A magnitude 7.8 earthquake that struck offshore of Mindanao, the second largest island in the Philippines, at 7:37 A.M. local time is the largest earthquake to hit anywhere in the world so far this year, according to U.S. Geological Survey records. But the temblor, which has killed at least 35 individuals and injured scores of people, was actually on the smaller side for the type of fault it occurred at, seismologist Lucy Jones says.
An earthquake’s size is determined by the area of the fault rupture—the larger the area, the larger the quake. The highest magnitudes only happen at plate boundaries, Jones says, “because that’s the only place you have a big enough fault.” For context, there is around one earthquake of magnitude 8.0 or higher somewhere in the world and some dozen in the range of magnitude 7.0 to 7.9 each year on average; this year there have been six of the latter so far.
Subduction zones—places where one plate is diving below another—are where the very largest quakes occur because the fault dips down at a shallow angle, creating a larger area of slip, Jones explains. In comparison, though the strike-slip faults of California can produce a magnitude 8.0 or higher earthquake, the faults there go straight down and quickly hit hotter rocks below, halting the movement and constraining the quake.
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Amanda Montañez
Some subduction zones, such as one off Chile, can create the largest earthquakes ever measured because they have younger rocks that dip at a particularly shallow angle. But in the part of the Philippines where the recent temblor occurred, the rock is older and colder, so the area that slipped wasn’t as large as it might have been.
This area is “a bit of a mess” in that it’s a jumble of pieces of tectonic plate, Jones says, meaning there isn’t as clean of a line of fault to slip, “and therefore you don’t have a big a piece that can move at once.” As a result, you get a somewhat smaller quake. Monday’s earthquake does, however, appear to be among the largest on record for the Cotabato Trench, according to seismologists Judith Hubbard and Kyle Bradley.
There are reports that the Philippines quake caused considerable damage, including to buildings. About one million people experienced severe (intensity VIII) shaking—the third highest ranking on the Modified Mercalli Intensity Scale—and the quake caused a landslide that resulted in most of the related deaths.
And because the fault is a subduction zone and is underwater, it displaces water, which causes tsunamis. In this case, the largest tsunami wave measured was about one meter—considerably smaller than the nearly 40-meter tsunami that was generated by the 2011 Tohoku quake in Japan.