Earthquakes and Faults

The California Geological Survey studies earthquakes to help Californians plan and build earthquake resistant communities. We record the strong ground motion from earthquakes, study the distribution of historic earthquakes and evaluate faults that are the source of earthquakes. We combine that information to prepare maps showing the potential for ground shaking, fault rupture, liquefaction and seismically induced landsliding.

Recent Earthquakes

Maps of recent earthquakes and ShakeMaps of the intensity of earthquake shaking are available from California Integrated Seismic Network.

Historical Earthquakes

Studies of historical earthquakes provide basic background for projecting future seismic hazards and losses. Search for more information on historical earthquakes and earthquake catalogs, as well as a list of significant California earthquakes.

Faults

Faults are planes of weakness in the earth’s crust where one side has moved relative to the other. They are recognized and mapped by sheared and displaced rock units and by the distinctive landforms created by repeated rupture of the earth’s surface. Descriptions of significant active faults are included in the USGS Quaternary fault and fold database. Digital maps of Quaternary faults are available for download from CGS. Faults that represent a hazard of surface rupture are included in Alquist-Priolo Earthquake Fault Zones. Also see theFault Activity Map of California.

Earthquake Probabilities

Earthquake probabilities are calculated by projecting earthquake rates based on earthquake history and fault slip rates. The result is expressed as the probability that an earthquake of a specified magnitude will occur on a fault or within an area. The Uniform California Earthquake Rupture Forecast represents the best available science for authoritative estimates of the magnitude, location, and likelihood of potentially damaging earthquakes in California.

Earthquake Shaking Hazards

Earthquake shaking hazards are calculated considering earthquake magnitudes and rates, the decrease in earthquake shaking with distance, and amplification of shaking by soils. The result is expressed as the level of ground shaking (as a percentage of gravity) that on average occurs every 2,500 years. These calculations for California are part of a cooperative project between USGS and CGS, and are part of National Seismic Hazard Maps.

Earthquake Loss Estimation

The effects of potential earthquakes are described by statewide earthquake loss estimation and planning scenarios. An earthquake planning scenario is a description of a hypothetical earthquake, including projected ground shaking, damage, social disruption and economic losses. The long term effects of possible earthquakes also can be estimated and described as annualized losses: the average loss or casualties per year. Earthquake loss estimation and planning scenarios provide useful information in preparing emergency response plans, developing earthquake hazard mitigation strategies, and evaluating the nature and scope of response and recovery efforts prior to the earthquakes.

What to do Before, During, and After an Earthquake

Knowing how to prepare for and survive a major earthquake will be critical in California sometime in the coming years.

How to Prepare

  • Plan on fending for yourself for at least three days, preferably for a week. Electricity, water, gas and telephones may not be working after an earthquake. The police and fire departments are likely to be tied up.
  • Stock your emergency supplies.You'll need food and water (a gallon a day per person); a first aid kit; a fire extinguisher suitable for all types of fires; flashlights; a portable radio; extra batteries, blankets, clothes, shoes and money (ATMs may not work); medication; an adjustable or pipe wrench to turn off gas or water, if necessary; baby and pet food; and an alternate cooking source (barbecue or camp stove). This list can also be applied to other disasters, such as floods or wildfires.
  • Decide in advance how and where your family will reunite if separated during a quake.Do in-home practice drills. You might choose an out-of-the-area friend or relative that family members can call to check on you.
  • Secure hazards and big appliances.Including water heaters, major appliances and tall, heavy furniture to prevent them from toppling are prudent steps. So, too, are storing hazardous or flammable liquids, heavy objects and breakables on low shelves or in secure cabinets.
  • Discuss earthquake insurance with your agent.Depending on your financial situation and the value of your home, it may be worthwhile.

What to do During an Earthquake

  • If you're indoors, stay there.Get under -- and hold onto --a desk or table, or stand against an interior wall. Stay clear of exterior walls, glass, heavy furniture, fireplaces and appliances. The kitchen is a particularly dangerous spot. If you’re in an office building, stay away from windows and outside walls and do not use the elevator. Many are certain that standing in a doorway during the shaking is a good idea. That’s false, unless you live in an unreinforced adobe structure; otherwise, you're more likely to be hurt by the door swinging wildly in a doorway or trampled by people trying to hurry outside if you’re in a public place.
  • If you're outside, get into the open.Stay clear of buildings, power lines or anything else that could fall on you.
  • If you're driving, move the car out of traffic and stop. Avoid parking under or on bridges or overpasses. Try to get clear of trees, light posts, signs and power lines. When you resume driving, watch out for road hazards.
  • If you're in a mountainous area, beware of the potential for landslides. Likewise, if you're near the ocean, be aware that tsunamis are associated with large earthquakes. Get to high ground.
  • If you’re in a crowded public place, avoid panicking and do not rush for the exit. Stay low and cover your head and neck with your hands and arms.

What to Do After an Earthquake

  • Check for fire or fire hazards. If you smell gas, shut off the main gas valve. If there's evidence of damage to electrical wiring, shut off the power at the control box.
  • If the phone is working, only use it in case of emergency.Likewise, avoid driving if possible to keep the streets clear for emergency vehicles.
  • Be aware.Items may fall out of cupboards or closets when the door is opened, and also that chimneys can be weakened and fall with a touch. Check for cracks and damage to the roof and foundation of your home.
  • Listen to the radio for important information and instructions. Remember that aftershocks, sometimes large enough to cause damage in their own right, generally follow large quakes.
  • If you leave home, leave a message telling friends and family your location.

Earthquake Frequently Asked Questions

What is an earthquake and what causes earthquakes?

An earthquake is a shaking of the ground caused by the sudden breaking and shifting of rock beneath the Earth's surface. Earthquakes occur when the two sides of a fault slip suddenly against each other. In California, the Pacific and North American plates creep past each other in opposite directions, about 1.5 inches per year. Friction between the plates causes some parts to snag, then break free in sudden, jerking movements. Those movements emit waves of energy that travel through the ground, causing the shaking you feel.

What is a fault?

A fault is a fracture in the crust along which one side has moved relative to the other side. Faults can be very small or hundreds of miles long. The earth's crust is composed of huge plates that are in slow but nearly constant motion. Part of California is on the Pacific Plate, and part is on the North American Plate. The San Andreas Fault, which runs from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County, is the boundary between these plates. Sometimes one block of the crust moves up while the other moves down, sometimes they move horizontally in opposite directions (that's what's happening with the San Andreas Fault; Los Angeles is creeping closer to San Francisco). Some faults are well known and easy to spot, such as the San Andreas. Others are underground, with nothing on the surface revealing their presence (a blind thrust fault). The 1994 Northridge earthquake was caused by a blind thrust fault.

How common are faults in California?

There are hundreds of identified faults in California; about 200 are considered potentially hazardous based on their slip rates in recent geological time (the last 10,000 years). More than 70 percent of the state's population resides within 30 miles of a fault where high ground shaking could occur in the next 50 years.

Does California have more and bigger earthquakes than anyplace else?

Actually, Alaska has more earthquakes and the biggest ones in the U.S. The largest earthquake in history occurred in Chile (a magnitude 9.5 on May 22, 1960). The biggest earthquake in the contiguous 48 states was the magnitude 7.9 Fort Tejon quake on Jan. 9, 1857.

How often do earthquakes happen?

The National Earthquake Information Center (U.S.) reports 12,000-14,000 earthquakes a year around the world, or 35 a day. Throughout the world, there are one "great" (magnitude 8.0 or more), 18 "major" (7.0-7.9), 120 "large" (6.0-6.9) and 1,000 "moderate" (5.0-5.9) earthquakes in an average year. Each year, California generally gets two or three earthquakes large enough to cause moderate damage to structures (magnitude 5.5 and higher). Earthquakes can occur at any time of the year.

What is an earthquake's hypocenter? What is the epicenter?

Earthquake ruptures usually begin far under the surface of the Earth. The point on a fault where rupture initiates is referred to as the focus or hypocenter of an earthquake. The hypocenter is described by its depth in kilometers, its map location in latitude and longitude, its date and time of occurrence, and its magnitude (a measure of the amount of energy radiated as seismic waves). The term epicenter, which is more commonly used to refer to an earthquake location, is the point on the earth's surface directly above the hypocenter. The description of an epicenter is the same as for a hypocenter except the depth is omitted.

How are earthquakes recorded and measured?

There is a network of seismic monitoring stations throughout California and in other parts of the country and world. Sensors and recorders in each station measure and record ground vibrations produced by earthquakes. They measure acceleration, velocity, and displacement on three axes of motion--horizontal (x, y), and vertical (z). They transmit the information to computers which further process the data. The data from multiple stations enable us to determine the time, epicenter, and focal depth of an earthquake. Also, estimates can be made of an earthquake's relative intensity and amount of energy it released.

How much power does an earthquake release?

The strength of an earthquake is generally expressed in two ways: magnitude and intensity. The magnitude is a measure that depends on the seismic energy radiated by the earthquake as recorded on seismographs. An earthquake's magnitude is expressed in whole numbers and decimals (e.g., 6.8). The intensity at a specific location is a measure that depends on the effects of the earthquake on people or buildings. Intensity is expressed in Roman numerals or whole numbers (e.g., VI or 6). Although there is only one magnitude for a specific earthquake, there may be many values of intensity (damage) for that earthquake at different sites.

A magnitude 6.0 quake releases approximately as much energy as 6,270 tons of TNT, a magnitude 7.0 199,000 tons, a magnitude 8.0 6.27 million tons and a magnitude 9.0 99 million tons. Of course, all that energy is not focused in one particular spot, but spreads out in waves.

What factors influence what you feel in an earthquake?

There are three major factors: magnitude, your distance from the fault, and local soil conditions. Magnitude is discussed above. As for distance, the seismic waves that cause the shaking become less intense farther from the fault. Certain soil conditions amplify the shaking; generally, the looser the soil, the greater the amplification. Although most of San Francisco escaped serious damage in Loma Prieta, those with unconsolidated landfill or soft soils (such as the Marina District) suffered serious damage. The ground motion in such areas was 10 times stronger than at neighboring sites on rock.

What is a tsunami?

Sometimes called seismic sea waves (or, incorrectly, tidal waves), a tsunami is a series of waves generated by large earthquakes that create vertical movement on the ocean floor. Tsunamis can reach more than 50 feet in height, move inland several hundred feet and threaten life and property. Often, the first wave of a tsunami is not the largest. Tsunamis can occur on all coastal regions of the world, but are most common along margins of the Pacific Ocean. Tsunamis can travel from one side of the Pacific to the other in a day, at a velocity of 600 miles an hour in deep water. A locally generated tsunami may reach the shore within minutes.

Can earthquakes be prevented?

There's no way to stop an earthquake, but there are ways to build safer buildings and structures and otherwise be prepared for them. Building codes in California are updated often as new information comes in. While earthquakes are a deadly threat, there have been few earthquake-related deaths in California relative to places with less stringent codes or enforcement, such as Turkey and China.

Sources: California Geological Survey/California Geology, USGS, Arkansas Center for Earthquake Education and Technology Transfer, Governor's Office of Emergency Services, Federal Emergency Management Agency, National Science Foundation, Southern California Earthquake Center, American Red Cross, Center for Earthquake Research and Information/University of Memphis.

Earthquake Myths

If the Earth moves beneath our feet, is it because Atlas shrugged? In a bygone era, some folks may have believed that theory, but even today there are earthquake anecdotes that aren't exactly the stuff of modern science. Let's take a look at them:

MYTH: Dogs and other animals can “sense” when an earthquake is going to strike

It's impossible to determine whether a dog is behaving in an unusual manner because it smells an earthquake coming or a cat across the street. Changes in animal behavior sometimes have been observed prior to earthquakes, but that behavior is not consistent, and sometimes there's no perceptible behavior change prior to an earthquake.

MYTH: Earthquakes occur during "earthquake weather"

The common misconception that earthquakes occur during hot and dry weather dates to the ancient Greeks. Earthquakes take place miles underground, and can happen at any time in any weather.

MYTH: Big earthquakes always occur early in the morning

Just as earthquakes don't care about the weather, they can't tell time. The 1940 Imperial Valley quake was at 9:36 p.m., the 1989 Loma Prieta quake at 5:02 p.m. People who perpetuate the time and weather myths tend to remember the earthquakes that fit the pattern and forget about the ones that don't.

MYTH: California could fall into the sea because of an earthquake

The San Andreas Fault System is the dividing line between two tectonic plates. The Pacific Plate is moving in a northwesterly direction relative to the North American plate. The movement is horizontal, so while Los Angeles is moving toward San Francisco, California won't sink. However, earthquakes can cause landslides, slightly changing the shape of the coastline.

MYTH: The ground can open up and swallow people

You've seen the image in books, movies and TV shows. That's not how it works. If a fault could open up, there wouldn't be any friction. Without friction, there's no earthquake. But earthquakes cause settling and other ground deformation that can include open fissures into which people, cars, etc., can fall.

MYTH: The safest place to be in an earthquake is under a doorway

That's true only if you live in an unreinforced adobe home. In a modern structure the doorway is no stronger than the rest of the building. Actually, you're more likely to be hurt (by the door swinging wildly) in a doorway. And in a public building, you could be in danger from people trying to hurry outside. If you're inside, get under a table or desk and hang on to it.

MYTH: Small earthquakes keep big ones from happening

Each magnitude level represents about 31.6 times more energy released. It takes 32 magnitude 3s to equal the energy released in a magnitude 4, 1,000 magnitude 3s to equal a magnitude 5 … and a billion magnitude 3s to equal a single magnitude 9. So while a small quake may temporarily ease stress on a fault line, it does not prevent a large temblor.

MYTH: The magnitude of an earthquake determines whether disaster assistance is forthcoming

A magnitude 7 quake in the middle of the desert is likely to do less damage than a magnitude 6 in downtown Los Angeles or San Francisco. It is the magnitude of the damage, not the earthquake, which determines the level of response.

MYTH: We have good building codes, so we must have good buildings

That's true -- provided you're talking about buildings constructed under current building codes. In the case of older buildings, retrofitting -- bringing the building up to modern standards -- is up to the building's owners. There are plenty of buildings in areas of California prone to seismic activity which were built under older codes.

MYTH: Earthquakes are becoming more frequent

Research shows that earthquakes of magnitude 7.0 or greater have remained fairly constant throughout recorded history and have actually decreased in recent years. However, since there are a greater number of seismological centers and instruments capable of locating many small earthquakes that went undetected in earlier years, it may seem as if there are more.

MYTH: There's nothing we can do to protect ourselves from earthquakes

It's true that earthquakes can't be stopped, but you can be prepared. You can put together an earthquake kit (food, water, flashlight, etc.), practice "drop, cover and hold on" drills at home with your family and at work, and develop an earthquake plan (where would you meet family members if you weren't together when an earthquake hit?). Also, remember that standard homeowners insurance doesn't cover damage from earthquakes. For information about earthquake insurance, visit the California Earthquake Authority.

Sources: California Geological Survey/California Geology, USGS, Arkansas Center for Earthquake Education and Technology Transfer, Governor's Office of Emergency Services, Federal Emergency Management Agency, National Science Foundation, Southern California Earthquake Center, American Red Cross, Center for Earthquake Research and Information/University of Memphis.