Evidence for tidal triggering for the earthquakes of the Ionian geological zone, Greece

Spyrous D Spyrous D,Dimitrios N Dimitrios N,Michael E Michael E

doi:10.4401/ag-5314

Abstract

We here investigate the evidence for tidal triggering of the earthquakes of the seismic area of the Ionian geological zone in Greece, using the cumulative histogram method that was introduced recently by Cadicheanu et al. (Nat. Hazards Earth Syst. Sci., 2007, 7, 733-740). We analyzed the series of earthquakes that occurred in the area bounded by 19˚E # { #22˚E and 36˚N # m # 40˚N from 1964 to 2006. Over this time, there were 19.916 shallow and intermediate depth earthquakes with magnitudes ranging between 2.5 and 6.2. The great majority of these earthquakes, including those with M ≥ 5.0, were shallow. The results of our analysis indicate that the monthly variations in the frequencies of the earthquake occurrence are in agreement with the period of the tidal lunar monthly variations. The same is true for the corresponding daily variations of the frequencies of earthquake occurrence and the diurnal lunisolar (K1) and semidiurnal lunar (M2) tidal variations. In addition, the confidence levels for the identification of such periodical agreement between the frequency of earthquake occurrence and the tidal periods varies according to the seismic activity; i.e. the higher confidence levels correspond to the periods with stronger seismic activity. These results are in favor of a tidal triggering process of earthquakes when the stress in the focal area is near the critical level.

Highlights

The question of possible connections between the Earth tides and earthquake occurrence is a very old one, and it has been tackled in a number of studies since more than 100 years ago
Since the periods of the Earth tides component are very well known and relatively accurately predictable in the local coordination system, we assigned a unique phase angle within the period of variation of a particular tidal component for which the effect of earthquake triggering is under investigation, with the simple relationship: zi where z is the phase angle of the time occurrence of the i earthquake in degrees, ti is the time of occurrence of the i earthquake in modified Julian days (MJD), to is the period that we chose in MJD, and Td is the period of the particular tidal component in MJD
Taking into account the low accuracy in the phase determination, due to the relatively large extent of the stacking bins (i.e. 30 ̊), we can read the phase lags of the maximum seismic activity from Figures 6 and 7, which are +90 ̊ and +270 ̊ for the anomalistic month, and +30 ̊ and +210 ̊ for the synodic month. Both of these follow the periodic variations of the maximal tidal forces, with different phase lags, but with a confidence level for a possible correlation between seismic activity and monthly tidal variation, which is much higher for the synodic month

Summary

Introduction

The question of possible connections between the Earth tides and earthquake occurrence is a very old one, and it has been tackled in a number of studies since more than 100 years ago. The results have been contradictory, with most of the outcomes not able to prove the possibility of any correlations between earthquake occurrence and the Earth tides [see for instance: Schuster 1897, Knopoff 1964, Simpson 1967, Shudde and Barr 1977, Rydelek et al 1992, Vidale et al 1998, and many others]. Recent analyses indicate that this is so [Tanaka et al 2002, 2006, Cadicheanu 2007] In these previous studies, was tidal triggering at the global [Tanaka et al 2002] and local [Tanaka et al 2006, Cadicheanu 2007] scales found, but in addition, in the last two studies, an increase in the reliability of the tidal-earthquake occurrence correlation was shown to be a precursory phenomenon for strong earthquakes

Methods

Results

Conclusion