Earth Planets Space, Vol. 63 (No. 7), pp. 725-730, 2011
Shinji Toda1, Jian Lin2, and Ross S. Stein3
1Disaster Prevention Research Institute, Kyoto University, Japan
2Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
3U.S. Geological Survey, Menlo Park, California, USA
(Received April 7, 2011; Revised May 11, 2011; Accepted May 13, 2011; Online published September 27, 2011)
The 11 March 2011 Tohoku Earthquake provides an unprecedented test of the extent to which Coulomb stress transfer governs the triggering of aftershocks. During 11-31 March, there were 177 aftershocks with focal mechanisms, and so the Coulomb stress change imparted by the rupture can be resolved on the aftershock nodal planes to learn whether they were brought closer to failure. Numerous source models for the mainshock have been inverted from seismic, geodetic, and tsunami observations. Here, we show that, among six tested source models, there is a mean 47% gain in positively-stressed aftershock mechanisms over that for the background (1997-10 March 2011) earthquakes, which serve as the control group. An aftershock fault friction of 0.4 is found to fit the data better than 0.0 or 0.8, and among all the tested models, Wei and Sladen (2011) produced the largest gain, 63%. We also calculate that at least 5 of the seven large, exotic, or remote aftershocks were brought ≥0.3 bars closer to failure. With these tests as confirmation, we calculate that large sections of the Japan trench megathrust, the outer trench slope normal faults, the Kanto fragment beneath Tokyo, and the Itoigawa-Shizuoka Tectonic Line, were also brought ≥0.3 bars closer to failure.
Key words: Coulomb stress change, Tohoku Earthquake, stress triggering, remote aftershocks.