San Fernando Earthquake series, 1971: Focal mechanisms and tectonics

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Abstract
The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis from the first three months of activity, 87 events in all. C. R. Allen and his co‐workers assigned the main shock parameters: 34°24.7′N, 118°24.0′W, focal depth h = 8.4 km, and local magnitude ML = 6.4. The initial rupture location coincides with the lower, northernmost edge of the main north‐dipping thrust fault and the aftershock distribution. The best focal mechanism fit to the main shock P wave first motions constrains the fault plane parameters to: strike, N67°(±6°)W; dip, 52°(±3°)NE; rake, 72° (67°−95°) left lateral. Focal mechanisms of the aftershocks clearly outline a down step of the western edge of the main thrust fault surface along a northeast‐trending flexure. Faulting on this down step is left lateral strike slip and dominates the strain release of the aftershock series, which indicates that the down step limited the main event rupture on the west. The main thrust fault surface dips at about 35° to the northeast at shallow depths and probably steepens to 50° below a depth of 8 km. This steep dip at depth is a characteristic of other thrust faults in the Transverse ranges and indicates the presence at depth of laterally varying vertical forces that are probably due to buckling or overriding that causes some upward redirection of a dominant north‐south horizontal compression. Two sets of events exhibit normal dip slip motion with shallow hypocenters and correlate with areas of ground subsidence deduced from gravity data. One set in the northeastern aftershock area is related to shallow extensional stresses caused by the steepening of the main fault plane. The other set is probably caused by a deviation of displacements along the down step of the main fault surface that resulted in localized ground subsidence near the western end of the main fault break. Several lines of evidence indicate that a horizontal compressional stress in a north or north‐northwest direction was added to the stresses in the aftershock area 12 days after the main shock. After this change, events were contained in bursts along the down step, and sequencing within the bursts provides evidence for an earthquake‐triggering phenomenon that propagates with speeds of 5–15 km/day. Seismicity before the San Fernando series and the mapped structure of the area suggest that the down step of the main fault surface is not a localized discontinuity but is part of a zone of weakness extending from Point Dume, near Malibu, to Palmdale on the San Andreas fault. This zone is interpreted as a decoupling boundary between crustal blocks that permits them to deform separately in the prevalent crustal shortening mode of the Transverse ranges region.