Almost every year, a large earthquake occurs somewhere in the world and captures the public's attention. Meanwhile, every day thousands of smaller tremors often go unnoticed by most people. Although we usually consider the ground to be solid and stable, the earth is, in fact, constantly shifting under our feet.
What causes earthquakes?
Earth's crust ranges from 3 to 45 miles deep (5 to 70 kilometers). The crust is a thin, hard shell that floats on the denser, hotter rock of the mantle. The crust is divided into several pieces known as tectonic plates that are constantly in motion, grinding past one another at boundaries known as faults.
As they slide past one another, the tectonic plates snag on rough patches of rock. They lock together like Velcro. However, even though the fault boundaries are locked together, the plates still move, pulling at the entangled sections. This pulling can further crack the Earth's crust, creating more faults near the plate boundaries.
An earthquake occurs when the pressure built up along a fault becomes stronger than the pressure holding the rocks together. Then the rocks on either side of the fault suddenly rip apart, sometimes at supersonic speeds. The two sides of the fault slide past one another, releasing the pent-up pressure. Energy from this separation radiates outward in all directions, including towards the surface, where it is felt as an earthquake. [The 10 Biggest Earthquakes in History]
乐动体育开户Even though the tectonic plates slide at a regular rate over time, the way that faults release stored energy is different with each earthquake, said Shimon Wdowinski, a geophysicist at the University of Miami's Rosentiel School of Marine & Atmospheric Sciences. "Every earthquake doesn't repeat exactly what happened before. Sometimes there is a big earthquakes, sometimes there are two or three together," he told Live Science. "There is no regularity."
A large earthquake is often followed by aftershocks, which are smaller quakes that result from the crust adjusting to the main shock. These aftershocks can help scientists target the origin of the main quake, but can create problems for those suffering its aftermath.
If the earthquake occurs in the ocean, it can push up powerful waves, known as tsunamis乐动体育开户. The sudden upward or downward movement of the seafloor during an earthquake creates large tsunami waves, similar to a child splashing in the bathtub. Earthquakes can also trigger tsunamis by unleashing underwater landslides, which also displace huge amounts of seawater.
An earthquake's size, or magnitude, depends on how large its parent fault is and how much it has slipped. Because these faults extend from the surface down to several miles deep, geologists can't simply visit the source to calculate these numbers. Instead, they rely on a tool known as a seismograph, which measures seismic waves, or vibrations, from an earthquake.
An earthquake's magnitude is ranked on the moment magnitude scale, not the Richter scale. The moment magnitude scale provides a better idea of the shaking and possible damage from earthquakes of all kinds around the world. [Related: Whatever Happened to the Richter Scale?]
Earthquakes with magnitudes less than 3 occur every day, and are generally not felt by people. A magnitude of 3 to 5 is considered minor, while a quake with a magnitude of 5 to 7 is moderate to strong. At the higher end, these quakes can be destructive to cities. Earthquakes from 7 to 8 are major; about 15 of these occur annually. Every year, at least one earthquake with a magnitude over 8 — a "great" quake — wreaks havoc. An earthquake with a magnitude of 10 has never been measured, but it would create widespread devastation. 
乐动体育开户By using the readings from at least three seismographs, geologists can triangulate the origin of the earthquake. At the fault, that origin is called the hypocenter; on the surface, the epicenter.
Most of the major earthquakes strike on well-known fault lines. Californians, for instance, are unlikely to be shocked if they feel the ground shuddering beneath their feet. But a draft map released by the U.S. Geological Survey in 2014 reveals that 42 out of the 50 states have a moderate to high seismic hazard risk.
Preparing for disaster
Scientists have not yet come up with a way to forecast earthquakes. Although animals are reputed to have a sixth sense when it comes to these vibrations, no research has confirmed it, much less determined how such predictions might occur. In many cases, animals are simply sensing the arrival of earthquake waves that go unnoticed by people.
However, scientists can identify locations that are likely to experience earthquakes in the future. For instance, along the tectonic plate boundary off the western shore of South America, researchers have mapped the location of historical earthquakes and found "seismic gaps" — zones with no recent large earthquake quakes. And indeed, scientists had forecast a major earthquake in a seismic gap near Maule, Chile, which was hit by a magnitude-8.8 temblor in 2010. "There have been some very successful forecasts based on the seismic gap theory," Wdowinski said.
乐动体育开户Even without such forecasts, there are some basic things that can be done to prepare for an earthquake. The Federal Emergency Management Agency recommends that all families everywhere should have an emergency kit in their home and car, and communicate with your loved ones a plan for any type of disaster (not just for earthquakes). Such preparation can make a difference not only physically, but also emotionally.
If you live in known earthquake territory, make sure your shelves are firmly attached to the walls, with heavy objects on lower shelves. Keep heavy objects away from beds and sitting areas, and fasten fixtures such as large televisions to the walls. Locate a safe place in each room, under a sturdy desk or table, where you can seek refuge from falling objects. Reinforced doorways can be a safe shelter, but most indoor doorways are not strong enough; a sturdy desk is likely to provide more protection, .
"Earthquakes don't kill, buildings kill," Wdowinski said.
If you are outside, get into an open area, away from structures or bridges. According to FEMA, many deaths in the 1933 Long Beach earthquake occurred when people ran outside, only to be crushed by falling debris from collapsing structures. Remember that the shaking ground rarely causes injury or death; instead, it is the falling objects that result from the quake. If you are in a car, stop as soon as you are able, but stay inside the car. If you are at or near the beach, move quickly inland to avoid potential waves from tsunamis.
After an earthquake, proceed with caution. Remember that most earthquakes are generally followed by aftershocks. Keep an eye (and a nose) out for gas leaks. If you were inside during the quake, move outside. Listen for public service announcements; a battery-powered radio is ideal for your emergency kit.
1811-1812 — Missouri乐动体育开户. In the early 19th century, the New Madrid series of earthquakes rattled the central United States. No seismographs existed at the time, so researchers used historical eyewitness accounts to determine that the magnitudes of the quakes ranged between 7 and 8. Huge waves formed on the Mississippi, causing some portions of the river to appear to flow backward.
1906 — San Francisco, California, Magnitude: 8. About 3,000 people died from the earthquake, on the San Andreas Fault乐动体育开户, and resulting fire.
1923 — Tokyo, Japan, Magnitude: 7.9乐动体育开户. One of the world's most destructive earthquakes, more than 142,000 people died from collapsing buildings and the resulting firestorm. The quake also resulted in enormous tsunami waves.
1960 — Chile, Magnitude: 9.5乐动体育开户. The largest earthquake ever recorded, the 1960 quake in Chile killed more than 1,600 people, with many of the deaths resulting from tsunamis. Waves reached 38 feet (11.5 meters) and carried debris as far as 2 miles (3.2 kilometers) inland.
1970 — Peru, Magnitude: 7.9乐动体育开户. Approximately 66,000 people died, many from collapsed buildings and a post-earthquake avalanche.
2004 — Indonesia, Magnitude: 9.1. The third largest earthquake in the world in this century, the quake killed more than 227,000 people. Powerful tsunami waves crisscrossed the Indian Ocean and ravaged 12 Asian countries.
2011 — Japan, Magnitude: 9.0. More than 15,000 people were killed when an earthquake in northern Japan triggered a giant tsunami. The tsunami also overwhelmed a nuclear reactor, creating new problems for people in the midst of destruction. [Japan Earthquake & Tsunami]
Additional reporting by senior writer Becky Oskin