1857 Quake May Be Having Ripple Effect

An Arizona seismologist reports a 130- to 150-year link among strong earthquakes in Southern California, saying that the great 1857 Ft. Tejon temblor on the San Andreas Fault may have sent a pulse of underground strain moving about a mile a year and triggering eight successive quakes along the San Jacinto Fault.

Writing in today’s issue of Science, Christopher O. Sanders of Arizona State University says the latest quake in the series was a magnitude 6.6 jolt that struck on the northern side of the Imperial Valley in 1987. He says another could occur farther to the southeast, near the Mexican border, in 20 to 30 years.

The San Jacinto Fault branches off from the San Andreas near Cajon Pass and passes through the San Bernardino and Hemet areas, bearing to the southwest of the San Jacinto and Santa Rosa mountains and continuing to the Imperial Valley. The fault was also the site of quakes, mostly in the 6-magnitude range, in 1899, 1918, 1923, 1937, 1954, 1968 and 1969.

Each was progressively farther to the southeast, and Sanders writes that there is evidence not only of a triggering mechanism from the San Andreas to the San Jacinto, but in the reverse direction as well, with large earthquakes on the San Jacinto triggering successive jolts on the Mojave segment of the San Andreas.

He adds that the magnitude 6.5 Desert Hot Springs earthquake of 1948, along the San Andreas, may have derived from a similar pulse of strain emanating from Ft. Tejon.

The Ft. Tejon quake along the Ridge Route north of Los Angeles was the most recent magnitude 8 “big one” to be centered in Southern California. Some scientists believe that another, perhaps southeast of San Bernardino, will occur in the next 25 years.