Deep low-frequency earthquakes (LFEs) in Northeast Japan occur beneath active volcanoes at depths of 20-40 km. LFEs radiate low-frequency seismic waves, with most energy at 2-8 Hz, despite their low magnitudes (M < 2). Although many previous studies have obtained various focal mechanisms with non-double-couple components and suggested physical processes related to magma, the universal physical process is poorly understood. Therefore, we comprehensively determined the focal mechanisms of 264 LFEs for 26 volcanic regions in Northeast Japan using S/P amplitude ratios. Many of the obtained solutions have large double-couple components with small compensated linear vector dipole components. Such source mechanisms can be explained by shear slip on the bending fault. We also find that the plunge of the null axis is as small as 20-30º at depths of 15-25 km, whereas it becomes larger and more various at deeper depths. We interpret that the regional stress field is relatively homogeneous in the middle of the crust, whereas it may be altered near the Moho discontinuity by thermal stress or other effects related to potential magmatic process. In addition, we quantitatively investigated the possible triggering of LFEs due to static stress change of both the 2008 Iwate-Miyagi earthquake and the 2011 Tohoku earthquake. The weak correlation between triggering potentials and activity change suggests that the activity of LFEs is more or less sensitive to temporal stress changes.