The role of tides and LOD in the case of earthquake triggering

We analysed the anomalously excited microseisms at the OKC seismic station (Czech Republic) during the period between 2007 – 2011. We showed that the time distribution of the anomalous microsesims is similar to the LOD (Length Of the Day) variations. The biggest number of anomalous microsesisms was excited in the period when LOD (differential) was higher than 0.001 s, i.e. mostly in two periods from the middle of February till the middle of April, and from the middle of September till the beginning of November. The second biggest peak of anomalous microseisms coincides with the increasing amplitude of tides at the end of the year, with the peak being at the beginning of December.

Since the microseisms are connected with the stress variations in the Earth´s crust, and should be expressed in the triggering process, we analysed the ANCC catalogue of the biggest earthquakes. We used all earthquakes with a magnitude bigger than 7 since 1900 till 2016. We found that their distribution in the phase space is almost homogenous until M8.1. We also found two groups of the strongest EQs bigger than M8.2: a) the events, which are triggered before tidal maximum. There is a proportional relationship between tidal amplitude and the magnitude of the triggered earthquake. b) The triggered events before and after LOD maximum. In that case, there is an inversion relationship between LOD value and EQ magnitude (i.e. the strongest EQs are triggered after the maximal deceleration of the Earth rotation, which is after February the 15^th and after September the 1^st, and the maximal number of triggered events is around the maximum of LOD, i.e. March the 21^st and September the 21^st).

It is possible to find (though difficult to detect) four global cycles of earthquakes: before 1915, before 1945, before 1980, and after 1980. The recent cycle is the most active cycle in the entire catalogue. The LOD plays a more important role after 2005 in the triggering process than the tidal amplitude. Therefore we observed the strongest EQs at the end of February (Chile 2010, M8.8) and in the beginning of March (Tohoku 2011, M9). We can expect the next big one in the same period of tidal cycle, but with larger amplitude, by 2020.