Earthquakes in Southern California with geologic relations*

ABSTRACT A brief historical outline concerning the investigation is followed by description of the geologic faulting in the region. Data of the instrumentally recorded earthquakes of recent years, giving shock magnitudes and the location of epicenters, are tabulated, with notes, comments, and discussion of the quality of the location of the shocks and the incompleteness of the information. Summaries of the tabulation are made in different ways. Maps show the geographic interrelationships of the shocks tabulated, and the faults. These relationships are also discussed in the text. Earthquake magnitude is discussed with relation to the method used in mapping the epicenters, and its energy equivalence is considered. A table shows the geographic distribution of the seismic energy and the corresponding variation in the “density” of activity as well as the corresponding number of shocks. The correlation of the earthquake origins with the faulting is discussed. The prevailing depth of the origins is considered and, based upon the findings, a hypothesis is advanced to account for the random, or scatter, distribution of many small shocks away from the mapped faults—i.e., repeated, migrating slips of small area on nearly horizontal planes at or near the depth of origin, with resultant growth of regional strain. As yet this is only a hypothesis, but the importance of continuing consideration of it is emphasized. The possibility of such action elsewhere, with the production of great earthquakes having large areas of high intensity, is suggested. No basis for earthquake prediction has resulted from the investigation. Future needs are discussed briefly.