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Earthquakes in Southern California with geologic relations

Part two

SEISMOLOGICAL LABORATORY, PASADENA, CALIFORNIA

Abstract

The contents of the paper have been summarized in the Abstract. There is needed only a brief résumé of the gist of the study and its conclusions.

Only tentative conclusions are warranted. The earthquakes instrumentally registered in the Southern California region over a span of less than twenty years, with several interruptions (especially in the earlier years), exhibit a very uneven geographic distribution with respect to number, and also to size (and strength) and thus to total energy.

The larger shocks appear definitely to be associated with the major geologic faults (and so do many of the smaller shocks as well), and this relation is considered causal, slipping along these faults giving rise to the vibration in such earthquakes.

However, a great many others of the smaller shocks do not cluster along any of the mapped faults, but exhibit a random distribution, singly or in groups.

There is strong suggestion that most of all the shocks, both large and small, originate within a narrow range of depth at about 18 kilometers beneath the surface, and this is considered to be near the base of the "granitic" layer here (except in some parts of the region beneath the higher mountains). Though the evidence for this suggestion is strong it cannot be considered sufficient for proof. Notwithstanding this, such a concentration of the places of origin at such a depth appears so probable that it has led the writer to the hypothesis that many of the small shocks originate there as a result of small slips over small areas along nearly horizontal planes with migration of the successive slippings from one small area to another and another neighboring one, with resulting transfer of rock material at depth in such a way as to build up compression and shear along the major faults. With such a mechanism the small shocks in question need have no relation of clustering to these major faults, or to any of the mapped faults.

In order to test either this hypothesis, or other or earlier views, the need is clearly indicated or still more intensive investigation with seismologic stations spaced more closely.

This article has been cited by other articles:

				C. R. ALLEN, P. ST. AMAND, C. F. RICHTER, and J. M. NORDQUIST Relationship between seismicity and geologic structure in the Southern California region Bulletin of the Seismological Society of America, August 1, 1965; 55(4): 753 - 797. [Abstract] [PDF]

				P. DEHLINGER Shear-wave vibrational directions and related fault movements in Southern California Earthquakes Bulletin of the Seismological Society of America, April 1, 1952; 42(2): 155 - 173. [Abstract] [PDF]