Relationship between temporal variation of Earth's flattening and spatial distribution of global earthquake energy

The secular change in the flattening of Earth and its effect on global tectonics is a subject still to be investigated.

Tidal friction causes a constant despinning of the rotation of Earth. It happens at a rate of Δω = – (5.4 ± 0.5) ∙ 10^-22s^-2, resulting in a change of the length of day with ∆LOD = (2.3 ± 0.1) ms/century (Stacey, 1992). The slowly decreasing rotational speed creates a change in the flattening of the Earth, that produces a latitude dependent stress field. The meridional stress component is always positive (i.e. tensional), while the azimuthal stress is negative (i.e. compressional) from the equator, up to the critical latitudes (~ ±48.2°), and positive poleward. This means two major tectonic provinces: in the equatorial region a strike-slip province and towards the poles, a normal fault province (Denis & Varga, 1990).

From the 1960s reliable seismological catalogues are available. ISC GEM Catalogue contains re-computed moment magnitude (Mw) values, what is essential for calculating the released seismic energy, since at higher magnitudes, it doesn’t go into saturation. One can obtain the energy released by an event with the formula logE = 5.2 + 1.44Mw (Båth, 1966). Based on this catalogue, a 52-year period with moment magnitudes higher than 5.8, all in all 8799 events were used.

Our study shows that the effect of the despun Earth is reflected in the latitudinal distribution of earthquake energy, which is symmetric with respect to the equator and there are clear maxima at mid-latitudes. The distribution of seismic energy released by either normal fault or strike-slip earthquakes also follow a pattern previously described. Especially on the northern hemisphere normal fault events occur dominantly towards the poles while strike-slip earthquakes tend to happen at lower latitudes. We can conclude that tidal friction actually creates the proposed stress field on Earth, and is visible if we observe how global seismicity behaves with respect to latitude.

 

Båth, M. (1966). Earthquake energy and magnitude. Physics and Chemistry of the Earth, 7, 115-165.

Denis, C., Varga, P. (1990). Tectonic consequences of the Earth’s variable rotation, In: Brosche P, Sündermann J (eds.) Earth rotation from eons to days. Springer, pp. 146-162.

Stacey, F. D. (1992). Physics of the Earth, Brookfield Press, Australia, ISBN 0-646-09091-7.