It Shakes

Earth's surface is broken into plates that move a few centimeters a year. Almost every earthquake happens at their edges, and the largest ones at one kind of edge: the trenches where one plate sinks beneath another.

Around the Pacific Ocean those trenches form an almost continuous ring about 40,000 km long, from Chile to New Zealand by way of Alaska and Japan: the Ring of Fire. According to the USGS, about 90% of the world's earthquakes and 81% of the largest ones happen there.

The map shows the 9,121 earthquakes of magnitude 6 or more recorded since 1960. 69% happened less than 300 km from a subduction trench, and those quakes add up to 91% of the energy released. The boxes are the windows of the four reels.

The ocean floor is born at mid-ocean ridges, then cools and grows denser as it ages. When it meets a lighter continent, it slides underneath: that is subduction. The trench marks where it starts to sink.

The plate doesn't slide smoothly. At the contact it locks and builds up stress for decades or centuries, until it slips all at once: these are interplate earthquakes, shallow, and they include the largest ones. Further down, the plate keeps breaking inside as it sinks.

When the plate reaches about 100 km deep, it releases the water it carries; that water lowers the melting point of the mantle above, which partly melts, and the magma rises: that is how the volcanoes of the Andes, Mexico and Japan are born.

Seen from the side, earthquakes aren't scattered at random: they form a slanted band that starts at the trench and dips toward the continent. It is called the Wadati-Benioff zone, after Kiyoo Wadati, who showed in 1928 that there were earthquakes more than 300 km beneath Japan and traced the slanted band in 1935, and Hugo Benioff, who followed it around the Pacific in 1949 and 1954.

These are the cross-sections of the four reels, at the same scale and with no vertical exaggeration: a kilometer across is as long as a kilometer down. Each quake is measured from the front of shallow quakes along its stretch of trench. Hover over a dot to see its magnitude, depth and year.

What changes most from one country to another is how cold the plate arrives. An old plate has been cooling for longer, and if it also sinks fast it has no time to warm up: it stays rigid and keeps breaking hundreds of kilometers down. A young plate arrives warm, heats up quickly and stops shaking sooner. Since 1979 we have known that the length of the seismic zone grows with the plate's age multiplied by its speed.

The Pacific Plate sinking beneath Japan is about 130 million years old, the fastest of the four, and shakes down to more than 600 km. The Cocos Plate is 10 to 25 million years old: no quake in the catalog goes deeper than 274 km, and under central Mexico there are hardly any intraplate quakes below 150 km.

The shape changes too. Under central Mexico, under Peru and between 27° and 33° S (under north-central Chile and, above all, Argentina), the plate runs almost flat for hundreds of kilometers before sinking further: this is flat-slab subduction. In South America it lines up with undersea relief reaching the trench: the Nazca Ridge and the Inca Plateau off Peru, and the Juan Fernández Ridge off Chile.

Magnitude is logarithmic: each extra point means about 32 times more energy, and two points about a thousand. That is why a handful of quakes hold almost all the energy in a catalog.

Valdivia 1960, the largest earthquake ever recorded by instruments, released about 85% of the energy of all 4,326 Chilean quakes since 1960 on its own, and Tōhoku 2011 (magnitude 9.1), 60% of Japan's. In Peru and Mexico the energy is more spread out: their largest quakes since 1960 were magnitude 8.4 and 8.2.

Each curve climbs in steps: years of calm and sudden jumps. When the next step comes can't be known from this data or any known method: the USGS states that no one has ever predicted a major earthquake.