The second of two seismic field experiments designed to study the precision with which the arrival of compressional body waves could be timed over paths up to 42 km in length was performed seven months after the first at the same location in the Gabilan Range near Salinas, California. Results of the second experiment show that the timing of the compressional waves could be carried out to the same order of precision as in the first experiment, i.e., to ±1 msec. In addition, the travel times over certain path lengths increased by about 6 msec within the intervening period. This observation is discussed in terms of the possible release of stress at depth in the region adjacent to the San Andreas Fault.

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