Plate Tectonics and Earthquakes

Earth’s broken outer shell

The rigid outer layer of the Earth — the crust plus the top of the mantle, together called the lithosphere — is not one continuous piece. It is broken into about 15 large tectonic plates and dozens of smaller ones. Each plate is roughly 100 km thick and rides on the hotter, slowly flowing rock of the asthenosphere beneath it.

The plates move, driven mainly by heat escaping from Earth’s interior: hot mantle rock rises, spreads, cools and sinks, and the weight of cold, dense oceanic plate sinking back into the mantle at subduction zones pulls the rest of the plate along behind it. Typical speeds are 2–10 cm per year — about as fast as a fingernail grows (U.S. Geological Survey).

Where plates meet, they push against, pull away from, or grind past one another. That is where stress accumulates, and that is where almost all earthquakes happen. Set the map to “everything up to here” and the plate boundaries draw themselves in dots.

Three kinds of plate boundary

Every plate boundary is one of three types, and each produces a characteristic earthquake pattern.

• Divergent boundaries — plates pull apart and new crust forms in the gap. Most run along the mid-ocean ridges, an underwater mountain chain more than 60,000 km long. Earthquakes here are frequent but shallow and rarely larger than about magnitude 6. On land, divergence forms rift valleys such as the East African Rift.
• Convergent boundaries — plates collide. If one is oceanic it usually subducts (dives) beneath the other; if both are continental, the crust piles up into mountains like the Himalayas. Convergent boundaries produce the largest and deepest earthquakes on Earth.
• Transform boundaries — plates slide horizontally past each other with little crust created or destroyed. The San Andreas Fault in California and the North Anatolian Fault in Turkey are examples. Earthquakes are shallow and can be large (up to roughly magnitude 8), but not as large as the greatest subduction earthquakes.

Subduction zones and the biggest earthquakes

Where an oceanic plate bends and slides beneath another plate, the contact between them is a gently sloping fault called a megathrust. It can be locked by friction for centuries while stress builds, then rupture over hundreds or even more than a thousand kilometres in a single earthquake. Every recorded earthquake of magnitude 9 or greater has been a subduction megathrust: Chile 1960 (M9.5), Alaska 1964 (M9.2), Sumatra 2004 (M9.1), Tōhoku 2011 (M9.1), Kamchatka 1952 (M9.0) (U.S. Geological Survey).

The subducting slab keeps producing earthquakes as it descends. These define an inclined zone of activity — the Wadati–Benioff zone — that can be traced down to about 700 km. On the map, this is why deep (blue and violet) earthquakes sit inland of the ocean trench, on the side the sea floor is sinking toward: behind Japan, the Andes, the Tonga arc, and the Sunda arc.

The Ring of Fire

The rim of the Pacific Ocean is almost entirely subduction zones and transform faults. This belt, the Ring of Fire, is where roughly 90% of the world’s earthquakes and about 75% of its active volcanoes occur (U.S. Geological Survey). It runs from New Zealand up through Tonga, Indonesia, the Philippines and Japan, across to the Aleutians, and down the west coasts of North and South America.

The second great belt, the Alpide belt, carries most of the rest. It stretches from the Azores through the Mediterranean, Turkey, Iran and the Himalayas to Indonesia, where it joins the Ring of Fire. It is the result of Africa, Arabia and India pushing north into Eurasia.

Earthquakes far from any boundary

A small fraction of earthquakes are intraplate — they occur in the stable interior of a plate, sometimes more than 1,000 km from the nearest boundary. They are much rarer, but because they are unexpected and the crust there transmits shaking efficiently, they can be very damaging.

Most intraplate earthquakes happen on ancient faults — old rifts or failed plate boundaries — that are weak enough to slip under the stresses still transmitted through the plate. The 1811–1812 New Madrid earthquakes in the central United States, the 2001 Bhuj (M7.7) earthquake in western India, and the 1989 Ungava earthquake in northern Canada are examples.

How we know all this

Alfred Wegener proposed continental drift in 1912, pointing to the fit of the continents and matching fossils and rock formations across oceans, but he had no mechanism and the idea was rejected for decades. The evidence that settled it came in the 1950s and 1960s:

• Symmetric magnetic “stripes” in the ocean floor on either side of mid-ocean ridges, recording reversals of Earth’s magnetic field as new crust formed and spread (Vine & Matthews, 1963).
• Ocean-floor rock that is youngest at the ridges and progressively older toward the trenches, exactly as sea-floor spreading predicts.
• The global pattern of earthquakes and volcanoes, which outlines the plate boundaries.
• Direct GPS measurement, which today tracks plate motion to within a millimetre per year.