MANILA — The magnitude 5.4 earthquake off the coast of Maburao, Occidental Mindoro on Friday morning rattled not only parts of Mindoro and the Calabarzon region but was also felt in Metro Manila.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) reported Intensity III shaking in parts of Quezon City, while employees of the Supreme Court in Manila briefly evacuated as a precaution after the tremor.
PHIVOLCS said the earthquake originated along the Manila Trench, a massive underwater trench located west of Luzon and Mindoro.
It said the quake was caused by subduction—the movement of one tectonic plate beneath another.
Unlike the more frequently discussed West Valley Fault, the Manila Trench lies beneath the ocean floor.
Although less talked about, it is also capable of generating strong earthquakes that can be felt across a wide area, including Metro Manila.
"The Manila Trench is another active earthquake source. Even if it's offshore, earthquakes generated there can still be felt in Metro Manila depending on their magnitude and depth,” PHIVOLCS Senior Science Researcher Bhenz Rodriguez told ABS-CBN News.
MANILA TRENCH VERSUS THE WEST VALLEY FAULT
Major earthquakes often revive public concern about the so-called "Big One"—the long-anticipated movement of the West Valley Fault, which scientists say could produce a magnitude 7.2 earthquake capable of causing widespread destruction in Metro Manila and nearby provinces.
However, PHIVOLCS emphasized that Friday's earthquake was not related to the West Valley Fault.
The two earthquake sources are different.
The West Valley Fault is an active strike-slip fault that cuts through several cities in Metro Manila and nearby provinces. Located only about 10 kilometers east of Manila, it runs on land, meaning earthquakes generated by it are relatively close to highly populated communities.
The Manila Trench, on the other hand, is a subduction zone beneath the West Philippine Sea where one tectonic plate dives beneath another. Earthquakes generated by subduction zones can occur offshore and may be felt over greater distances.
Rodriguez said the Manila Trench regularly releases energy through earthquakes, making it different from faults that remain locked for long periods before rupturing.
"Based naman dun sa seismicity nung area o dun sa mga nangyari dun, so far, nagkakaroon naman siya lagi ng paggalaw. So hindi natin masabi na nahihinog kasi lagi naman siyang gumagalaw," Rodriguez said.
Unlike the West Valley Fault, movement along the Manila Trench also has the potential to generate tsunamis, especially during much stronger offshore earthquakes that significantly displace the seafloor.
"Sa West Valley Fault, since nasa lupa, wala tayong ina-expect tsunami," Rodriguez explained.
NO MAJOR DAMAGE REPORTED
As of Friday afternoon, PHIVOLCS had recorded three aftershocks. No injuries or significant structural damage have been reported.
Authorities nonetheless continue to remind the public to remain prepared for aftershocks and to practice earthquake safety measures like the "duck, cover and hold" and move to open areas only after the shaking has stopped.