In ancient Greece, earthquakes were thought to occur when Poseidon, the god of the sea, got angry and banged his trident on the ocean floor. Modern seismologists have improved considerably on that theory.
Building on the research of German meteorologist Alfred Wegener, who in 1912 postulated that the continents are drifting apart, scientists developed the now widely accepted theory of plate tectonics. Its basic premise holds that the Earth's crust and upper mantle, or lithosphere, is made up of gigantic plates that are like rocky pieces of a global jigsaw puzzle. The plates slide over a semisolid, and relatively slippery, layer called the asthenosphere. The plates meet along faults, or areas of weakness in the Earth's crust.
Earthquakes occur when tectonic plates move in one of three main ways: They push into each other, pull away from each other or slide sideways against each other. Plate tectonic motion occurs slowly and continuously (typically rates are an inch or a few inches per year) due to convection deep within the Earth. However, the faults that define the plate boundaries are often “locked,” building up massive strain energy that can be suddenly released in a large earthquake.
When tectonic plates collide, several things may happen. If both plates carry continents, they may create mountains as the Earth's crust folds and pushes upward. That process formed the Himalayas and Alps, scientists say.
Another type of collision — subduction — produces the biggest earthquakes. In subduction, a dense oceanic plate slides underneath a lighter continental plate. That phenomenon caused this year's massive quake in Chile, as the oceanic Nazca plate slid below the South American plate, lifting a big section of seafloor.
Subduction zones also produce most of the world's active volcanoes, many of which are found along the so-called “Ring of Fire” — a band rimming the Pacific plate from the west coast of North and South America to Indonesia and beyond; the catastrophic 2004 Indian Ocean quake and tsunami were born in this subduction zone.
“We have subduction zones … off of our Pacific Northwest, starting in Northern California, all the way up to British Columbia, and also off of Alaska, and indeed, our territories in the Caribbean as well,” David Applegate, senior science adviser for earthquake and geologic hazards at the U.S. Geological Survey (USGS), told the PBS “NewsHour.” “Back in 1700, the Pacific Northwest had a magnitude 9 earthquake, a very similar sort of subduction-zone quake.” Another example of a subduction quake, he noted, was a magnitude 9.2 quake in Alaska in 1964, among the biggest on record.
When tectonic plates move apart, the motion generates moderate-size quakes and reshapes parts of the ocean floor. Molten rock moves up where the gap between plate boundaries occurs, gradually flowing outward from a central ridge. Such ridges usually remain below the ocean's surface but may emerge above sea level. The Mid-Atlantic Ridge, which divides almost the entire Atlantic Ocean from north to south, forms the island of Iceland.
Tectonic plates may grind horizontally against each other along what is known as a strike-slip fault, as occurred in this year's magnitude 7.0 earthquake in Haiti. In that case, the Caribbean plate slid westward relative to the North American plate along a fault just south of Port-au-Prince. Another fault is California's San Andreas, where the Pacific plate grinds northwestward relative to the North American plate.
Strike-slip faults can produce violent shaking as pent-up energy along the fault line is suddenly released.
When a large quake causes an offset of the ocean floor — such as occurred in the subduction zone earthquakes in Sumatra, Chile, Alaska and in Cascadia in 1700 — it can generate huge waves, or tsunamis, that can traverse the ocean at the speed of a jetliner. By the time they reach land, tsunami waves can be as tall as a five-story building.
An earthquake's magnitude is based on the range of its seismic shock waves and the energy released. A magnitude 9 quake, for example, is roughly 1,000 times more powerful than a magnitude 7 quake. Still, a quake's intensity, or extent of ground shaking, and the damage it does can depend on a variety of factors: for instance, the quake's proximity to built-up areas and the relative softness of the soil near the surface (soft ground, such as that of valleys and basins, amplifies shaking).
When a big quake does occur, it offers no reassurance that another won't follow in the same area. “Earthquakes tend to reoccur along faults,” the USGS notes. “Even if a fault zone has recently experienced an earthquake …, there is no guarantee that all the stress has been relieved. Another earthquake could still occur. Furthermore, relieving stress along one part of the fault may increase stress in another part.”
— Thomas J. Billitteri