The Philippines earthquake is massive, but it could have been bigger: here’s why

A magnitude 7.8 earthquake that struck off the coast of Mindanao Island in the Philippines at 7:37 a.m. local time is the largest earthquake to strike the world so far this year, according to US Geological Survey records. But the temblor, which killed at least 35 people and injured dozens, was actually smaller for the type of fault at which it occurred, says seismologist Lucy Jones.

The size of an earthquake is determined by the fault rupture zone: the larger the zone, the larger the earthquake. The largest magnitudes only occur at plate boundaries, Jones says, “because that’s the only place where there is a large enough fault.” As a reminder, there is about one magnitude 8 earthquake somewhere in the world and about 12 magnitude 7 earthquakes every year on average; this year there have been six magnitude 7s so far.

Subduction zones – places where one plate dips beneath another – are where the largest earthquakes occur because the fault dips at a shallow angle, creating a larger area of ​​slip, Jones explains. In comparison, although California’s strike-slip faults can produce a magnitude 8 earthquake, the faults there run straight down and quickly strike the hotter rocks below, stopping the movement and limiting the earthquake.

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Some subduction zones, like the one in Chile, can create the largest earthquakes ever measured because they contain younger rocks that dip at a particularly shallow angle. But in this part of the Philippines, the rock is older and colder, so the area that slipped wasn’t as big as it could have been.

This area is “a little messy” in that it’s a jumble of pieces of tectonic plate, Jones says, meaning there’s not as clear a fault line to slide “and so you don’t have a big piece that can move at the same time.” As a result, you get a slightly weaker earthquake. Monday’s quake, however, appears to be the largest earthquake ever recorded in the Cotabato Trench, according to seismologists Judith Hubbard and Kyle Bradley.

Reports indicate extensive damage caused by the earthquake in the Philippines, particularly to buildings. About 1 million people suffered severe shaking, the second largest quake, and the quake triggered a landslide that caused most of the deaths.

And because the fault is a subduction zone and it’s underwater, it moves the 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 earthquake in Japan.