Geohazard mapping and the Guinsaugon landslide

Unlike typhoons and volcanic eruption, there is no telling when geohazards will strike

(Editor’s Note: This story was first published by Newsbreak magazine weeks after the landslide that buried a significant portion of the village of Guinsaugon in St. Bernard, Southern Leyte happened. Its original title in the print version is “Without Warning.”

Rappler is republishing this story under Project Agos to provide a glimpse of the state of geohazard mapping and awareness of geohazards at the time the landslide happened. It must be noted that hazard mapping technology in general and the Philippine government’s geohazard mapping efforts in particular have improved significantly since the disaster struck 9 years ago.)

 

MANILA. Philippines – The earthquake was not that strong. But 30-year-old Lonie Dayula, who was harvesting abaca in his farm, got frightened when he saw boulders the size of houses tremble and move. Then he heard a loud explosion. “It was as if a helicopter crashed.”

He saw huge rocks bounce up and down before rolling down the mountain slope and hurtling at great speed toward houses in his barangay. After that, it was as if the mountain had dissolved right before his eyes.

When Dayula looked at the barangay, houses and other structures were no longer there. He lost his uncle and a sibling and their families to the avalanche. “I alone survived.” Other witnesses said it was all over in just about two minutes.

A whole section of the mountain that overlooked the village of Guinsaugon in St. Bernard, Southern Leyte, had collapsed. The avalanche carried 1.2 billion cubic meters of mud and boulders up to about three kilometers, according to UP geologists. These now cover a wasteland of about 300 hectares.

Because of the speed of text messages and the fact that the landslide was seen by those traversing the national highway, response from the authorities came almost immediately. Petronillo Maraon, 54, a resident of neighboring Sub-angon, recalled seeing a dump truck from St. Bernard town within an hour or two after the incident. Gov. Rosette Yñiguez Lerias was informed of the tragedy less than 15 minutes after it struck. Military troops and rescuers came almost immediately afterward. But in the end, the thick mud defeated rescue efforts.

The island of Leyte is no stranger to disasters and landslides. In the early 1990s, a flashflood killed 4,000 to 5,000 people in Ormoc City. A few months before Guinsaugon, a landslide struck the nearby town of Sogod, rendering some roads impassable up to now.

Along the road to St. Bernard, one still sees scars of previous landslides. Romeles “Teteng” Punio said that minor landslides had hit Guinsaugon almost every year since he was a child.

Bad weather contributed much to the Guinsaugon mudslide. It had been raining almost incessantly for over two weeks; the rainfall was four to five times the normal amount that used to fall on the area. Rainwater seeped through the mountain’s crevices and caused pulverized rocks to dissolve, geologists said.

But the area was geologically hazardous to begin with, said Malyn Tumonong, senior science research specialist at the Mines and Geosciences Bureau (MGB) and spokesperson for the team of geologists the bureau sent to St. Bernard. It goes far, far back in time. Tumonong said rocks that made up the mountain overlooking the village were shattered when the Philippine Fault—which runs from Luzon to Mindanao—cut across Leyte Island around five million years ago. This made the rocks prone to weathering, erosion, and alteration. Movements in the fault zone also caused the rocks to continuously grind against each other, pulverizing them in the process.

Like the landslides that devastated Quezon, Aurora, and Nueva Ecija in late November 2004, the Guinsaugon mudslide was only waiting to happen, Tumonong said. “No amount of tree planting could have prevented it.”

The most that could have been done was to relocate people to minimize the loss of lives and property. Geohazard maps could have guided planning on where to relocate human settlements.

The MGB has been working on such maps for years. Geographic information systems (GIS) technology is helping speed up the process. But lack of funds has hampered the bureau’s work. In a special report commemorating the Cherry Hills (Antipolo) landslide, NEWSBREAK (Aug. 5, 2002) reported that the bureau would need P90 million to map the entire country for geohazards.

It was only recently that geohazard mapping became an urgent concern. Funding for geohazard maps—amounting to P90 million—was only made available in December 2005, Eloisa Legaspi of the MGB’s finance department told NEWSBREAK. Of this amount, P60 million went to the MGB. The rest—P30 million—went to the National Mapping and Resource Information Office (NAMRIA) to update its topographical maps, which the bureau will use as basis for geohazard mapping.

The MGB has initially drawn up indicative maps at a scale of 1:250,000 to get a general idea of where the hazards in the Philippines may be located. This will be used to determine which areas should get priority for geohazard mapping.

But this is just for starters. “You don’t ask people to relocate on the basis of a 1:250,000 map,” said Tumonong. To draw up geohazard maps, the MGB will have to generate more detailed maps of a 1:10,000 scale. Then geologists will have to do extensive groundwork to assess specific hazards and make recommendations.

Even if the maps are in place, the question of whether people will heed the danger signs will still remain. Unlike PHIVOLCS and PAG-ASA, the MGB is not a warning agency, Tumonong pointed out. There are signs, she explained, that could indicate when a typhoon is coming or when a volcano is going to erupt. Geohazards, however, are a totally different matter. “We can tell you where a particular hazard could happen, but we cannot tell you when it will happen.”

Authorities and planners have to race against time. Despite the recent disasters and repeated warnings on existing geohazards, and while MGB people were briefing officials of St. Bernard on their findings, villagers who were evacuated were already begging to go back to their farms. – Rappler.com

Add a comment

Sort by

There are no comments yet. Add your comment to start the conversation.