824 aftershocks in 24 hours after double Davao quakes
MANILA, Philippines — State seismologists have recorded 824 aftershocks within 24 hours after two strong earthquakes jolted Davao Oriental on Friday, October 10.
PHIVOLCS Director Teresito Bacolcol said the aftershocks were from the twin quakes off the province — a magnitude 7.4 offshore tremor in the morning followed by a 6.8 quake in the early evening — both originated along the Philippine Trench.
“As of 10 a.m., we recorded in Davao 824 aftershocks,” Bacolcol said on dzMM on Saturday, October 11. He noted that the count excludes the 6.8-magnitude event, which is classified as another major earthquake.
The strongest aftershock so far measured magnitude 5.8, while six registered magnitudes above 5, one of which caused Intensity 4 ground shaking in nearby areas.
Bacolcol said the series of tremors were all concentrated in the same region.
“Nasa isang lugar lang sila, hindi na natin ma-distinguish which is which,” he said, referring to the difficulty of separating aftershocks linked to each of the two main quakes. (They were just in one area, so we're unable to distinguish which quake caused which aftershocks.")
Aftershocks, he said, can persist on a diminishing frequency for 10 days.
Movement. Bacolcol also explained that the ground shaking episodes were caused by a reverse fault movement on the trench, in which the hanging wall moves upward in a thrusting motion.
Most of the recent tremors, showed vertical or thrust-type movements, Bacolcol added. This is typical of quakes along subduction zones like the Philippine Trench. Horizontal or sideways motion, he said, is rare in such settings.
Fewer than Cebu's. The Phivolcs chief described the number of aftershocks as lower compared to those logged after the September 30 Bogo City earthquake, where 1,853 aftershocks were recorded in the first 24 hours following the main event.
At least 72 people were killed in Cebu areas in late September, while at least 7 died during the Davao doublet earthquake on October 10.
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