Experiencing an earthquake can be frightening and confusing. Knowing what just happened can reduce our fear and help us understand what to expect next. This page describes information that will be available from various organizations after an earthquake, and how you can also provide valuable information.
Modern seismic networks can automatically calculate an earthquake's magnitude and location within a few minutes. Local networks of the Advanced National Seismic System (ANSS) have websites with automatically generated maps and lists of recent earthquakes in their region.
For recent Southern California earthquakes, visit the Southern California Earthquake Data Center at www.data.scec.org.
Because waves from large earthquakes travel throughout the world, networks both near and far will calculate the magnitude and location of an earthquake. These networks will sometimes report different magnitudes for the same earthquake, because of differences in seismometers and techniques. This has become less likely as moment magnitude becomes more commonly used (see Locating and Measuring Earthquakes).
The ShakeMap and "Did You Feel It?" maps shown on this page express the level of shaking experienced in terms of a range of intensities similar to the Modified Mercalli Intensity Scale. While magnitude describes the total energy released by the earthquake, intensity describes the level of shaking produced by the earthquake at a certain location. A single earthquake will have one magnitude value but will have many values for intensity, usually decreasing with distance from the epicenter. ShakeMap uses instruments to measure this shaking, while "Did You Feel It?" uses input from people about how strongly they were shaken and observations of how much damage was caused. Both systems map shaking according to increasing levels of intensity that range from imperceptible shaking to catastrophic destruction. The level of intensity is designated by Roman numerals.
Modern seismic networks, with digital instruments and high-speed communications, have enabled seismic data to be used in new and innovative ways. A product of these new networks is ShakeMap, which shows the distribution of ground shaking in a region. This information is critical for emergency management. ShakeMaps are automatically generated and distributed on the Internet for most felt earthquakes (visit www.cisn.org/shakemap/sc/shake to view maps for Southern California earthquakes). This information may save lives and speed recovery efforts.
ShakeMap was first developed for Southern California as part of the TriNet Project, a joint effort by the U.S. Geological Survey (USGS), California Institute of Technology (Caltech), and the California Geological Survey (CGS).
Not long ago, the first thing that most people did after feeling an earthquake was to turn on their radio for information. Now many people are getting this information via the Internet and are also sharing their experience of the earthquake online. "Did You Feel It?" is a web site developed by the USGS (and regional seismic networks) that allows people to share information about the effects of an earthquake. Visitors to the site enter their ZIP code and answer a list of questions such as "Did the earthquake wake you up?" and "Did objects fall off shelves?" These responses are converted to equivalent intensities for each ZIP code and within minutes a map is created on the Internet that is comparable to ShakeMaps produced from seismic data. The map is updated frequently as thousands of people submit reports. Such "Community Internet Intensity Maps" contribute greatly in quickly assessing the scope of an earthquake emergency, especially in areas lacking seismic instruments. To report your experience of an earthquake, visit earthquake.usgs.gov/eqcenter/dyfi.php.