Aftershocks of the magnitude 6.5 earthquake of 9 April 1968 near Borrego Mountain, California, were recorded by 20 portable seismographs during a two-month period beginning two days after the main shock. Near the end of the recording period, three explosions were set off in the region to obtain station time corrections and a crustal model for use in locating the aftershocks. Ten truck-mounted seismic-refraction units supplemented the existing seismograph network for the shots. The data were analyzed by the time-term method using Pg arrivals in the interval 15 to 82 km. The time-term values, which range from 0 to 1 ¼; sec, correlate well with the lithologic foundation type: low values for stations on or near pre-Tertiary granitic rocks, intermediate values on Tertiary sediments, and high values on Quaternary alluvium. A consistent relationship is exhibited between the time terms and the basement depths as determined from other seismic-refraction studies and well data. This suggests that basement depths throughout the region can be estimated from time terms. A velocity of 5.93 km/sec was found for Pg waves. Later arrivals indicate the presence of an intermediate layer at a depth of 14 km.

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