San Fernando rupture mechanism and the Pacoima strong-motion record

Abstract The extent of fault rupture in the main San Fernando earthquake of February 9, 1971 is well determined. This paper explains the time history of the unique Pacoima accelerogram in terms of the rupture mechanism and elastic wave theory. The Pacoima strong-motion record was obtained on the crystalline rocks of the San Gabriel Range which were thrust southward. The three recorded components are resolved along the strike, dip and normal to the fault plane. Filtered accelerograms are given for 0.5, 0.75, 1.0, 1.5, 2, 4, and 10 Hz. These time histories are very different. The first half of the records (with the longer spectral components) show clearly P and S onsets from the focus, and the fault dislocation passing with average rupture velocity 3.0 km/sec under Pacoima. The latter part of the record consists predominantly of 1.5 to 10 Hz waves which cease 9 to 10 sec after P . These waves are the superposition of high-frequency waves (mainly S ) which propagated with little attenuation through the crystalline crustal rocks. As the rupture progressed, waves were produced in a random manner as bursts of high-frequency energy near the rupture front.