An active experiment was attempted in the San Jacinto fault zone near Anza, California, during the summer of 1969 for the purpose of measuring the tectonic stress which is believed to exist there. The experiment consisted of a “trigger experiment” and a “shaker experiment.” In the former, we measured the near field and far field of seismic waves from a buried explosion in an attempt to detect a microearthquake which might have been triggered by the explosion. The radiation pattern of seismic waves from the microearthquake should indicate the nature of existing tectonic stress. In the shaker experiment, a precise measurement of change of apparent seismic velocity was made before and after the explosion in the nearby granitic rocks by the use of a special hydraulic oscillator. Explosives totaling 1000 lb were detonated in four holes, 20 m deep, and spaced linearly 20 m apart. The results were negative, indicating that the above size of the experiment is not large enough for our purpose, and also that rocks at shallow depths in the fault zone are cracked in place and may not be capable of sustaining tectonic stress.

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