Influence of external forces on the triggering of quakes

Tidal stresses are generated in any three-dimensional body influenced by external inhomogeneous gravity field. In this paper stresses caused by the body tides are discussed in more detail for the case of the Earth and to some degree in case of Moon and Mars. Distribution of tidal normal, horizontal (tangential) and also shear and hydrostatic stresses within the Earth were calculated for the PREM and also for comparison purposes for a homogenous mantle and for a mantle with constant density. It can be concluded that increase of stress as a function of depth is practically independent from the structure of the Earth and reaches a value around 10³ N/m² at the depth of 900-1500 km, well below the zone of deep earthquakes. At the depth of the overwhelming part of seismic energy accumulation (around 50 km) the stresses of lunisolar origin are only (0.5-1.0 10² N/m²), therefore the influencing effect of the  solid earth tides on earthquake occurrences is small. Every conclusion concerning the influence of the tides on the break out time of earthquakes must be interpreted very carefully because the tidal and tidal load stresses (of the order of 10¹-10² N m ^-2) are much smaller than the earthquake stress drops (10⁶ -10² N m ^-2)----

 It should be mentioned that since the tidal potential and its derivatives are coordinate dependent and the zonal, tesseral and sectorial tides have different distributions on and within the Earth, the lunisolar stress cannot influence directly the break-out of every seismological event in the same degree. A correlation between earthquake energy release and the lunisolar effect can exist theoretically in some cases where the seismic area is well determined and has either one seismic source or severe similar ones. Particularly in volcanic areas, where the seismic activity is connected to the volcano's activity, or in the case of some aftershock swarms, significant correlation was found by different authors.

Between 1972 and 1977 seismometers were installed on the Moon in the frame of Apollo project. These instruments recorded in total 12 000 moon quakes. The reason of 28 biggest tremors ((M≥5) is still unclear. In addition, beside the shallow events, due to thermal influences and meteor impact activity there are around 7000 small deep quakes (M≤2) in the middle of lunar mantle characterized by 27 day periodicity. It is likely that they are excited by the tide generating force of the Earth, which is 130 times greater than the lunar one. In the case of the not yet known seismicity of Mars significant seismicity inside its mantle cannot be expected because the lack of a satellite with significant gravity effect.

This research was supported by the Hungarian Scientific Research Fund under the project OTKA K 109060.