New Map of Daily Probability of Earthquake Shaking in California

The U.S. Geological Survey (USGS) has announced the release of new public web pages that show the probability of earthquake shaking in the next 24 hours in California. These maps graphically illustrate the change in earthquake probability during aftershock and possible foreshock sequences.

The maps are not intended to be used to predict an upcoming earthquake; however, based on previous earthquake sequences, an increase in probability will be seen before about half of California’s larger earthquakes. The maps are updated at least once an hour and are available to the public at http://pasadena.wr.usgs.gov/step/.

"The new maps represent a synthesis of our knowledge about earthquakes in California," said Dr Lucy Jones, Scientist-in-charge of the USGS Earthquake Hazards Program in Southern California. "They factor in our knowledge of faults, underlying geology and expected rates of earthquake aftershocks."

In almost all cases, said Jones, the probabilities on this map will be very low. The background probability for strong to moderate earthquake shaking in most of California is between 1-in-10,000 and 1-in-100,000. By comparison, the average American’s risk of being in a car accident in any 24-hour period is 1-in-2,500. "The only time that probabilities become large enough to cause concern is after a significant earthquake that may have already caused damage," explained USGS seismologist Dr Matt Gerstenberger. "Aftershocks are likely in this situation, and the new maps show where those aftershocks are most likely to be felt and how the hazard changes with time."

In the system used to create the maps, the probability of strong earthquake shaking (with a Modified Mercalli Intensity of VI or greater) in the next 24 hours is calculated, based upon the known behaviour of aftershocks and the possible shaking pattern predicted from historical patterns on the fault or in the area. The system considers all earthquakes, large and small, that are recorded by the California Integrated Seismic Network (CISN), the California element of the Advanced National Seismic System (http://pasadena.wr.usgs.gov/recenteqs/. These probabilities apply both to aftershocks smaller than the first event and to the possibility that the 'aftershock' will, in fact, be larger than the first event – in which case it becomes a 'foreshock'.

This product has been reviewed by the California Earthquake Prediction Evaluation Council, which determined that this methodology represents the state of the art in evaluation and illustration of aftershock probabilities. Implementation of these maps for other areas outside California depends upon both a robust real-time seismic network and the research to establish the average rate of aftershocks for that area, Jones said.