constantly updating our Active Faults Map,” Solidum told The STAR.

Several earthquakes jolted different parts of the country last week, including the magnitude 6.1 temblor in Luzon and the magnitude 6.5 quake in Eastern Samar.

Sixteen people were killed when a supermarket in Porac, Pampanga collapsed after the magnitude 6.1 quake, which originated in Castillejos, Zambales on April 22.

At 11:59 a.m. yesterday, another temblor shook Surigao del Norte. The magnitude 4.7 quake struck 50 kilometers southeast of General Luna in the province. It was felt at Intensity 2 in Surigao City.

Solidum said most of the active faults the agency has mapped are located on land.

“All of our mapped active faults are based on visible evidence, meaning these faults have ground surface manifestations,” the official said. “However, there are also many active faults that pass through the seas, considering that the Philippines is archipelagic. Many of these offshore faults are unmapped.”

Blind faults

The magnitude 6.1 tremor that rocked Luzon, including Metro Manila, on April 22 was generated by a blind or hidden fault, according to Solidum.

“Blind active faults are active faults which have not pierced through the ground surface. They are ‘blind’ faults because although they may have moved in the past, the magnitudes these faults generated were not enough to break into the ground surface,” he said.

A blind fault also triggered the magnitude 7.2 quake that hit Bohol and Cebu in October 2013, killing nearly 200 people.

“In some cases, blind active faults may manifest only during earthquake swarms or only when they generate major earthquakes. Since these faults are buried, its evidence on the surface are limited,” Solidum, a geologist, said.

Cutting-edge technology

Phivolcs is currently working on the project “Mapping of Earthquake-Related Landforms in the Philippines,” which aims to come up with an updated 1:50,000 scale Active Faults maps for all provinces of the Philippines, Solidum said.

The project uses high-resolution satellite images in interpreting landforms that show evidence of active faulting.

In addition, Phivolcs conducts a yearly project that targets mapping of earthquake hazards in a particular province aside from paleoseismic studies or the study of past earthquakes. Geologists estimate the recurrence interval of earthquakes as well as the amount of displacements an active fault may generate during earthquakes.

“Through this activity we can estimate the number of times it generated earthquakes in the past and the number of years in between earthquake events,” Solidum said.

Paleoseismic studies have been conducted on the West Valley Fault and on the Philippine Fault’s Digdig Segment, Masbate Segment, Guinayangan Segment, Leyte Segment, Surigao Segment, Compostela Valley Segment and Mati Segment.

One of the newer technologies Phivolcs is using to monitor active faults is the Global Positioning System or GPS that allows geologists to identify which fault is locked or not moving and which fault is creeping or sliding, said Solidum.

“If the fault is active but is locked, it means that it is storing up energy which would be released in future earthquake events,” he said. “However if the fault is creeping or sliding, it means that the probability of a strong earthquake to happen is quite small as the fault is constantly releasing energy.”

To date, the agency has more than 250 GPS campaign and 100 continuous GPS points all over the country, he added.

In Metro Manila, the agency maintains 20 continuous GPS to monitor the West Valley Fault, which Phivolcs warned is ripe for movement.

The 100-kilometer West Valley Fault is capable of generating a magnitude 7.2 quake, which could kill up to 48,000 people in Metro Manila, Bulacan, Cavite, Rizal, Laguna and Pampanga.

Solidum said the agency has also manufactured its own Unmanned Aerial Vehicles (UAV) or drones.

“Since UAV technology is already available and there are now extensive choices for parts and components in the market, we think that it is more advantageous and cheaper if we build the drones rather than purchasing off the shelf ready-to-use commercial drones,” he said.

He said manufacturing their own drones would ensure low maintenance cost and sustainability of the equipment “since we know how to build, repair and troubleshoot them.”

“The agency is using these drones to assist us in mapping of active faults especially in areas that are not easily accessible. These drones are also useful in our assessment of damage following an earthquake or volcanic eruption,” he added.

As for active faults that pass through the sea, Solidum revealed that Phivolcs map them using geophysical instruments – multi-beam, 2D seismic profiles and acoustic sub-bottom profiles.

The agency also regularly conducts walk-the-fault activities, where state geologists help local government units identify the location of active faults that pass through their communities.

“This activity is useful especially in the preparation of LGUs’ Comprehensive Land Use Plans,” Solidum said.

To make the active fault maps easily accessible to everyone, Phivolcs created the web-based application, FaultFinder. It estimates the distance of one’s location to the nearest active faults.

Solidum earlier called for a manual structural audit of all structures especially those built before 1992 after two powerful tremors hit the country last week.

He noted that buildings constructed after the 1992 National Building Code are more earthquake resistant.

“Engineers considered a change in the design code of buildings to be more resistant to earthquakes in 1992. We use it as a first criteria to prioritize which to inspect. But other buildings constructed after 1992 should also be checked especially after a strong shaking,” Solidum said.