The Philippines earthquake is the largest this year, but it could’ve been bigger—here’s why

The Philippines earthquake is massive, but it could’ve been bigger—here’s why

The magnitude 7.8 earthquake that hit the Philippines happened at a subduction zone. Such places are capable of producing the largest earthquakes possible

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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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 .