Foreshocks and Aftershocks

Abstract

There was little prelude to the major earthquake of July 21, 1952. Small shocks had occurred sporadically in the area. The one true foreshock occurred 2 hours earlier. Aftershocks were studied using seismograms from stations previously existing, from new stations set up in Kern County, and from portable seismographs operating at numerous locations for short intervals. On September 3-5, 1952, three portable units were in the field. Epicenter locations were begun assuming wave speeds determined in earlier investigations. These are consistent with the new data, so that epicenters are accurate in general within about 2 miles. However, the assumed velocities can be improved, especially at short distances. For the first 36 hours all located epicenters lie on or south of the White Wolf fault, tending to diverge from it toward Tehachapi. This agrees with the known dip of the fault. Beginning with a large aftershock after 36h 46m, aftershocks occurred both north and south of the White Wolf fault. Two large ones on July 25 northeast of Caliente were followed by many small ones from the same point, continuing for months. On the night of July 28-29 a large shock and several small ones occurred along a line parallel to the White Wolf fault but passing near Bakersfield; this is almost exactly transverse to the known surface structures there. On August 22, 1952, a shock of comparatively minor magnitude (5.8) on this line added greatly to the damage at Bakersfield. Even considering smaller shocks, epicenters of the group are confined to an area with sharp straight boundaries on at least three sides; these boundaries presumably indicate faults. To the south, the boundary runs appreciably north of the Garlock fault; westward, it lies only a few miles west of the epicenter of the main earthquake, so that the activity nowhere gets near the San Andreas fault; to the north it is marked by the line near Bakersfield. The complexity of this distribution in space and time is probably not exceptional ; but on this occasion the data are better than for any preceding major event, so that the details are established with unprecedented clearness. The mechanical unity of the whole phenomenon is indicated by a tendency for successive shocks to occur in different parts of the active area, rather than repeating from the same point; this is illustrated by a special type of scatter plot. The effect of the root of the Sierra Nevada in modifying the paths of seismic waves is shown clearly, especially in the times of arrival at the Tinemaha station. Most of the shocks have been assigned to a depth of 16 kilometers (10 miles); a large fraction, especially to the northeast, having been worked out for a depth of 10 kilometers, and others, mostly small, are still shallower. Depth determination is less accurate than epicenter location. One hundred ninety-nine shocks of magnitude 4.0 and over are listed to the end of June, 1953. Location for these is incomplete, since overlapping recording presents difficulties in the first few hours. Additional smaller shocks which have been investigated are also catalogued, bringing the listed total to 267. Twenty-one further shocks of magnitude 4.0 and over occurred to the end of June 1955 (table 1).

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