The Borrego Mountain earthquake of 9 April 1968 produced ground shaking having 5 per cent g maximum acceleration at the 450 MWe San Onofre Nuclear Generating Station site. The recorded motion showed two distinct phases separated by about six seconds. The spectra of these phases differ markedly, presumably reflecting differences in source generation. There was no damage at the plant. More recently, a special seismic central accelerograph system has been installed to record the earthquake-induced motion of the concrete base of the containment structure and the response motion of the steam generators and pressurizer. Experience with the installation and operation of the expanded accelerograph system is reviewed with suggestions offered for alleviating problems due to system malfunction.

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