If you’ve ever felt the earth moving under your feet or seen the light fixtures overhead swaying without any wind, you’ve experienced an earthquake. Earthquakes are actually shock waves transmitted through the ground, which we sense as movement or shaking. In that way they’re much like sound waves. We’ve all felt the vibration of the neighbor’s subwoofer when the sound system is cranked to 10, right? The shock waves of an earthquake are much more powerful, and there’s nobody twisting a dial to make them happen. So, you ask, what does cause earthquakes?
To begin at the beginning, we need to talk about what earth scientists call plate tectonics. According to the scientists, the earth has several layers and the outermost layer of the earth –- called the crust –- is made up of rigid, continent-sized plates that float on the next layer down, the mantle. They float because they’re cool, rigid, and relatively light; while the mantle is hot, dense, and kind of gooey. To make things more fun, those crust plates are all moving. They move very slowly, maybe a few millimeters each year, but they move. Since they’ve been moving for millions of years, even a few millimeters a year adds up!
Where those plates bump into each other, several things can happen: one can slip under the other, one can skid along the edge of the other, or both can bang together like two SUVs in a head-on collision. Some of those plates are breaking in half, right down the middle. Where all this action is taking place, friction keeps the two plates from moving smoothly along the boundary. Instead of just gliding along, the plates stay “locked” together at their edges while forces build up for decades or centuries. Then, suddenly, there’s a LURCH! and all that pent-up force is released – as an earthquake.
The boundaries between plates aren’t a giant cracks on the surface. Instead, the plate boundaries are kind of mushy, made up of networks of smaller cracks. Geologists call these cracks faults. Most people know the names of some famous faults, like the San Andreas Fault in California or the Zagros Fault in Iran; but they are only two of the thousands of faults that have been identified. In fact, both of them are mushy in their own way, because both of those famous faults are made of networks of smaller faults. This is the earth’s way of spreading out the forces because if there were only one fault; the energy released as the fault let go would be much, much larger than even the biggest recorded earthquakes.
This pattern holds true for all types of plate boundaries, though the movement on faults is different depending on the kind of plate intersection. Where plates move toward each other, faults tend to lie nearly horizontal and movement along them causes the adjacent rocks to crumple and fold. This is the sort of movement that has built mountains like the Andes and the Himalayas. Where the plates slide against each other, the faults are nearly vertical. This is the type of movement observed in the San Andreas system, where the plate under the Pacific Ocean is moving northwest along the western edge of North America. Where plates are cracking down the middle, called rifting or spreading, the faults are also close to vertical, and rocks tend to slide “downhill” toward the middle. That process is happening now at the center of the Atlantic Ocean, down the length of the Red Sea, and also at Africa’s Olduvai Gorge.
If you were to place a dot on a map of the world everywhere an earthquake is reported, the dots would eventually join together to make a map of the continental plate boundaries. There would be one line around most of the Pacific Ocean, another one where India bumps up against the underbelly of China, and one across the top of the Mediterranean Sea. In fact, you can see such a map here at the U. S. Geological Survey’s earthquake hazards site – a map that’s continuously updating with current information. According to scientists at the USGS, 90% of all earthquakes occur at or near plate boundaries. Most of the other ten per cent occur at old zones of weakness in the crust, often the site of ancient, healed plate boundaries. The latter includes the New Madrid Fault of the central USA, site of a nineteenth-century earthquake so powerful that it caused the Mississippi River to run backwards temporarily.
Earthquakes, then are caused by faults, cracks in the earth that let the rocks on either side slip past each other like cards in a card deck. But what causes the faults in the first place? Faults, my friends, are the fault of a restless Earth and the plates of crust that wander about its surface. They may not move very fast, but nothing stands in their way.
Some interesting sites to explore: the British Geological Survey Earthquake Hazard site. the United States Geological Survey Earthquake Hazard site, and a Stanford University performance piece depicting plate motion.