Annual changes in atmosphere and ocean appear to be among numerous reasons, able to cause an earthquake. Such conclusion was made by astronomers of Crimea Astrophysics Observatory and RAS Moscow Astronomy Institute after analyzing more than 200 thousand of impacts registered on our planets for the period of 27 years, 1964-1990.
 

Earth is irregularly revolving round its axis. Neighbouring celestial bodies, the Sun and The Moon, first of all, influence its movement. It results in periodic behaviour of events, including earthquakes, occurring on our planet. Once a year angular torque moment of atmosphere and solid Earth changes. This seems to be the reason for light earthquakes annually repeating in the same places.
 

To make such a conclusion the scientists have generated a time sequence – they took daily number of earthquakes happening all around the world and transformed it using Fourier transform. Mathematicians usually use this transform to find some cyclic patterns in a set of statistical data. They find out that the most pronounced cycle among various cycles lasting from 10 days to 13,5 years is the cycle of 365 says, i.e. one year. However, this cycle can be distinctly seen only for medium earthquakes with magnitude of 3,5-4,8 points. Such earthquakes cause buildings to shake, sleeping ones to wake up and glass and plaster to crackle. The fact that earthquakes with higher magnitude do not have such annual cycles is proved by statistical data.
 

Earthquakes with the annual cycle occur on the surface of our planet irregularly. Such pattern is the most indistinct in the Eastern Hemisphere, and the most distinct – in the Northern Hemisphere, with the annual cycle reliability 1000 times higher than average for the Earth. If one compares southern and northern hemispheres, he finds out that for the period of 27 years the stations have detected almost the same number of earthquakes, but annual cycle reliability differs 100 thousand-fold. This can be explained by the fact that annual increase in earthquake number is common for mid-latitude (among 40 and 60 degree parallels) territories. In the north of this zone 20 thousand earthquakes had been detected and in the south – 30 thousand. Southern hemisphere has suffered only 700 earthquakes for 27 years, while equatorial territories were detected to have 20 thousand. Equatorial earthquakes have no annual cycle and thus concealed the real cyclic behaviour. Such difference is not surprising because mid-latitudes of the Southern Hemisphere are mainly covered with water.
 

The depth of an earthquake also affects the cycling. The annual cycle is indicated for those earthquakes, which occur not deeper than 650 km or between 90 and 140 km. The second interval is of particular interest because at these depth earthquakes happen due to lithosphere plates’ interactions.