Abstract

The Desert Hot Springs earthquake of December 4, 1948, was one of the larger recorded earthquakes of southern California, and its aftershocks have continued into 1957. The assigned epicenter is 33°56'.4N,116°23'.1 W; origin time, 15:43:16.7 P.S.T.; magnitude 6 1/2. Arrival times at local and distant stations are consistent with existing travel-time curves, except for anomalous S – P intervals at very near-by temporary stations; these unexplained anomalies cannot be attributed to varying depth of focus.

Epicenters of the 72 aftershocks that have been accurately located are concentrated in a zone 18 km. long, parallel to the Mission Creek fault trace indicated by older scarps, but 5 km. north of it. Aftershock activity is markedly concentrated toward the two ends of this line. Location of the main shock suggests that fracturing started near the southeast end and progressed northwest-ward. The ground surface was not broken, except by landslides.

Offset of the line of seismic activity from the trace of the Mission Creek fault suggests that the fault plane dips north. This attitude is substantiated not only by field observations, but also by first motions at stations within 6° of the epicenter, which require a combination of thrust-slip and right lateral-slip on a fault dipping north less than 66°. Inasmuch as this fault is not parallel to regional San Andreas trend, such oblique displacement is reasonable and is consistent with the tectonic pattern of other faults in this region.

Five groups of earthquakes represent more than 85 per cent of the total strain release since 1933 in the 3,000 sq. km. area surrounding Desert Hot Springs. These earthquakes, in addition to the Desert Hot Springs shock, are: Morongo Valley (1947), Kitching Peak (1944), Covington Flat (1940), and San Gorgonio Mountain (1935); all are associated with known faults. The Morongo Valley earthquakes probably represent fracturing on the segment of the Mission Creek fault adjacent to that broken during the subsequent Desert Hot Springs shock.

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