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About earthquakes

The California Governor’s Office of Emergency Services, Earthquake, Tsunami and Volcano Program is continuously researching, learning, and developing new ways to improve the safety of California’s residents and visitors before disaster strikes. Through planning and improving building codes, transportation, communications, and education, our staff works with our partners to prepare the people of California to decrease the loss of life, protect the environment, and property. If you live in an area that can be impacted by an earthquake, preparation is vital, as these events can strike suddenly, at any time, and anywhere. Visit My Hazards to see if you live or work in an area prone to earthquakes.

What causes earthquakes?

The earth has four major layers: the inner core, outer core, mantle and crust. The two outer layers make up a thin skin on the surface of our planet. This skin is made up of many pieces like puzzle pieces (known as plates) covering the surface of the earth. These puzzle pieces continuously move around, slowly sliding past one another and bumping into each other. The surface where the pieces slip and slide is called the fault or fault plane. The boundaries of these pieces are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they can get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is a sudden release of energy which we call an earthquake.

Sometimes there are smaller earthquakes that happen before a larger earthquake in the same location (called foreshocks). Scientists can’t tell if an earthquake is a foreshock until a larger earthquake happens. The largest earthquake in a series is called the mainshock. All mainshocks are followed by numerous aftershocks (smaller earthquakes in the same place as the mainshock). After a large earthquake, aftershocks can continue for weeks, months, and even years.

Earthquake Magnitude

The magnitude of an earthquake describes its size. There is only one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

The magnitude of an earthquake depends on the size of the fault and how far the plates move. This is not something scientists can measure with a ruler or measuring tape, since faults are deep beneath the earth’s surface. So how do they measure an earthquake? They use recordings (known as seismograms) made on the surface of the earth to measure motion and determine how large the earthquake was.

The recordings look like a series of wiggly lines that appear during ground vibration (see image below). A series of short wiggly lines means a small earthquake, and a series with long wiggly lines means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.

Seismogram at Weston Observatory

The magnitude of an earthquake describes how much energy was released based on the maximum motion recorded. There are different scales used to convey this information, but the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally.

Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released).

Earthquake Intensity

Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment. This means, how strong an earthquake FEELS to people in a certain location or was there damage to buildings or other structures. Intensity is measured using a descriptive scale called the Modified Mercalli Intensity Scale. One earthquake will have numerous values for intensity, as it will feel stronger and cause more damage in areas near the epicenter and lessen for areas further away. Information gathered from people near the earthquake, combined with damage reports, is gathered by zip code and assigned an intensity value (see chart below). This information is then mapped to get an overall picture of the earthquake's strength.

Real Time Earthquake Information

As soon as an earthquake shakes, an enormous amount of information is gathered from people, equipment and first response organizations. This information is interpreted by scientists and computers, and then a number of products are available from warnings to maps of the impacted area. Many of these products are available in seconds or minutes, what is referred to as “real-time”. Alerts may even be produced before your location is impacted (see earthquake early warning).

In most cases, you can sign up to receive real time information through your cell phone, social media, email, etc. Check with your local county emergency management office (see Tools and Resources below for websites) to see how to sign up. Other information is accessible virtually immediately online, on television or on the radio.

Did You Know?

Doorways are no stronger than any other part of a structure so don’t rely on them for protection! During an earthquake, get under a sturdy piece of furniture and hold on. It will help shelter you from falling objects that could injure you during an earthquake.
To date, no one has predicted an earthquake. To do so requires to identifying the fault, giving the magnitude, and limiting the time period when the event would occur.
Most homeowners insurance does not cover damage caused by earthquakes. Read your policy carefully. Go to the California Earthquake Authority for more information on obtaining earthquake insurance in California. Emergency loans or grants from the Federal Emergency Management Agency (FEMA) do not take the place of earthquake insurance. Read up on it at FEMA.gov.
Mobile homes and homes not attached to their foundations are at particular risk during an earthquake.
Buildings with foundations resting on landfill and other unstable soils are at increased risk of damage.
Using a cell phone takes up the same bandwidth as 800 text messages. After a disaster, use of cell phones can shutdown wireless phone service and prevent 911 calls from getting through. To communicate after an earthquake, send a text, don’t call. Even if your text gets a “busy signal” the phone will continue attempting to send the message.