A sidereal period in earthquake occurrences reconfirmed

Abstract

The paper by Shimshoni and Dishon is the second to be published in the Journal of Geophysical Research which comments on our original finding [Sadeh and Meidao, 1973] of sidereal periodicity in earthquake occurrences. The first paper by Hunter [1975] found the same experimental fact but argues about its statistical significance. The present paper by Michael Shimshoni and Menachem Dishon reconfirms the existence of a sidereal period but suggests that this period is due to the lunisolar tide. In our 1973 paper we discussed the possibility that the effect found is due to tides, and we ruled it out because there are at least six more tidal periods and we searched for such periods in earthquake occurrences and none were found. Especially at the strongest tidal period, the lunar period, nothing was found. A model which claims that the luni-solar tidal force can trigger aftershocks must explain why a stronger tidal force does not do it.We are unable to argue with the claim made in Shimshoni and Dishon's paper that they found periods other than sidereal periods in their analysis. We do not know how significant those periods are, how many of them are present, and what their exact period is. In Figure 10 of our 1973 paper we plotted the whole spectrum of periods between 24 h -8 min/day and 24 h + 8 min/day, and only two new periods were significant.Hunter [1975] did not find any new periods. If Shimshoni and Dishon, using the same raw data, found some new periods, they should be plotted or tabled for everyone to examine. If, for example, one of their new periods coincides with a tidal period, it gives a strong support for their argument. It is important that three different and independent groups have found the sidereal period, and therefore it can be considered as an experimental fact. What is the interpretation of this fact is another problem. Roger Hunter believes that it is a coincidence due to the fact that two large earthquakes occurred at a certain time; Shimshoni and Dishon believe that if it exists, it is due to tidal forces.We realize that a sidereal period does not necessarily mean that the cause of the effect lies outside the solar system. A relevant fixed direction in space is formed by the intersection of the ecliptic plane (where the mass of the solar system is) and the normal to a fault plane. A detailed explanation of this model with statistical evidence is being published [Sadeh and Meidav. 1975]. An application of this model to lunar quakes and a strong correlation between lunar direction in the sky and earthquakes have been submitted to Science [Sadeh and Wood, 1975]. REFERENCES Hunter, R. N., Earthquakes and sidereal period reevaluated, J. Geophys. Res., 80, in press, 1975. Sadeh, D., and M. Meidav, Search for sidereal periodicities in earthquake occurrences, J. Geophys. Res., 78, 7709-7716, 1973. Sadeh, D., and M. Meidav, Possible solar effects on earthquake occurrences, J. Geophys. Res., 80, 3378-3380, 1975. Sadeh, D., and K. Wood, Periodicities in lunar seismic activity and earthquakes, submitted ...