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.
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]
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. 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.
In this photo taken by a tourist Eric Skitzi from England, tourists watch as tsunami waves hit the shore from a safe place inside Casuarina Beach Hotel resort in Penang, northwestern Malaysia around 1:00pm in local time (0500GMT) Sunday, Dec. 26, 2004. The resort hotel lifeguards noticed waves were huge and sounded warning to all tourists around the hotel beach area to run to the safety area. The Malaysian government on Wednesday canceled New Year celebrations nationwide, urging people to pray this weekend that the country will be spared from future disasters such as the recent tsunamis that killed at least 65 Malaysians.
In this photo taken by a tourist Eric Skitzi from England, tourists watch as tsunami waves hit the shore from a safe place inside Casuarina Beach Hotel resort in Penang, northwestern Malaysia around 1:00pm in local time (0500GMT) Sunday, Dec. 26, 2004. The resort hotel lifeguards noticed waves were huge and sounded warning to all tourists around the hotel beach area to run to the safety area. The Malaysian government on Wednesday canceled New Year celebrations nationwide, urging people to pray this weekend that the country will be spared from future disasters such as the recent tsunamis that killed at least 65 Malaysians.
Credit: AP Photo/Eric Skitzi
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. [Scary Scenario: Devastating Earthquake Visualized]
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