Earthquake clustering relative to lunar phases in Taiwan

The occurrence of local seismicity recorded in Taiwan for the period of 1973-1991 was converted to lunar time to reassess the notion that some earthquakes may be triggered by tidal forces. The abundant seismicity on the basis of lunar time provides not only sufficient data for statistical analysis but also the most direct and reliable estimation of tidal effects on earthquake triggering. Significant clustering of smaller earthquakes (2.5<M_L<5) was found around Day 14, with clustering around the full moon day being about 30% higher in frequency than that on other non-full moon days. These phenomena indicate that tidal forces caused by lunar attraction can indeed trigger some smaller earthquakes. On the other hand, the paucity of any correlation between larger earthquakes (M_L>5) and lunar phases indicates that larger earthquakes are barely induced by tidal forces, but predominantly by tectonic forces. No clear correlation between micro-earthquakes (M_L<2.5) and lunar phases suggests that, in addition to tectonic and tidal forces, their occurrence must be primarily associated with other factors.