At 5:04 P.M. on Tuesday, October 17, 1989 local time (10/18/89 00:04:15.23 UT) a large earthquake ruptured a 40-km segment of the San Andreas fault in the Santa Cruz Mountains in northern California. The magnitude Ms was calculated at 7.1 by the National Earthquake Information Service using data from 18 stations. This report is based on information made available to geophysicists at the C. F. Richter Seismological Laboratory at the University of California at Santa Cruz (UCSC), as of 10 days following the main shock.The Santa Cruz Mountains (Loma Prieta) earthquake was the most severe in the continental U.S. since 1952, when a very large earthquake (Ms = 7.7) broke along the White Wolf fault near Bakersfield, Calif. It was the largest event on the San Andreas since the 1906 (M = 8.3) San Francisco event.