Focal mechanisms and stress field in the Nobi fault area, central Japan

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In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97° ± 23° E and N6° ± 32° E, and the averages of the dips of the P- and T-axes were 11° ± 10° and 32° ± 25°, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1) is oriented E–W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1 – σ 2)/(σ 1 – σ 3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3, and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1. The direction of the σ 1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released.