How to Respond to Earthquake Advisories

You may hear a variety of advisories and predictions of future earthquakes.
When deciding what action you should take:

• Determine whether the statement was made by a scientist from a reputable organization.
• Ask whether the statement has been reviewed and endorsed by the National and the California Earthquake Prediction Evaluation Councils.
• Evaluate how much risk you and your family are likely to face during the anticipated earthquake.

A primary goal of continuing research on earthquakes is to increase the reliability of probability estimates, especially to narrow the time period during which an earthquake is anticipated. For example, scientists would like to be able to specify a high probability for a specific earthquake on a particular fault during a particular year.

Some data suggest that scientists may eventually be able to predict not only the location, but the specific time when an earthquake is likely, hours to weeks in advance. When this type of information becomes available for an earthquake in California, it will be reviewed by the California and National Earthquake Prediction Evaluation Councils. Such review involves a thorough examination of the method and of the data. If the prediction is found to be reliable, the California Office of Emergency Services will issue the prediction. In California, public safety agencies only respond to predictions reviewed and endorsed by established scientific panels.

Even though specific predictions of earthquakes are not yet possible, potential earthquake hazards have been described in advisories issued by the California Office of Emergency Services and will be issued in the future.

• On June 27, 1988, and August 8, 1989, there were two earthquakes of magnitude 5.0 and 5.2 near the Santa Cruz Mountains segment of the San Andreas fault. In both cases, scientists were concerned that these events could be foreshocks to a larger earthquake because of their magnitude and their position. Foreshocks do occur less than 5 days before about half of the large earthquakes in California. For these reasons, the California Office of Emergency Services issued an advisory of an increased likelihood of a major earthquake within the next 5 days following those quakes. The Loma Prieta earthquake of October 17, 1989, was a little late, but it was the quake anticipated by the advisories.
• On April 4, 1990, a magnitude 4.5 earthquake shook the region near Walnut Creek, California. Scientists were concerned that this earthquake could be a foreshock to a magnitude 6.5 quake on the Calaveras fault. They decided, however, that this event, and two others like it on April 27, were probably not foreshocks because of their location and the sequence of the many smaller earthquakes that accompanied them. No advisory was issued and no major earthquake has hit yet in this region.

We cannot now predict earthquakes, nor can we control them, but we have some control over how much damage will result. We still have a great deal to learn about earthquakes, the response of buildings and other structures to earthquakes, and ways to reduce earthquake damage; yet enough is known already that each of us can take action now to reduce earthquake hazards.

Aftershocks

In the weeks and months after a strong earthquake, there will be many aftershocks, some strong enough to cause additional damage to structures already weakened in the main shock. A magnitude 7 earthquake in California is typically followed by about six aftershocks of magnitude 5 or larger. Most of these aftershocks strike during the first week, but some are possible as much as 3 to 6 months later. Because strong aftershocks impose additional hazards and may seriously affect emergency response efforts, scientists at the U.S. Geological Survey in California monitor aftershocks closely and regularly issue forecasts about the probability of large aftershocks in the near future. Following the Loma Prieta earthquake, the USGS was able to transmit radio signals at the instant large aftershocks struck, providing warning to rescue crews tens of miles away several seconds in advance of the onset of strong shaking. Such a warning is possible because radio waves travel much faster than earthquake waves. Because of the potential for large aftershocks, removal of belongings from damaged buildings may have to be delayed.