For this model, most of the aftershocks surrounding the center of slip are positively stressed.
We also calculate the ratios of three cases in which (1) both nodal planes are positively stressed; (2) one nodal plane is positively stressed, while the other is negatively stressed; and (3) both nodal planes are negatively stressed.
The red, green, and blue focal spheres show the positively stressed receiver faults for both nodal planes, the positively stressed receiver faults for either the first or second nodal plane, and the negatively stressed receiver faults for both nodal planes.
However, Fig. 4(b) shows that the normal-fault aftershocks beneath the trench is positively stressed at all depths.
They reported that 85% of these aftershocks were found to be positively stressed, also consistent with the CST hypothesis.
Most of these normal-faulting earthquakes were positively stressed due to the occurrence of the megathrust earthquakes.
For the Tohoku-Oki earthquake, Toda et al. (2011) showed that there is a mean 47% gain in positively stressed aftershock mechanisms over background (1997 2011 March 10) earthquakes.
For the slip distribution with the center of slip distant from the trench, 69% of the aftershocks are positively stressed (Fig. 3(a)).
The ratios of aftershocks that occurred in the positively stressed regions following the Sumatra Andaman, Maule, and Tohoku-Oki earthquakes were 47.6, 49.8, and 47.0%, respectively.
Even if a majority of aftershock nodal planes were positively stressed by the mainshock rupture, this would not necessarily prove the stress triggering hypothesis because the distribution of receiver faults is controlled primarily by predominant stress fields.
