Extremely weak fault planes: An estimate of focal mechanisms from stationary seismic activity in the San'in district, Japan

• Published in: Tectonophysics Vol. 723; pp. 136 - 148
• Main Authors: Iio, Yoshihisa, Kishimoto, Shinji, Nakao, Setsuro, Miura, Tsutomu, Yoneda, Itaru, Sawada, Masayo, Katao, Hiroshi
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 16-01-2018; Elsevier BV
• Subjects: Anomalies; Coefficient of friction; Earthquakes; Fault strength; Fluid pressure; Focal mechanism; Intraplate earthquake; Planes; Pore fluid pressure; Pressure anomalies; Seismic activity; Seismology; Sensors; Shear stress; Stress inversion; Velocity; Japan; Stress inversion; Intraplate earthquake; Pore fluid pressure; Focal mechanism; Fault strength
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2017.12.007

In this paper, using data from dense seismic observations in and around the seismic belt of the San'in district, Japan, we describe our analysis of focal mechanisms of stationary seismic activity, estimated stress states, and pore fluid pressures. We found these focal mechanisms to be described by the estimated stress field, suggesting that the stress field can be treated as uniform in each sub-region of the study area. We also found that events occurred even on unfavorably oriented fault planes with small shear stress. Further, we inferred that pore fluid pressures of approximately 20% of the faults analyzed are greater than the magnitude of minimum principal stress, when we assume μ=0.6. A possible explanation is localized high pore fluid pressure anomalies; another is the coefficient of friction of at least a part of faults analyzed in the study area being substantially smaller than 0.6, which we view as more plausible, since it is difficult to maintain pore fluid pressures higher than the magnitude of minimum principal stress for long periods of time.
•The stress field is treated as uniform within a region with a length of a few tens of kilometers.•Earthquakes occurred even on faults with small shear stress.•Pore pressures are computed to be over σ3 on about 20% of faults analyzed, if μ=0.6.•The coefficient of friction can be much smaller than 0.6.