The M
 JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL.

/pdf/seismicity-controlled-by-resistivity-structure-the-2016-4xbi1gfbou.pdf

Seismicity controlled by resistivity structure: the 2016 Kumamoto earthquakes, Kyushu Island, Japan

We used a field analysis of rock deformation microstructures and mesostructures to reconstruct
the long-term orientation of stresses around two major active fault systems in Japan, the Median Tectonic
Line and the Rokko-Awaji Segment Our study reveals that the dextral slip of the two fault systems, active
since the Plio-Quaternary, was preceded by fault normal extension in the Miocene and sinistral wrenching in
the Paleogene The two fault systems deviated the regional stress field at the kilometer scale in their vicinity
during each of the three tectonic regimes The largest deviation, found in the Plio-Quaternary, is a more fault
normal rotation of the maximum horizontal stress to an angle of 79° with the fault strands, suggesting an
extremely low shear stress on the Median Tectonic Line and the Rokko-Awaji Segment Possible causes of this
long-term stress perturbation include a nearly total release of shear stress during earthquakes, a low static
friction coefficient, or lowelastic properties of the fault zones comparedwith the country rock Independently of
the preferred interpretation, the nearly fault normal orientation of the direction of maximum compression
suggests that the mechanical properties of the fault zones are inadequate for the buildup of a pore fluid
pressure sufficiently elevated to activate slip The long-term weakness of the Median Tectonic Line and the
Rokko-Awaji Segment may reside in low-friction/low-elasticity materials or dynamic weakening rather than in
preearthquake fluid overpressures

/pdf/stress-rotations-and-the-long-term-weakness-of-the-median-pvd5pjk1ne.pdf

Stress rotations and the long-term weakness of the Median Tectonic Line and the Rokko-Awaji Segment

Kinematic Analyses and Radiometric Dating of the Large-Scale Paleogene Two-Phase Faulting Along the Median Tectonic Line, Southwest Japan

3D fault-zone architecture across the brittle–plastic transition along the Median Tectonic Line, SW Japan: Fault-rock characterization

Fluid-filled fractures and fissures often determine the pathways and volume of fluid movement. They are critically important in crustal seismology and in the exploration of geothermal and hydrocarbon reservoirs. We introduce a model for tube wave scattering and generation at dipping, parallel-wall fractures intersecting a fluid-filled borehole. A new equation reveals the interaction of tube wavefield with multiple, closely spaced fractures, showing that the fracture dip significantly affects the tube waves. Numerical modeling demonstrates the possibility of imaging these fractures using a focusing analysis. The focused traces correspond well with the known fracture density, aperture, and dip angles. Testing the method on a VSP data set obtained at a fault-damaged zone in the Median Tectonic Line, Japan, presents evidences of tube waves being generated and scattered at open fractures and thin cataclasite layers. This finding leads to a new possibility for imaging, characterizing, and monitoring in situ hydraulic properties of dipping fractures using the tube wavefield.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017JB014681

Munemaru Kishimoto

Papers

Magnetotelluric imaging of the Median Tectonic Line in western Shikoku, southwest Japan: Implications of the fault-related low-resistivity zone