Extension of the energy-to-moment parameter Θ to intermediate and deep earthquakes

We extend to intermediate and deep earthquakes the slowness parameter Θ originally introduced by Newman and Okal (1998). Because of the increasing time lag with depth between the phases P, pP and sP, and of variations in anelastic attenuation parameters t∗, we define four depth bins featuring slightly different algorithms for the computation of Θ. We apply this methodology to a global dataset of 598 intermediate and deep earthquakes with moments greater than 1025 dyn∗cm. We find a slight increase with depth in average values of Θ (from −4.81 between 80 and 135 km to −4.48 between 450 and 700 km), which however all have intersecting one-σ bands. With widths ranging from 0.26 to 0.31 logarithmic units, these are narrower than their counterpart for a reference dataset of 146 shallow earthquakes (σ=0.55). Similarly, we find no correlation between values of Θ and focal geometry. These results point to stress conditions within the seismogenic zones inside the Wadati-Benioff slabs more homogeneous than those prevailing at the shallow contacts between tectonic plates.