FACT: Earthquakes are sudden rolling or shaking events caused by movement under the Earth’s surface.
An earthquake is the ground shaking caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake.
Faults are caused by the tectonic plates grinding and scraping against each other as they continuously and slowly move. In California, for example, there are two plates - the Pacific Plate (which extends from western California to Japan, including much of the Pacific Ocean floor) and the North American Plate (which is most of the North American continent and parts of the Atlantic Ocean). The Pacific Plate moves northwestward past the North American Plate along the San Andreas Fault at a rate of about two inches per year.
Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other parts, strain can build up for hundreds of years, producing great earthquakes when it finally releases. Large and small earthquakes can also occur on faults not previously recognized; recent earthquakes in Alabama and Virginia are good examples.
FICTION: “Mega Quakes” can really happen.
The magnitude of an earthquake is related to the area of the fault on which it occurs - the larger the fault area, the larger the earthquake. The San Andreas Fault is 800 miles long and only about 10-12 miles deep, so that earthquakes larger than magnitude 8.3 are extremely unlikely.
The largest earthquake ever recorded by seismic instruments anywhere on the earth was a magnitude 9.5 earthquake in Chile on May 22, 1960. That earthquake occurred on a fault that is almost 1,000 miles long and 150 miles wide, dipping into the earth at a shallow angle. The magnitude scale is open-ended, meaning that scientists have not put a limit on how large an earthquake could be, but there is a limit just from the size of the earth. A magnitude 12 earthquake would require a fault larger than the earth itself.
FICTION: Earthquakes only occur on the West Coast in the United States.
Earthquakes can strike any location at any time. But history shows they occur in the same general patterns over time, principally in three large zones of the earth. The world's greatest earthquake zone, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of the world's largest earthquakes occur. That belt extends from Chile, northward along the South American coast through Central America, Mexico, the West Coast of the United States, the southern part of Alaska, through the Aleutian Islands to Japan, the Philippine Islands, New Guinea, the island groups of the Southwest Pacific, and to New Zealand.
The second important belt, the Alpide, extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world's largest earthquakes, including some of the most destructive.
The third prominent belt follows the submerged mid-Atlantic ridge. The remaining shocks are scattered in various areas of the world. Earthquakes in these prominent seismic zones are taken for granted, but damaging shocks occur occasionally outside these areas. Examples in the United States are New Madrid, Missouri, and Charleston, South Carolina. Many decades to centuries, however, usually elapse between such destructive shocks.
FICTION: The 1906 San Francisco earthquake was the deadliest ever.
Though well known, the magnitude 7.8 San Francisco earthquake and ensuing fire killed 3,000 and razed large sections of the city. It was the most deadly in U.S. history, but that doesn’t make it the worst the world has seen, by far. The deadliest earthquake in recorded history struck Shensi province in China in 1556, killing about 830,000 people. The 1976 magnitude 7.8 earthquake which struck Tangshan, China killed somewhere between 250,000 and 800,000 people. In 2003, the magnitude 6.5 earthquake in Bam, Iran killed more than 40,000 people.
The earthquake in Chile on May 22, 1960, is the strongest to be recorded in the world with magnitude 9.5, and killed more than 4,000. For the record, the largest U.S. earthquake occurred on March 28, 1964, in Alaska. It was a magnitude 9.2 quake and took 131 lives.
PARTIALLY FACT: California has the most earthquakes in the United States.
Alaska registers the most earthquakes in a given year, with California placing second, until 2014 when a sudden increase in seismicity in Oklahoma pushed it well past California as the second most active in terms of magnitude (M) 3.0 and greater earthquakes. In 2014 there were 585 M3 and greater earthquakes in Oklahoma and about 200 in California. As of April 2015 Oklahoma (260 events) is still well ahead of California (29 events).
California, however, has the most damaging earthquakes, including a M6.0 earthquake near Napa in August 2014, because of its greater population and extensive infrastructure. Most of Alaska’s large earthquakes occur in remote locations such as along the Aleutian Island chain. Florida and North Dakota have the fewest earthquakes each year.
FACT: Earthquakes can occur near the surface or deep below the surface.
Earthquakes occur in the crust or upper mantle, from the earth’s surface to about 400 miles below the surface. But the very deepest earthquakes only occur at subduction zones where cold crustal rock is being pushed deep into the earth. In California, earthquakes are almost all in the top 15 miles of the crust, except in northern California along the Cascadia Subduction Zone, which extends into Oregon, Washington, and British Columbia.
Seismologists use earthquakes to study the interior of the earth and to pinpoint faults and geologic structures such as the core-mantle boundary, subduction zones, and the subsurface extent of the San Andreas Fault. Using earthquakes and waves from earthquakes, scientist can see all of the earth’s interior.
FICTION: The ground can open up during an earthquake.
A popular cinematic and literary device is a fault that opens during an earthquake to swallow up an inconvenient character. But unfortunately for principled writers, gaping faults exist only in movies and novels. The ground on the two sides of the fault slide past each other, they do not pull apart. If the fault could open, there would be no friction. Without friction, there would be no earthquake. Shallow crevasses can form during earthquake induced landslides, lateral spreads, or other types of ground failures. Faults, however, do not gape open during an earthquake.
FICTION: California will eventually fall into the ocean.
The ocean is not a great hole into which California can fall, but it is itself land at a somewhat lower elevation with water above it. It’s absolutely impossible that California will be swept out to sea. Instead, southwestern California is moving horizontally northward towards Alaska as it slides past central and eastern California. The dividing point is the San Andreas fault system, which extends from the Salton Sea in the south to Cape Mendocino in the north. This 800 mile long fault is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving to the northwest with respect to the North American Plate at approximately 46 millimeters (two inches) per year (the rate your fingernails grow). At this rate, Los Angeles and San Francisco will one day (about 15 million years from now) be next-door neighbors, and in an additional 70 million years, Los Angeles residents will find themselves with an Alaska zip code!
FICTION: An earthquake on the San Andreas fault can cause a large tsunami.
The San Andreas fault cannot create a big tsunami like the ones that happened in Sumatra in 2004 or Japan in 2011. Those earthquakes happened on subduction zone faults, on which fault slip caused vertical uplift of the sea floor. While a part of the San Andreas fault near and north of San Francisco is offshore, the motion is mostly horizontal, so it will not cause large vertical motions of the ocean floor that would generate a tsunami. Earthquakes on other faults offshore California as well as underwater landslides triggered by strong shaking can create local tsunamis, some of which may be locally damaging.
PARTIALLY FACT: An “Aftershock” can be greater than the initial earthquake.
“Foreshock”, “mainshock”, and “aftershock” are relative terms, all of which describe earthquakes. Aftershocks are smaller earthquakes that occur in the same general area during the days to years following a larger event or “mainshock”. They mostly occur within 1-2 fault lengths of the mainshock. For the largest earthquakes, this is a long distance; it is thought that the 1906 San Francisco earthquake triggered events in southern California, western Nevada, southern central Oregon, and western Arizona, all within 2 days of the mainshock.
As a general rule, aftershocks represent readjustments in the vicinity of a fault that slipped at the time of the mainshock. The frequency of these aftershocks decreases with time. If an aftershock is larger than the first earthquake then we call it the mainshock and the previous earthquakes in a sequence become foreshocks. About 5% to 10% of earthquakes in California are followed by a larger one within a week and then are considered a foreshock.
It is possible to have two earthquakes of about the same size in a sequence. There is a 5% chance of having the two largest earthquakes in a sequence be within 0.2 units of magnitude, during the first week of a sequence. Given that very large earthquakes are rare to begin with, it is not surprising that we have not yet observed two very large earthquakes so close together in time in California.
NOT LIKELY: Two major earthquakes occurred on the same day, so they must be related.
Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over long distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress efficiently over thousands of miles. There is evidence to suggest that earthquakes in one area can trigger seismic activity within a few hundred miles, including aftershocks clustered near the main shock. There is also evidence that some major earthquakes manage to trigger seismicity over much greater distances (thousands of miles), but these triggered quakes are small and very short lived.
PARTIALLY FACT: People can cause earthquakes.
Earthquakes induced by human activity have been documented in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the filling of large reservoirs for water supplies. Most of these earthquakes were minor. Deep mining can cause small to moderate quakes and nuclear testing has caused small earthquakes in the immediate area surrounding the test site, but other human activities have not been shown to trigger subsequent earthquakes.
Within the central and eastern United States, the number of earthquakes has increased dramatically over the past few years. Between the years 1973-2008, there was an average of 21 earthquakes of magnitude three and larger in the central and eastern United States. This rate jumped to an average of 99 M3+ earthquakes per year in 2009?2013, and the rate continues to rise. In 2014, alone, there were 659 M3 and larger earthquakes . Most of these earthquakes are in the magnitude 3?4 range, large enough to have been felt by many people, yet small enough to rarely cause damage. There were reports of damage from some of the larger events, including the M5.6 Prague, Oklahoma earthquake and the M5.3 Trinidad, Colorado earthquake.
The increase in seismicity has been found to coincide with the injection of wastewater in deep disposal wells in several locations, including Colorado, Texas, Arkansas, Oklahoma and Ohio. Much of this wastewater is a byproduct of oil and gas production and is routinely disposed of by injection into wells specifically designed and approved for this purpose. Hydraulic fracturing, commonly known as “fracking”, does not appear to be linked to the increased rate of magnitude 3 and larger earthquakes.
See also: USGS Induced Earthquakes Research
FICTION: People can stop earthquakes.
We cannot prevent earthquakes from happening (or stop them once they’ve started). However, we can significantly mitigate their effects by characterizing the hazard (e.g., identifying earthquake faults, unconsolidated sediment likely to amplify earthquake waves, and unstable land prone to sliding or liquefying during strong shaking), building safer structures, and preparing in advance by taking preventative measures and knowing how to respond.
There are many things being done now by the USGS and other agencies to protect people and property in the United States in the event of a major earthquake. These include Earthquake Early Warning, Earthquake Rupture Forecasts and Probabilistic Seismic Hazard Assessments.
FICTION: Nuclear explosions can start or stop earthquakes.
Scientists agree that even large nuclear explosions have little effect on seismicity outside the area of the blast itself. The largest underground thermonuclear tests conducted by the United States were detonated in Amchitka at the western end of the Aleutian Islands, and the largest of these was the 5 megaton test code-named Cannikin that occurred on November 6, 1971 that did not trigger any earthquakes in the seismically active Aleutian Islands.
On January 19, 1968, a thermonuclear test, code-named Faultless, took place in central Nevada. The code-name turned out to be a poor choice because a fresh fault rupture some 4,000 feet long was produced. Seismograph records showed that the seismic waves produced by the fault movement were much less energetic than those produced directly by the nuclear explosion. Locally, there were some minor earthquakes surrounding the blasts that released small amounts of energy. Scientists looked at the rate of earthquake occurrence in northern California, not far from the test site, at the times of the tests and found nothing to connect the testing with earthquakes in the area.
FICTION: You can prevent large earthquakes by making lots of small ones, or by “lubricating” the fault with water.
Seismologists have observed that for every magnitude 6 earthquake there are about 10 of magnitude 5, 100 of magnitude 4, 1,000 of magnitude 3, and so forth as the events get smaller and smaller. This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event. It would take 32 magnitude 5's, 1000 magnitude 4's, OR 32,000 magnitude 3's to equal the energy of one magnitude 6 event. So, even though we always record many more small events than large ones, there are far too few to eliminate the need for the occasional large earthquake.
As for “lubricating” faults with water or some other substance, if anything, this would have the opposite effect. Injecting high-pressure fluids deep into the ground is known to be able to trigger earthquakes—to cause them to occur sooner than would have been the case without the injection. This would be a dangerous pursuit in any populated area, as one might trigger a damaging earthquake.
FICTION: We can predict earthquakes.
There is no scientifically plausible way of predicting the occurrence of a particular earthquake. The USGS can and does make statements about earthquake rates, describing the places most likely to produce earthquakes in the long term. It is important to note that prediction, as people expect it, requires predicting the magnitude, timing, and location of the future earthquake, which is not currently possible. The USGS and other science organizations are working to better understand earthquakes in the hope of eventually being able to predict the size, location and time that an earthquake will happen. The USGS does produce aftershock forecasts that give the probability and expected number of aftershocks in the region following large earthquakes.
FICTION: Animals can predict earthquakes.
Changes in animal behavior cannot be used to predict earthquakes. Even though there have been documented cases of unusual animal behavior prior to earthquakes, a reproducible connection between a specific behavior and the occurrence of an earthquake has not been made. Because of their finely tuned senses, animals can often feel the earthquake at its earliest stages before the humans around it can. This feeds the myth that the animal knew the earthquake was coming. But animals also change their behavior for many reasons, and given that an earthquake can shake millions of people, it is likely that a few of their pets will, by chance, be acting strangely before an earthquake.
MAYBE: Some people can sense that an earthquake is about to happen.
There is no scientific explanation for the symptoms some people claim to have preceding an earthquake, and more often than not there is no earthquake following the symptoms.
FICTION: It’s been raining a lot, or very hot--it must be earthquake weather!
Many people believe that earthquakes are more common in certain kinds of weather. In fact, no correlation with weather has been found. Earthquakes begin many kilometers (miles) below the region affected by surface weather. People tend to notice earthquakes that fit the pattern and forget the ones that don't. Also, every region of the world has a story about earthquake weather, but the type of weather is whatever they had for their most memorable earthquake.
NOT LIKELY: The Golden Gate Bridge, Seattle Space Needle and other buildings will all eventually fall during an earthquake.
Damage in earthquakes depends on the strength of the ground shaking and the ability of a structure to accommodate this shaking. Building codes define the guidelines for how strong structures need to be to perform well in earthquakes and continue to evolve as engineers and scientists better understand earthquakes and how structures respond to ground shaking.
Based on the type of construction and the building code at the time when they were built, we have a pretty good understanding of what buildings are likely to be damaged in future earthquakes. A detailed scientific assessment of the likely damage in a big San Andreas earthquake in southern California (The ShakeOut Earthquake Scenario - A Story That Southern Californians Are Writing) estimated that 300,000 buildings in southern California would be damaged at a moderate level (losing at least 10% the value of the building) as modeled in the M7.8 ShakeOut scenario earthquake. Although this is a large number, it is only 1 out of every 16 buildings in the region. Most buildings will not have significant damage. Moreover, only 1,500 of those buildings will actually collapse. That is less than 1 out of 30,000 buildings in southern California. Widespread collapse of many buildings is not realistic.
FACT: Earthquakes don’t kill people, buildings and their contents do.
The greatest risk in an earthquake is the severity of the shaking it causes to manmade and natural structures and the contents within these that may fail or fall and injure or kill people. There have been large earthquakes with very little damage because they caused little shaking and/or buildings were built to withstand that shaking. In other cases, smaller earthquakes have caused great shaking and/or buildings collapsed that were never designed or built to survive shaking.
Much depends on two variables: geology and engineering. From place to place, there are great differences in the geology at and below the ground surface. Different kinds of geology will do different things in earthquakes. For example, shaking at a site with soft sediments can last 3 times as long as shaking at a stable bedrock site such as one composed of granite.
Local soil conditions also play a role, as certain soils greatly amplify the shaking in an earthquake. Seismic waves travel at different speeds in different types of rocks. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard rock at the same distance from the same earthquake. The looser and thicker the soil is, the greater the energy movement will be. Fires are another major risk during earthquakes as gas lines may be damaged and particularly hazardous.
FICTION: During an earthquake you should head for the doorway.
That’s outdated advice. In past earthquakes in unreinforced masonry structures and adobe homes, the door frame may have been the only thing left standing in the aftermath of an earthquake. Hence, it was thought that safety could be found by standing in doorways. In modern homes doorways are no stronger than any other parts of the house and usually have doors that will swing and can injure you.
YOU ARE SAFER PRACTICING THE “DROP, COVER, AND HOLD ON” maneuver under a sturdy piece of furniture like a strong desk or table. If indoors, stay there. Drop to the floor, make yourself small and get under a desk or table or stand in a corner. If outdoors, get into an open area away from trees, buildings, walls and power lines. If in a high-rise building, stay away from windows and outside walls, stay out of elevators, and get under a table. If driving, pull over to the side of the road and stop. Avoid overpasses and power lines. Stay inside your car until the shaking is over. If in a crowded public place, do not rush for the doors. Crouch and cover your head and neck with your hands and arms. You should practice the “DROP, COVER AND HOLD ON” method at work and at home at least twice a year.
FICTION: Everyone will panic during the Big One.
A common belief is that people always panic and run around madly during and after earthquakes, creating more danger for themselves and others. Actually, research shows that people usually take protective actions and help others both during and after the shaking. Most people don't get too shaken up about being shaken up!
FICTION: You can’t plan ahead for an earthquake.
There are plenty of things you can do right now to prepare if you live in an earthquake-prone area.
- Make sure each member of your family knows what to do no matter where they are when earthquakes occur:
- Establish a meeting place where you can all reunite afterward.
- Find out about earthquake plans developed by children's school or day care.
- Remember transportation may be disrupted, so keep some emergency supplies--food, liquids, and comfortable shoes, for example--at work.
- KNOW where your gas, electric and water main shutoffs are and how to turn them off if there is a leak or electrical short. Make sure older members of the family can shut off utilities.
- LOCATE your nearest fire and police stations and emergency medical facility.
- TALK to your neighbors--how could they help you, or you them after an earthquake?
- TAKE Red Cross First Aid and CPR Training Course.
- MAKE your disaster supply kit. Beyond the usual flashlights, batteries and radios, include a first-aid kit; work gloves; sturdy shoes or boots; a week’s supply of any presciption medications you or your family might need; credit card and cash; personal identifcation; extra set of keys; matches in a waterproof container; map of your area; phone numbers of family and other important people (doctors, veterinarians, etc.); copies of insurance policies and other important documents; special needs equipment (diapers, baby formula, hearing aid batteries; spare eyeglasses, etc.); three gallons of water per person; three-day supply of food per person; hand tools; a portable ABC fire extinguisher; sanitation supplies for you and your family; entertainment (toys, books, coloring books and crayons, playing cards)
- BOLT bookcases, china cabinets, tall furniture, file cabinets, etc. to wall studs. Brace or anchor heavy electronics and other heavy items. Secure items that might fall. Move heavy or fragile items to lower shelves. Fasten drawers and cabinet doors with latches or locks. Brace overhead light fixtures. Strap your water heater to wall studs and bolt down any gas appliances. Look for other non-structural steps you can take in your home and workplace to reduce your chances for injury and loss.
- ASK AN ENGINEER about the seismic safety of your home and/or business. It’s well known that unreinforced masonry structures can fail quickly during earthquakes. An inspection by a structural engineer now can help you decide if retrofitting will help your property withstand shaking.
FACT: The U.S. Geological Survey is conducting research to better forecast the effects of potentially damaging earthquakes throughout the United States and mitigate their effects.
Basic and applied scientific research is being carried out to predict the types of ground shaking expected from future large earthquakes based upon the probabilities (or likelihoods) of those earthquakes occurring, the physics of the earthquake source, the propagation of seismic waves through the Earth’s crust and local site effects. Together with rupture scenarios for specific faults, these hazard assessments are essential for multiple applications, including:
- Probabilistic seismic hazard assessments such as the National Seismic Hazard Map that underlie seismic provisions of building and other regulatory codes; as well as detailed urban seismic hazard maps that include the effects of rupture directivity, 3D basin response and soil nonlinearity. These urban hazard maps will be included in code updates for selected regions.
- Development of credible earthquake scenarios for specific faults with synthetic ground-motion time histories for evaluating current engineering design practice, improving building codes and for emergency planning and public education.
- Other uses of these hazard products include: 1) site-specific designs and retrofits of critical and major facilities such as bridges, hospitals, nuclear power reactors, dams and tall buildings, 2) modeling damage patterns and damage to specific structures after earthquakes, 3) assessing secondary earthquake hazards such as liquefaction and landslides and 4) computing actuarially sound earthquake insurance premiums.