Table 1. Results of stress tensor inversion before the mainshock (left columns of Fig. 4). The stress ratio φ is defined as φ = (σ₂ –σ₃) / (σ₁ –σ₃). The stress difference D is related to the angular stress distance ω on a five-dimensional unit sphere as (D/2) = sin(ω/2) (Sato and Yamaji, 2006). D ranges from 0 to 2. D = 0 and 2 mean the identical and opposite stress states, respectively. and represent the mean stress difference and mean angular stress distance from an optimal solution, respectively. Stress tensor inversion was performed only for those regions with more than 20 focal mechanisms.

From: Detailed stress fields in the focal region of the 2011 off the Pacific coast of Tohoku Earthquake—Implication for the distribution of moment release—