Stronger Mati quakes possible, warns Phivolcs

MANILA — Earthquakes are a very real threat to Davao Oriental province’s coastal city of Mati and its vicinity, including waters bordering the area.

According to experts, local faults can generate earthquakes not only on land but offshore as well.

Last Sept. 24, a magnitude 6.5 offshore quake was felt at varying intensities, not only in Mati but as far as Misamis Oriental, Bukidnon, Sarangani and Agusan del Sur provinces.

Experts expect future earthquakes in waters off Mati and even within the city.

Some of those earthquakes may be stronger than Mati’s 6:53 a.m. offshore tremor last month, they also warned.

“Mati and its waters are in a seismically active location so stronger quakes are possible there,” said science research analyst John Deximo of the state-run Philippine Institute of Volcanology and Seismology (Phivolcs).

He noted that a segment of the Philippine Fault Zone (PFZ) might trigger a “big” earthquake there.

Also capable of triggering a quake is the Philippine Trench east of Mindanao where Mati is located, he said, warning, “Communities concerned must be alert and prepare accordingly.”

According to Phivolcs, PFZ, a 1,200 km.-long major tectonic feature transecting the Philippines from northwestern Luzon to southeastern Mindanao, poses a “high seismic risk”.

“This arc-parallel, left-lateral strike-slip fault is divided into several segments and has been the source of large-magnitude earthquakes in recent years, such as the 1973 Ragay Gulf earthquake (magnitude 7.0), 1990 Luzon earthquake (magnitude 7.7) and 2003 Masbate earthquake (magnitude 6.2),” Phivolcs said.

Experts noted that the Philippine Trench is an underwater depression in one of Earth’s deepest abysses.

Citing historical records, Deximo said a number of earthquakes already rocked part of Mati waters where last month’s temor also struck.

He noted that among such earthquakes was the magnitude 8.3 offshore tremor on April 15, 1924.

“The 1924 rupture happened almost in the same area where the 2016 one occurred,” he said.

Deximo said a still-unnamed local fault there may have triggered such earthquakes.

That fault is most probably a segment of PFZ, he noted.

“Mati and its surroundings lie in a geologically complex area,” he said.

Tremors also occurred in the vicinity of both earthquakes’ location, he continued.

The Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE) described the 1924 earthquake as among “the greatest submarine disturbances originating in the West Pacific”.

Mati was nearest to that 12:22 a.m. earthquake’s epicenter and suffered its most disastrous effects, SEASEE said.

It further said that the earthquake mainly hit Mati’s coast, damaging structures there, aside from producing cracks, fissures and subsidence in hilly and alluvial grounds.

“A tsunami followed, which flooded low coastal shores but without considerable damage,” SEASEE also said in its 1985 catalog.

The earthquake likewise affected Surigao’s coast and Agusan Valley’s northern part.

“Although no loss of life was reported, the material damage was great — cracks and landslips were conspicuous in alluvial ground and in steep surfaces chiefly along water courses,” said SEASEE.

A series of aftershocks already followed last month’s earthquake off Mati.

“We monitored 382 aftershocks from that earthquake since then,” Deximo said, citing data as of Monday morning.

He noted such number of aftershocks is still normal, given the earthquake’s magnitude.

Aftershocks will continue until geologic conditions that triggered the earthquake stabilize, he said.

Phivolcs science research analyst Julius Galdiano agreed, saying he expects aftershocks to still occur over time but at increasingly lesser frequency and magnitude.

“We don’t expect future aftershocks to be as frequent and intense as what these were immediately after the earthquake,” he said.

It’s unclear when the aftershocks will stop, however, he clarified.

Phivolcs data show the earthquake’s strongest aftershock reached magnitude 4.9 and occurred less than an hour after this tremor struck.

In succeeding days, Phivolcs monitored aftershocks of lesser magnitude and frequency. Catherine Teves/