EXPLAINER: The series of earthquakes in Mindanao, and why Cebu can feel the tremors
The building owned and occupied by the state weather bureau Pagasa in Hinatuan sustained damages following the 7.4-magnitude earthquake that struck near the town on December 2, 2023. | Photo from Local Govenment Unit of Hinatuan
CEBU CITY, Philippines – There is roughly a 480-kilometer distance between the island of Cebu and Hinatuan in Surigao del Sur, the town closest to the epicenter of the 7.4-magnitude earthquake that rocked Mindanao on Saturday, December 2.
But residents in Cebu still felt the tremors of the quake.
Even experts from the Philippine Institute of Volcanology and Seismology in Central Visayas (Phivolcs-7) urged Cebuanos not to be complacent.
So, what triggered last December 2’s strong earthquake, and why do people in Cebu still feel the ground shaking?
Movement in the Philippine Trench
The 7.4-magnitude earthquake on Saturday was due to the subduction along the Philippine Trench, Phivolcs said.
In geology, subduction refers to when an oceanic plate runs into a continental plate and slides beneath it.
This movement of rock materials underneath the earth’s surface can also cause stress, said Engr. Robinson Jiorgio, director at Phivolcs-7.
“And if there’s been too much stress, it will rupture and will release a huge amount of energy,” he explained in Cebuano.
Tremors
When measuring the energy of an earthquake, seismologists use magnitude and intensity.
Magnitude refers to the energy released at the focus or center of the quake while intensity means the strength felt and perceived on the surface.
The higher the magnitude and intensity, the more violent an earthquake can be. As a result, even far-flung areas can experience some slight shaking on the ground.
“Remember, this is a 7.4-magnitude earthquake, stronger than the 7.3-magnitude earthquake in Bohol in 2013,” Jiorgio pointed out.
Domino Effect
The major earthquake last Saturday not only resulted in aftershocks but also new strong ones in the area.
Around 3 a.m. on Monday, December 4, a strong, 6.8-magnitude earthquake hit Cagwait town, a few towns north of Hinatuan in Surigao del Sur, in what experts described as a ‘domino effect’.
According to Jiorgio, major earthquakes like the one in Hinatuan can trigger strong, new ones within its vicinity.
“It’s a domino effect. You have to remember the huge energy of the major earthquake as well as its location,” Jiorgio added.
Surigao del Sur, as well as neighboring provinces in Mindanao, are located in one of the seismically active regions in the Philippines.
“The presence of active faults and trenches, such as the Philippine Fault and its segments, as well as the Philippine Trench, plays a significant role in generating seismic events,” said Phivolcs.
Local fault lines nearby also have the capability to generate minor to strong earthquakes.
How long?
As of 2 p.m. on Monday, or barely 48 hours since the major earthquake occurred on Saturday night, Phivolcs has recorded over 1,600 aftershocks.
The strongest measured 6.2-magnitude on the Richter scale.
Due to these aftershocks, netizens, particularly those residing in areas near the epicenter, reported getting dizzy and frightened more often.
And it looks like they might need to brace for the sudden movement of the ground in the coming weeks or months.
The aftershocks following the 7.4-magnitude earthquake may persist from several days to months. They may also be felt in nearby provinces, Phivolcs added.
For comparison, the aftershocks generated by the 7.3-magnitude earthquake that hit Bohol and Cebu in 2013 lasted for six months, said Jiorgio.
“Considering the earthquake in Hinatuan was stronger than the one in Bohol, the duration of aftershocks may last longer than six months,” he added.
Meanwhile, Phivolcs, including its counterpart in Central Visayas, continue to monitor the seismic activities in Mindanao.
“We’re collecting data to form analysis that will be helpful in the future,” Jiorgio said.
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