Why not shake intensity maps instead of fault zones?

A MAN walks past the ruins of the viewing deck of the Chocolate Hills in Carmen town in Bohol after the killer 7.2-magnitude earthquake hit the province in October. MARIANNE BERMUDEZ

What did the Bohol earthquake teach the world? Like the 1990 Luzon earthquake, the 7.2-magnitude temblor that shook Bohol province in October has reinforced a school of thought which urges communities to measure the extent by which buildings shake in a given settlement instead of simply building away from fault lines, said

Dr. Arthur Saldivar-Sali, an engineering geologist.

The impact of the Bohol quake was felt kilometers away from the fault, which suggests that land-use development must take into account the theory that a quake’s impact could reach farther away from the fault, Sali told participants in the 60th National Mine Safety and Environment Conference in Baguio City last month.

Saldivar-Sali, president of Geotecnica Corp., said he was commissioned by Bohol officials to study the impact of the Oct. 15 quake that killed more than 150 people.

The quake had been attributed to an undocumented fault system near Inabanga town, which is notable for a wall of rock thrust up three meters by the quake’s energy, he said.

“Common folk and many building engineers believe that the intensity of the temblor’s shock wave diminishes if it gets farther away from the fault line,” he said.

But he said the path of destruction in Bohol worsened in areas that were far from the fault line. This was a phenomenon he also detected in Baguio City, which was devastated by the 7.7-magnitude quake despite the fact that the epicenter was in the area bordering Nueva Ecija and Nueva Vizcaya.

“The distance [of an inhabited area from] an earthquake does not automatically reduce its effect. Distance used to mean decreasing intensity of a quake,” Sali said.

But the Bohol and Baguio quakes suggested that “the energy created by the quake does not emerge from the epicenter or the fault itself,” he said.

He said no technology has been able to predict when a fault system would move, but engineers and geologists can draw up shake intensity maps instead to make sure buildings survive strong earthquakes. These maps determine the shake intensity threshold of areas on which they construct facilities, he said.

Often, engineers design buildings to withstand a given intensity or magnitude of earthquakes, but Salvidar-Sali said the formula should now include surface geology because building owners should also know “what is holding the building in place.”

“If the ground soil is fragile, a building should use materials that make it more rigid,” he said.