The tectonic environments of seismically active and inactive areas along the San Andreas fault system

Abstract

On several recent occasions the author has made the statement that in California the geologist is in a somewhat better position than is the seismologist to delineate areas of high seismic hazard from great earthquakes, simply because of the inadequacy of the statistical sample based on the relatively short seismic history as compared to the much greater time span that the geologist is able to interpret from the Quaternary geologic history. A number of recent studies have pointed out the lack of correlation between areas of current seismic activity and areas of great earthquakes in the historic past (Niazi, 1964; Allen and others, 1965; Ryall and others, 1966; Brune and Allen, 1967a). The purpose of this paper is to go one step further and ask if the geologist can say anything about the maximum size of earthquakes that might be generated in any area on the basis of its distinctive geologic features. More specifically, are different segments of the San Andreas fault-which currently show markedly varying mechanisms of strain release ranging from continuous creep to infrequent great earthquakes--characterized by contrasting geologic features that might suggest that these differences in current behavior are permanent rather than temporary characteristics of these individual fault segments? That is, are there geologic features that might somehow allow us to predict that some segments of the fault will be the repeated sites of great earthquakes, whereas other segments might never experience great earthquakes but instead be characterized by continuous creep or by numerous smaller shocks? If those same segments of the fault that show distinctive strain-release characteristics also show distinctive geologic characteristics, then one must suspect that the answer to this question is indeed "yes," although such an affirmative answer must be contingent on a demonstration that this is mechanically reasonable and possible.

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