Successive Abra quakes due to ‘stress transfer’–DOST chief

BAGUIO CITY—The magnitude 7 earthquake that struck Abra province on July 27 may have disturbed adjacent fault lines that triggered the magnitude 6.4 temblor on Oct. 25, or within a span of three months, Science Secretary Renato Solidum Jr. said here this week.

In both incidents, the epicenters were located in Abra towns. The province will be with aftershocks that may last for weeks, months or even years but with lower intensities, Solidum said at a briefing after he launched a Philippine Science High School athletic competition in Baguio.“The period between earthquakes is often wide apart and would not be too damaging,” said Solidum, who served as director of the Philippine Institute of Volcanology and Seismology (Phivolcs) from 2003 to 2017.But Abra and the Ilocos region are filled with faults “that have neighbors,” which may explain why the northern Cordillera province and many Ilocos towns suffered a succession of earthquakes, he said.

The Abra River Fault was believed to have caused the July quake, with its epicenter traced to the town of Tayum, according to Phivolcs.

Solidum said the Oct. 25 quake might have been triggered by a fault that branches out from the Abra River Fault, this time with its epicenter at Lagayan town, also in Abra.

The Abra River Fault built up underground stresses that were transferred to other faults, he said.Calling this phenomenon “stress transfer,” Solidum said the path taken by the recent quakes had led northward.

But Solidum noted that fault lines that moved were no longer considered risks in the future. “The Abra River Fault has already moved. What has not moved is its southern segment,” he said.

Recovery

But Abra, as well as Mountain Province, could begin recovery efforts by rebuilding damaged houses or constructing new homes that are quake-resistant, Solidum said.

Authorities have yet to estimate the cost of recovery from the Oct. 25 quake, but National Economic and Development Authority Cordillera director Susan Sumbeling in September said Abra would require P4.5 billion to rebuild from the July catastrophe, while Mountain Province needed P1.5 billion.Unlike other areas, the faults traversing Abra and the Ilocos region have adjoining faults, Solidum explained.

“The Abra River Fault has at least two branches. It is also near faults linked to the West Ilocos Fault System, as well as faults leading to the mountains,” he said.He added: “When the ground moves, the buildup of stress transfers to a fault line that has not moved. If you observe the July 27 and Oct. 25 quakes, the epicenter of the recent quake moved northward.”

Since stress tends to migrate, more quakes are likely in the future, he warned, adding that structures equipped to withstand the strongest known intensities of earthquakes would save lives.

“There is time to reinforce your homes,” Solidum said. —Vincent Cabreza INQ

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