A simple, uncontrived model of the rupture during the San Fernando earthquake can explain the main features of the particle velocity traces derived from the accelerograms recorded at Pacoima Dam. This result, combined with the probable small effect of surface topography on the velocity traces, strengthens the case for acceptance of the peak particle velocity at Pacoima Dam (115 cm/sec) as a valid ground-motion parameter for design purposes in earthquake engineering.

Most of the conspicuous motion on the velocity traces during the first 4 sec after triggering seems to result from thrust faulting, starting at a focal depth within several kilometers of 14 km, on a fault surface dipping at least 50° and extending only part way to the surface at a velocity near 2.5 km/sec. The data also indicate that this faulting continued to the surface at a slower rupture velocity (less than 2 km/sec) along a less steeply dipping surface.

The amount of relative offset across the fault surface is difficult to determine, both because of inherent limitations in the two-dimensional model and because of nonuniqueness in the fitting of the data. The estimates of this dislocation, however, are consistent with the wide range of values reported by other authors in studies using various types of data. The data are also consistent with a model suggested by Alewine and Jordan (1973) and Trifunac (1974) in which the total dislocation has a minimum near the center of the fault surface, with approximately equal amounts of total offset on the fault near the hypocenter and near the Earth's surface.

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