Depth-dependent change in the focal mechanism of shallow earthquakes: Implications for brittle-plastic transition in a seismogenic region

An analysis is conducted of the focal mechanism solutions of 2150 shallow earthquakes that have occurred in the central part in the Kinki district, Japan. It is found that in regions shallower than the characteristic depth (the depth at which the frequency-depth distribution of earthquakes is maximum), T axis plunges are widely distributed in the horizontal and vertical directions. Furthermore, the distribution of the plunges does not change with depth in the region. In regions deeper than the characteristic depth, the plunges are concentrated horizontally. That is, both strike-slip and reverse fault events are found in regions shallower than the characteristic depth, while strike-slip events dominate in the deeper region. It is inferred from the above results that the minimum horizontal stress has almost the same magnitude as the vertical stress in regions shallower than the characteristic depth, while in the deeper regions the minimum horizontal stress is less than the vertical stress. This conclusion implies that the minimum horizontal stress increases proportional to the depth in regions shallower than the characteristic depth but does not increase in proportion to depth in the deeper regions. It was also concluded that the brittle-semibrittle transition could possibly occur at the characteristic depth.