Explainer: How Davao del Sur’s M6.9 quake may be linked to October temblors

Kristine Sabillo, ABS-CBN News

Posted at Dec 16 2019 10:34 PM

While another fault has been identified as most likely responsible for the 6.9-magnitude earthquake that rocked Mindanao on Sunday, the Philippine Institute of Volcanology and Seismology (Phivolcs) believes it might be connected to the Cotabato Fault System that caused a series of strong quakes in October. 

At 2:11 p.m. on Sunday, a 6.9-magnitude quake, with a depth of three kilometers, hit Matanao town in Davao del Sur. It was felt throughout the province and nearby areas such as Digos City, Kidapawan City, General Santos City, and Sultan Kudarat. At least four people were killed and dozens were injured.

Ishmael Narag, officer-in-charge of Phivolcs’ Seismological Observation and Earthquake Prediction Division, said while the epicenter of the recent 6.9-magnitude quake is nearer the Tangbulan Fault, it is also near the Cotabato Fault System, which caused the series of major earthquakes (magnitude 6.3 and up) in Cotabato and surrounding areas last October.

“It appears whatever forces acting on these faults is also the same with the one in Tangbulan fault,” Narag told ABS-CBN on Monday.

Phivolcs defines an earthquake as “a weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface.” Such movements occur along plate boundaries or fault lines, which are fractures between two blocks of rock. When the rocks move against or apart from each other, an earthquake occurs. A fault system is a group of inter-related faults. 

In an earlier ABS-CBN News article, Phivolcs explained how faults can trigger each other because of “stress transfer.” 

Narag said the 69.2-kilometer Tangbulan Fault was initially identified as separate from that of the Makilala-Malungon Fault and others that are classified under the Cotabato Fault system. These include the M’lang Fault, North Columbio Fault, South Columbio Fault, and the Balabag Fault.

The last 6.9-magnitude earthquake is believed to have been generated by the Tangbulan Fault. Phivolcs is now investigating if it is part of the larger Cotabato Fault System. Graphic by Pam Ramos, ABS-CBN News. Source: Phivolcs.

However, the epicenter of the recent quake and aftershocks are not just in the area of the Tangbulan Fault but also in the equally extensive Makilala-Malungan Fault.

“We are deliberating on whether this (Tangbulan Fault) is part of the Cotabato fault system,” Narag said.

He said it will still have to go through a process and several assessments before they are able to confirm this. However, he said he personally believes this is highly possible.

“I would like to think they are interrelated. It could have been stress transfer,” Narag said when asked if the October quakes could have triggered the recent earthquake in Davao del Sur.

He said among the data they need are satellite images of the area and actual observations of tension cracks and landslides. He said the before and after images will show the direction of the movement of the ground.

Narag explained that it is not surprising that there is a lot of movement in the Mindanao area because the island is in between the Philippine Trench and the Cotabato Trench. He said that while the force coming from the trenches are strong, it is distributed or partitioned with most of the force directed towards the trench’s subduction or the sinking of one tectonic plate under another. Only a small amount of that force is transferred to the land mass.

Still, faults such as Tangbulan and Makilala-Malungan can generate an earthquake that is 7.2 in magnitude.

“We’re not discounting the possibility that it might have another major event,” Narag said. “We had a magnitude 6.9 which is not yet the maximum.”

However, he said the possibility is not as high, especially since the recent quakes in the Davao del Sur area are only occurring in the northern part of the Tangbulan Fault.

“Faults move in segments,” he explained, adding that there are areas along the fault line that resist movement. He said a 7.2-magnitude can only occur if the whole fault moves.

With the possibility of the recent quake being connected to the Cotabato Fault System, Phivolcs is advising the public to be ready for another earthquake by having their houses and buildings checked by engineers.