Earthquake energy distribution along the earth surface and radius

Abstract

The global earthquake catalog of seismic events with MW ≥ 7.0, for the time interval from 1950 to 2007, shows that the depth distribution of earthquake energy release is not uniform. The 90% of the total earthquake energy budget is dissipated in the first ~30km, whereas most of the residual budget is radiated at the lower boundary of the transition zone (410 km - 660 km), above the upper-lower mantle boundary. The upper border of the transition zone at around 410 km of depth is not marked by significant seismic energy release. This points for a non-dominant role of the slabs in the energy budged of plate tectonics. Earthquake number and energy release, although not well correlated, when analysed with respect to the latitude, show a decrease toward the polar areas. Moreover, the radiated energy has the highest peak close to (±5°) the so-called tectonic equator defined by Crespi et al. (2007), which is inclined about 30° with respect to the geographic equator. At the same time the presence of a clear axial coordination of the radiated seismic energy is demonstrated with maxima at latitudes close to critical (±45°). This speaks about the presence of external forces that influence seismicity and it is consistent with the fact that Gutenberg-Richter law is linear, for events with M>5, only when the whole Earth's seismicity is considered. These data are consistent with an astronomical control on plate tectonics, i.e., the despinning (slowing of the Earth's angular rotation) of the Earth's rotation caused primarily by the tidal friction due to the Moon. The mutual position of the shallow and ~660 km deep earthquake energy sources along subduction zones allows us to conclude that they are connected with the same slab along the W-directed subduction zones, but they may rather be disconnected along the opposed E-NE-directed slabs, being the deep seismicity controlled by other mechanisms.