Phivolcs records 2,329 aftershocks with more expected in the coming days, weeks
Since a Sept. 30 earthquake rocked Cebu with a magnitude 6.9 tremor, Phivolcs has recorded 2,329 aftershocks as of 4 a.m. on Oct. 2, with the magnitude of the aftershocks ranging from 1 to 5. More are even expected in the next few days, even weeks.
However, in an interview with DZMM, Phivolcs director Dr. Teresito Bacolcol assured these will taper off both in frequency and intensity.
Caused by an offshore active fault northeast of Cebu island that hasn’t moved for 400 years, the tremors reached as far as Sorsogon, more than 400 km from the epicenter.
Bacolcol explained the wide reach has to do with the depth of the earthquake source.
“Ang seismic energy kasi, as it travels to the surface, humihina. ‘Pag malalim ang pinanggalingan ng lindol, pagdating sa ibabaw, hindi na natin gaanong mararamdaman,” he said.
“In contrast, kung mababaw ang pinanggalingan ng seismic energy, pagdating sa ibabaw, malakas pa rin kasi hindi pa siya masyadong nakakapag-shed ng energy,” Bacolcol continued.
In the Sept. 30 earthquake, the distance between the source of the seismic energy and the surface was only 5 km.
“Kaya mas matindi ang naramdaman ng tao at mas malawak ang lugar kung saan ito naramdaman,” Bacolcol said.
Bacolcol shared Phivolcs considers two scenarios if another earthquake with around the same magnitude were to hit a more densely populated area, such as Metro Manila, which has a population of 13.4 million as of 2020, according to the Philippine Statistics Authority.
“For the west valley fault, one scenario we are looking at, for a magnitude 7.2, casualties would be around 33,000; for a 6.5 magnitude, casualties would be around 23,000. For a 6.9 magnitude, like this one, the casualty number would be between 23,000 and 33,000. ‘Yun ‘yung magiging scenario if we have a 6.9 [magnitude earthquake] in Metro Manila,” Bacolcol said.
He said the quarterly earthquake drills or the "Nationwide Simultaneous Earthquake Drill" organized by the National Disaster Risk Reduction and Management Council served their purpose in Bogo City, judging by circulating videos of people ducking under tables and stopping their vehicles.
“But some did still panic. We don’t recommend that. When you run while the ground is shaking, you can trip and fall, and cause a domino effect. You won’t be a casualty because of falling debris but because of a stampede,” Bacolcol said.