why the – A magnitude-7.8 earthquake struck offshore of Mindanao at 7:37 A.M. local time, killing at least 35 people and injuring scores. Though it ranks among the largest quakes recorded worldwide this year so far, seismologists say its fault system—older, colder rocks
At 7:37 A.M. local time. the offshore region of the Philippines island of Mindanao began shaking with a magnitude-7.8 earthquake—so large it stands as the largest quake to hit anywhere in the world so far this year. according to U.S. Geological Survey records. It was also. for many families. brutally close to the surface of daily life: at least 35 people have been killed. and scores have been injured.
Seismologists say the quake’s scale is both impressive and—when you look at the fault—strangely constrained. The earthquake was on the smaller side for the type of fault it occurred at, according to seismologist Lucy Jones. Earthquake size, she explained, is determined by the area of fault rupture: larger rupture area means a larger quake. The largest magnitudes typically form at plate boundaries. because that’s where there’s a big enough fault for the rupture to run.
There’s a useful yardstick for how unusual a quake like this can be. Around one magnitude-8 earthquake occurs somewhere in the world, and some 12 magnitude-7s happen each year on average. So far this year, there have been six magnitude-7s.
What made this event land in a middle ground comes down to subduction zones—the tectonic settings where one plate dives beneath another. Jones said these zones are where the very largest quakes occur because the fault dips down at a shallow angle. creating a larger area of slip. By comparison. California’s strike-slip faults can produce a magnitude-8 earthquake. but the faults there go straight down and quickly reach hotter rocks. which halt the movement and constrain the quake.
In the Philippines. the subduction setting is real—but the details of the rocks and the geometry aren’t as favorable for a giant rupture. Some subduction zones. like one off Chile. can generate the largest earthquakes ever measured because the rocks dip at a particularly shallow angle and are younger. In this part of the Philippines. the rock is older and colder. Jones said. so the slipped area wasn’t as large as it might have been.
The fault system itself adds another complication. Jones described the region as “a bit of a mess,” a jumble of pieces of tectonic plate. With no clean line to slip all at once, there isn’t “as big a piece that can move at once,” she said—so the earthquake ends up somewhat smaller.
Even with those constraints, the Monday quake still appears to have a local historical distinction. Seismologists Judith Hubbard and Kyle Bradley said it is the largest earthquake on record for the Cotabato Trench.
For residents, the story was never about theoretical fault areas—it was about shaking, damage, and the ground giving way. Reports of considerable damage include damage to buildings. About 1 million people experienced severe shaking, which was ranked second highest in severity. The quake also triggered a landslide, and that landslide caused most of the related deaths.
Then there was the ocean. Because the fault is a subduction zone and it’s underwater, the earthquake displaces water, which causes a tsunami. In this case. the largest tsunami wave measured was about 1 meter—considerably smaller than the nearly 40-meter tsunami generated by the 2011 Tohoku quake in Japan.
The rupture may have been limited compared with the biggest subduction events. but the impact still stretched across lives in Mindanao—proof that even a “somewhat smaller” quake for its fault type can still be massive enough to hurt communities. topple structures. and send water surging where people live along the coast.
Philippines earthquake Mindanao magnitude 7.8 Cotabato Trench subduction zone tsunami landslide Lucy Jones seismic rupture