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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.
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.
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.
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).
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.
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.
Uniform California Earthquake Rupture Forecast Version 3
Latest Earthquakes - Worldwide
Realtime Seismogram Displays (USGS)
USGS Realtime Feeds and Notifications
California Earthquake Information (USGS)
Did You Feel It? (USGS)
CGS Information Warehouse (Maps and Reports)
Ready.gov - Tribal Leaders (FEMA)
Mobile Homes in Earthquakes: How to protect your home and family
CGS Kids GeoZone
The Science of Earthquakes (USGS)
Earthquake Topics for Education (USGS)
Earthquake Publications for Teachers and Kids (FEMA)
ABC’s of Post EQ Evacuations – A Checklist for Administrators and Faculty (CalOES)
Resources for K-College Teachers (USGS)
Learn About Quakes and Get Prepared! (ABAG)
Non-structural Earthquake Hazards in California Schools (CalOES; DGS, Div. of State Architect; Seismic Safety Commission; Dept. of Education)
Ready Business - Prepare, Plan, Stay Informed (FEMA)
Helping Businesses Become Disaster Resilient (BICEPP)
United States Small Business Administration
Protect Your Business (FEMA)
Emergency Preparedness Checklist for Small Businesses (FedEx and ARC)
Emergency Preparedness Resources for Business (FEMA)
Open For Business EZ Toolkit (Insurance Institute for Business and Home Safety)
ShakeOut Publications, Multimedia Products, Flyers and Posters
Earthquake Country Alliance
Helping Children Cope with Disaster (FEMA/American Red Cross)
Earthquake Publications for Individuals and Homeowners (FEMA)
Earthquake Publications for Community Planners and Public Policy Makers (FEMA)
California Earthquake Preparedness Survey (CEPS) The Study of Household Preparedness:Preparing California for Earthquakes (Alfred E. Alquist Seismic Safety Commission)
California Earthquake Preparedness Survey (CEPS) Recommendations to Guide Future State Preparedness Efforts
California Earthquake Preparedness Survey (CEPS) Press Release, March 5, 2010
California Earthquake Preparedness Survey (CEPS) Findings
California Earthquake Preparedness Survey (CEPS) Facts
CGS Seismic Hazard Zonation Program
California Geological Survey Earthquake Program
Business and Industry Council for Emergency Planning and Preparedness