Fig. 2

Crustal stresses around Iwaki City prior to the 2011 Tohoku earthquake. a Lower-hemisphere, equal-area stereographic projections showing the results of the multiple inverse method (MIM; Otsubo et al. 2008) applied to selected events in Fig. 1 (solid circles). The multiple inverse method was applied with the combination number k _f  = 5 and the enhance factor e = 8 (see Yamaji (2000) for explanations of these parameters). The left and right stereograms show the results for the σ₁ and σ₃ axes, respectively. The colors of the symbols correspond to stress ratios. b Results of k-means clustering (after Otsubo et al. 2006) applied to the data in a. Black diamonds indicate the axes of the optimal stresses. White diamonds indicate the axes of the stress detected from the focal mechanism data earthquakes that occurred between March 12, 2011, and April 11, 2011 (Otsubo et al. 2013). Crosses (+) indicate the axes of the normal faulting stress around Iwaki City, as calculated by Imanishi et al. (2012) using the method of Michael (1984). Dashed circles show 95% confidence intervals. Triangles outside stereograms indicate the orientations of co- and post-seismic displacements of the M7 class earthquakes around Iwaki City (Suito et al. 2011). c Histogram of stress ratios of stress tensors calculated by the MIM technique. Diamond symbols indicate calculated mean stress ratios