Summary and Conclusions
Data were assembled of about 320 earthquakes, including nearly all those available for which the determinations of epicentral location, time of occurrence, and focal depth were considered reliable. They were analyzed collectively and in selected geographical groups by the method of periodogram analysis which appeared most suitable. Tests were made on periods of 6, 12, and 14 months and for correlation with twice the lunar and solar hour angles. None of the results could be interpreted as other than accidental. However, the Japanese group gave a slight indication of a correlation with twice the solar hour angle, and the South American earthquakes with twice the lunar hour angle. Many additional data must first be accumulated and tested before any special significance can be attached to these results.
The manner whereby small fluctuations in stress might be expected to determine the exact time of occurrence of an earthquake was reviewed. A study was then made of three possible sources of such trigger stresses which could be effective at great depth, namely, tidal stresses in the solid earth, stresses due to changes in sea level, and stresses due to variations in barometric pressure. Tidal stresses alone were considered in detail.
It was concluded that tidal stress variations approximate harmonic changes sufficiently well to warrant a search for lunar hour angle correlations by the methods of periodogram analysis, but they are nevertheless too irregular to make the statistical results of such studies valuable in supplying information concerning the mechanics and environment of deep-seated earthquakes. Periodicity investigations of this type, even if successful, can be expected to do little more than indicate the causes which can influence earthquake occurrence times.
The importance of other agencies in causing stress variations within the earth was studied. It was concluded that oceanic tidal loads, and to a much less degree erratic barometric fluctuations, are capable of producing stress changes comparable with those due to tides in the earth. The stresses from the earth tides in general dominate the others, although the effect of ocean tidal loading may be of primary importance under special circumstances. The irregularity of both ocean loads and barometric pressure fluctuations renders them unsuitable subjects for study.
For the reasons given, the only stress changes that appear to warrant study with reference to their effect on earthquakes are those due to earth tides.
The best chance of finding a definite correlation world be from a study of data of a large number of earthquakes which had occurred within relatively small regions, as only then is it justifiable to assume that all the earthquakes had been similarly affected by the triggering stress. Data at present available are far from sufficient to provide a satisfactory basis for such a study.