Earthquake early warning is the rapid detection of earthquakes, real-time assessment of the shaking hazard, and notification of people prior to shaking. Warning times range from a few seconds to a few minutes depending on your location and how big the earthquake is. The further you are away from the epicenter, the more warning time. The bigger the earthquake, the stronger the shaking at greater distances. An early warning should tell you how strong the shaking will be at your location, and how long until that shaking starts (the warning time).
An earthquake early warning system can provide a few seconds to tens of seconds warning prior to ground shaking during an earthquake. The information can be used to reduce damage, costs and casualties in an earthquake. Earthquake early warning systems are currently operating in Mexico, Taiwan and Japan, but not in the United States. In California, the California Integrated Seismic Network (CISN) is now operating a prototype production early warning system, called ShakeAlert.
The objective of earthquake early warning is to rapidly detect the initiation of an earthquake, estimate the level of ground shaking to be expected, and issue a warning before significant ground shaking starts. This can be done by detecting the first energy to radiate from an earthquake, the P-wave energy, which rarely causes damage. Using P-wave information, we first estimate the location and the magnitude of the earthquake. We use this to estimate the anticipated ground shaking across the region to be affected. The method can provide warning before the S-wave, which brings the strong shaking that usually causes most of the damage, arrives.
The CISN's tests of earthquake early warning in California have demonstrated that the warning time ranges from zero seconds to a few tens of seconds, depending on the distance to the epicenter of the earthquake. This is enough time to slow and stop trains and taxiing planes; to prevent cars from entering bridges and tunnels; to move away from dangerous machines or chemicals in work environments, and to take cover under a desk; or to automatically shut down and isolate industrial systems. Taking such actions before shaking starts can reduce damage and casualties during an earthquake. It can also prevent cascading failures in the aftermath of an event. For example, isolating utilities before shaking starts can reduce the number of fire initiations.
Image: The ShakeAlert early warning demonstration system caught the M 6.0 South Napa earthquake of 2014. This map shows the distribution of ground shaking intensity predicted using the first few seconds of data recorded by seismometers near the epicenter. The data used to generate this map was available a few seconds before the shaking was felt in San Francisco.
An end-to-end system for generating earthquake early warnings is currently being tested by the CISN as part of the real-time earthquake information system in California. From this "pre-prototype" implementation, the timeliness and accuracy of the warnings generated are being evaluated. Similar warnings will be provided when earthquake early warning is implemented for general use in California. The project partners are the California Institute of Technology, the University of California Berkeley, the Eidgenossische Technische Hochschule Zurich (Switzerland), the Southern California Earthquake Center, and the U.S. Geological Survey. The project is being funded by the U.S. Geological Survey. In the first phase of the project, three algorithms were tested in the CISN real-time environment: On-site, the Virtual Seismologist, and ElarmS. We are now in the second phase, the implemention and evaluation of an end-to-end real-time system that will quickly produce and update earthquake alerts after an earthquake has started. This project is a step on the path to the future, full implementation of earthquake early warning.
The public dissemination of earthquake early warning alerts is not part of the current project. There are two main reasons for this. The first is that there are many areas in California where there are not enough seismic stations to recognize and characterize a newly starting earthquake so quickly that an early warning would be possible. The second reason is that for early warning alerts to be useful, people, companies and institutions must know beforehand what they will do when they receive the information. This is an outreach effort that has not yet been part of the project due to the limited funding currently available.
Eventually, earthquake early warning alerts will arrive by all means possible - through email, applets, radio, and television, and by computer-to-computer messages for automatic control of systems like trains and production facilities.