Looking Back for a Long-Term View

While it may not help produce the kinds of forecasts that could give warning to an area about a specific earthquake immediately prior to its occurrence, one means of anticipating the seismic future of an area is to generate a long-term outlook for the rate of major earthquakes.

To do this requires information about the earthquake history of an area. What has happened in an area is usually a dependable guide to what will follow in the future. Following this principle, it makes sense to thoroughly examine the seismicity record present in the earthquake catalogs for a given area. For southern California, we have the catalog of events recorded by instruments in the Southern California Seismic Network (SCSN).

	Damage to the Pomengia Building in San Jacinto, caused by the M 6.5 earthquake on Christmas morning, 1899. (Photo: E.W. Claypole)

You may notice, upon studying earthquake catalogs, that you find fewer and fewer records of earthquakes as you look back in time. Modern networks of seismic instruments are generally larger and more precise than their earlier counterparts; certainly, this is true in southern California. Therefore, you will find fewer earthquake records in the past, and will probably notice that the smallest events recorded back then were generally larger than the smallest recorded today.

When you try to study earthquakes even farther back in time, you run into an even greater challenge -- the instrumental records disappear altogether! In southern California, the records in the SCSN earthquake catalog date back only to 1932. And yet, you can find information about the magnitude and location of events like the San Jacinto earthquake of 1918, or even the great Fort Tejon earthquake of 1857. How can this be? Earthquake catalogs are a wonderful research tool, but only as far back as instruments have been recording earthquakes. What do we do to study activity before that time?