Earthquakes & Tsunamis: Causes & Information

Almost every year, a large earthquake occurs somewhere in the world and captures the public's attention. But it's not the only one—thousands of smaller tremors happen on a daily basis and 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), a thin shell. The crust is divided into several pieces, known as tectonic plates, that are constantly in motion, sliding past one another in regions known as faults or fault planes.

But the plates are jagged, not smooth. As they slide, pieces from one plate often snag on another. As the plates continue moving, they pull at the entangled sections until finally tearing them apart. Energy from this separation radiates outward in all directions, including towards the surface, where it is felt as an earthquake.

A large earthquake is often followed by aftershocks, 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.

In this photo taken by 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 on Sunday, Dec. 26, 2004 as the greatest tsunami in recent history came ashore. 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.
Credit: AP Photo/Eric Skitzi

If the earthquake occurs in or near the ocean, it can push up powerful waves, known as tsunamis.

Measuring earthquakes

An earthquake's size, or magnitude, depends on how large its parent fault is and how much it has slipped. Because these faults are 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 how much the earth moves over the course of a quake.

An earthquake's magnitude is ranked on a scale. Earthquakes with magnitudes less than 3 occur daily, and are generally not felt by people at the surface, though millions occur annually. 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 fifteen of these occur annually. Every year, at least one earthquake with a magnitude over 8 – a "great" quake – wrecks 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.

Although minor earthquakes occur around the world, most of the major ones are centered on well-known fault lines. Californians, for instance, are unlikely to be shocked if they feel the ground shuddering beneath their feet. But a map released by the United States Geological Survey in 2011 reveals that 39 out of the 50 states have a moderate to high seismic hazard risk. Many of these are due to the 'New Madrid' fault in the center of the country, which runs from St. Louis to Memphis.

World map shows 105 years of earthquakes.
More than 100 years of earthquakes glow on a world map.
Credit: John Nelson, IDV Solutions.

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, there has been no research to confirm it, much less determine how such predictions might occur.

However, 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 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 hanging objects away from beds and sitting areas, and brace overhead lighting fixtures. 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.

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.

San Francisco Fire and Earthquake
Great San Francisco Fire and Earthquake - April 18, 1906
Credit: Records of the Office of the Chief Skinal Officer

Famous quakes

1811-1812 — Missouri. In the early nineteenth century, a series of several earthquakes spread through the center of the United States. No seismographs existed at the time, so researchers used data to determine that the magnitudes of the quakes ranged between 7 and 8. The ground rose and fell, and huge waves formed on the Mississippi, causing some portions of the river to appear to flow backwards.

1906 — San Francisco, California, Magnitude: 8. Approximately 3,000 people died from the earthquake and resulting fire. The fault line causing the quake was exposed at the surface, a rare occurrence.

1923 — Tokyo, Japan, Magnitude: 8.25. One of the world's most destructive earthquakes, over 142,000 people died from collapsing buildings and the resulting firestorm. The quake also resulted in enormous waves.

1960 — Chile, Magnitude: 9.5. The largest earthquake in the world, the 1960 quake in Chile killed over 1,600 people, with many of the deaths resulting from tsunamis along the coast. Waves reached 38 feet (11.5 meters) and carried debris as far as two miles inland.

1970 — Peru, Magnitude: 7.9. Approximately 66,000 people died, many from collapsed buildings and the resulting avalanche.

2004 — Indonesia, Magnitude: 9.1. The third largest earthquake in the world in the last century killed over 227,000 people. The shaking of the ground resulted in powerful waves that ravaged 12 Asian countries.

2011 — Japan, Magnitude: 9.0. Over 20,000 people were killed when an earthquake in northern Japan triggered a giant tsunami. The shaking damaged several nuclear reactors, creating new problems to people in the midst of destruction. [Japan Earthquake & Tsunami]

—Nola Taylor Redd

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