An earthquake is a geological event in which rock masses below the surface of the earth suddenly shift, releasing energy and sending strong vibrations to the surface.
Most earthquakes are caused by movement along a fault line, which is a fracture in the earth's crust. Thousands of earthquakes happen each day around the world, but most are too small to be felt. Major earthquakes strike both populated and unpopulated areas every year, killing hundreds, injuring thousands, causing electrical fires and destroying manmade structures resulting in hundreds of millions of dollars in damage. Earthquakes have many geological effects, including sparking volcanic activity, causing tidal waves and tsunamis, and shifting entire cities as the tectonic plate is shifted. Over time, earthquakes create mountains or valleys that did not previously exist. Despite millions of dollars and decades of research, seismologists (scientists who study earthquakes) are still unable to predict precisely when and where an earthquake will happen. Nevertheless, in 2012, in a controversial ruling, an Italian judge sentenced seven scientists to six years imprisonment for giving optimistic assessments prior to the L'Aquila quake of 2009.
The earth is covered by a crust of rock that is broken into numerous plates, sometimes referred to as tectonic plates. The plates float on a layer of molten (liquid) rock within the earth called the mantel. This molten rock moves and flows, and this movement is thought to cause the shifting of the plates. When plates move, they interact in a variety of ways that can cause earthquakes or other geological effects. Plates can slide past, bump into, overrun, or pull away from each other; this movement of plates is called plate tectonics. Boundaries between plates are called faults or fault lines.
Earthquakes can occur when any of the four types of movement take place along a fault. Earthquakes along the San Andreas and Hayward faults in California occur because two plates slide past one another. Earthquakes also occur if one plate overruns another. When this happens one plate is pushed under the other plate, as on the western coast of South America, the northwest coast of North America, and in Japan. If plates collide and neither is pushed downward, both are forced upward, creating mountain ranges. Many faults on the floor of the ocean are between two separating plates. Many earthquakes with centers at the floor of the ocean are caused by this kind of movement. Undersea earthquakes can result in large, destructive waves called tsunamis.
The relative size of earthquakes traditionally has been determined by the Richter Scale, which measures the energy an earthquake releases. Each whole number increase in value on the Richter scale indicates a tenfold increase in the energy released and a 30-fold increase in ground motion. An earthquake measuring 8 on the Richter scale is 10 times more powerful, therefore, than an earthquake with a Richter magnitude of 7. This familiar scale has been largely replaced by the moment magnitude scale. This scale measures movement of rock along a fault, which gives a more accurate account of larger earthquakes. Larger earthquakes release energy for a longer time and affect a broader area, making magnitude a better standard for large earthquakes. Another scale, the Mercalli scale, uses observations of damage (such as fallen chimneys) or people's assessments of effects (such as mild or severe ground shaking) to describe the intensity of a quake. The Modified Mercalli Intensity Scale is preferred in the United States. It assigns intensity levels in Roman numerals from I for no shaking felt to X for extreme shaking that destroys well-built structures.
Several million earthquakes occur each year, but most are small in size and strength. The U.S. Geological Survey (USGS) has reported on activity up to 2012. In that year, 32 earthquakes occurred in the United States, 27 of them between 5 and 5.9 magnitude, and five earthquakes between 6 through 6.9 magnitude. The agency's 2018 forecast showed most potential in California, areas of the Northwest, and Oklahoma. Still, the USGS noted a decline in the three years before 2018 in Oklahoma and southern Kansas earthquakes. These areas had seen increased earthquake activity from fluid injection (fracking). Globally, an average of about 140 earthquakes above magnitude 6.0 are expected each year, although 2017 reported only 111 at that magnitude or higher. About 700 earthquakes each year are strong enough and located close enough to a populated area to cause reportable damage.
Earthquakes have been recorded throughout written history as catastrophic events. When a strong earthquake hit in the ancient world, it could shatter stone walls, and destroy infrastructure, resulting in a massive death toll and widespread chaos. The ancient Mediterranean cities of Knossos, Chattusas, and Mycenae were located in tectonically active mountain ranges and were brought to ruin because of their geological instability. Reports of earthquakes appear in the chronicles of ancient civilizations.
The strongest earthquake ever recorded as of 2018 registered 9.5 on the Richter scale and 12 on the Mercalli Scale. It occurred on May 22, 1960, and was centered off the coast of Chile. The earthquake killed 2,000 people, injured 3,000, and caused $675 million in property damage. A resulting tsunami caused additional injuries, deaths, and significant property damage in Hawaii, Japan, and the West Coast of the United States.
The Long Beach earthquake struck on March 10, 1933, killing 120, injuring hundreds, and causing more than $50 million in property damage. It led to the passage of the state's Field Act, which established strict building code standards designed to make structures better able to endure strong earthquakes.
In the twentieth century, Alaska had many more severe earthquakes than California. However, because they occurred in mostly sparsely populated areas, deaths, injuries, and property damage were light, and thus these quakes were not as widely reported. Of the 15 strongest earthquakes ever recorded in the 50 states, 10 were in Alaska, with the strongest registering a 9.2 (the second strongest ever recorded in the world) on the Richter Scale and 12 on the Mercalli Scale. It struck the Anchorage area on March 28, 1964, killing 125 (most from a tsunami caused by the earthquake), injuring hundreds, and causing $311 million in property damage.
An underwater earthquake with a magnitude of 9.3 on the Richter Scale caused the December 2004 Indian Ocean tsunami, also known as the Boxing Day Tsunami, that killed an estimated 220,000 to 250,000 people and caused damage from Indonesia around the perimeter of the ocean to India and all the way to South Africa. This was the sixth deadliest natural disaster in recorded history. In fact, of the top ten deadliest natural disasters five were land-based earthquakes and one was a tsunami caused by an earthquake. Each of these disasters had a recorded death toll of between 230,000 and one million people.
On January 12, 2010, a 7.0-magnitude earthquake in Haiti killed an estimated 250,000 to 300,000 people and left around one million people homeless. A 2011 earthquake offshore led to the huge tsunami that hit the east coast of Japan and damaged the Fukushima nuclear power plant, which caused a massive environmental disaster and evacuation of 170,000 people.
Every major earthquake raises the question of whether scientists will ever be able to predict exactly when and where one will strike. This cannot prevent the earthquake but could save lives when a large earthquake hits. In the early 2000s, scientists could only make very broad predictions. For example, scientists believed there is at least a 50% chance that a devastating earthquake will strike somewhere along the San Andreas fault within the next 100 years. The USGS produces yearly forecasts based on trends and other data that estimate areas with potential for high earthquake activity in the coming year.
Ultra-sensitive instruments placed across faults at the surface can measure the slow, almost imperceptible movement of fault blocks. This measurement records the great amount of potential energy stored at the fault boundary. In some areas small earthquakes called foreshocks that precede an earthquake can help seismologists predict the larger earthquake.
Other instruments measure additional fault-zone phenomena that seem to be related to earthquakes. The rate at which radon gas issues from rocks near faults has been observed to change before an earthquake. The properties of the rocks themselves (such as their ability to conduct electricity) also change as tectonic forces exert pressure on them. Unusual animal behavior has been reported before many earthquakes, and research into this phenomenon is being pursued, though no definitive conclusions were drawn as of 2018. In April 2018, scientists in Chile discovered there is a relationship between magnetic fields and earthquakes that could lead to improve prediction of events.
See also Emergency prepardedness ; Fukushima Daiichi nuclear power station disaster ; Haiti earthquake ; Japanese earthquake (2011) .
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Lin II, Rong-Gong. “Here's What Earthquake Magnitudes Mean and Why an 8 Can Be So Much Scarier than a 6.” LA Times. http://www.latimes.com/nation/la-fg-mexico-earthquake-magnitude-20170921-htmlstory.html (accessed April 2, 2018).
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National Earthquake Information Center, PO Box 25046, MS 966, Denver, CO, 80225, (303) 273-8500, https://earthquake.usgs.gov .
Ken R. Wells
Alyson C. Heimer, MA
Revised by Teresa Odle, BA, ELS