Progress Report, Seismological Laboratory, California Institute of Technology, 1953

Changes in our seismograph installations during the past year are as follows: Barrett was equipped with three standard short‐period and three standard long‐period Benioff seismographs, one vertical‐component short‐period Benioff seismograph with low magnification, and one standard N‐S torsion seismograph. Woody now has one standard short‐period vertical Benioff seismograph and one standard E‐W torsion seismograph. Isabella, located at Φ = 35° 38.6′, λ = 118° 28.6′, h = 760 m, was established as a permanent station with one short‐period Benioff vertical‐component seismograph. The preliminary curve of the elastic strain rebound characteristic included in our previous report based on a small number of aftershocks has been substantially modified during the past year as a result of additional data on over 200 shocks measured and located by Richter and assistants. Benioff finds that shocks originating southeast of the fault trace form a compressional release series of the form a + b log t, whereas those on the northwest side form a shear series of the form A + B [1 ‐ exp(−αt 0.5) ]. The characteristic to January 15, 1954, is shown in Figure 1. In the original theory of aftershock generation, Benioff assumed that aftershocks occur along the principal fault. The observed evidence from the Kern County sequence indicates that aftershocks occur on auxiliary faults within the original strain zone at least as much as on the principal break. In the opinion of Benioff and Gutenberg, some aftershocks may possibly originate as slips between approximately horizontal zones in which the mechanical properties differ. The delay in onset of the shearing phase, also observed in other sequences, still remains unexplained.