5:15 PM - 7:15 PM
[MGI28-P06] Structure and Physical properties of fault fracture zone by Atotsugawa fault drilling, central Japan
Keywords:fault drilling, physical logging, electromagnetic survey, resistivity, Atotsugawa fault
Fault drilling research allows us to directly collect fault materials and conduct various in-situ experiments and measurements at deep underground. This is effective for physical, chemical, and geological modeling and verifying the process of strength recovery and stress accumulation on the fault during repeating earthquake cycles associated with structural changes. In order to investigate the relationship between the heterogeneous structure of the shallow part of the fault fracture zone and fault activity, NIED has conducted physical logging by drilling across the fault fracture zone for major inland active faults and clarified the resistivity structure within the fault fracture zone, and has also conducted electromagnetic survey along transects that crosses the fault strike to obtain the resistivity structure in a wider area of the fault. In this presentation, we introduce an example of fault drilling into the Atotsugawa Fault in central Japan, which is thought to have been activated by the 1858 Hida earthquake (M=7.0), and reconsider the relationship between the heterogeneous structure of the shallow part of the fault fracture zone and fault activity. The Atotsugawa Fault is clearly divided along its strike into three regions; a region of inactive shallow seismic activity in the center and active regions on either side of it. It is important issue to explore the cause of the difference in seismic activity patterns between the three regions for understanding fault activity not only of the Atotsugawa Fault but of active faults in general.
Among the physical properties of faults, resistivity is an important property for exploring the structure of a fault, because underground resistivity is an indicator of the presence of fluids and clay minerals and the distribution of pores, and fault fracture zones are expected to have low resistivity. Therefore, in addition to physical logging, electromagnetic survey was conducted along transects that crosses the fault strike. The position, shape and the characteristics of the fault fracture zone were estimated associated with fault activity, which cannot be understood by drilling alone.
Physical logging revealed resistivity of 100-600 ohm m, density of 2.0-2.5 g/cc, P-wave velocity of 3-4 km/sec, and neutron porosity of 20-40%, which were consistent with the physical properties of fault fracture zones found by other fault drillings. Furthermore, observation of the recovered cores showed fracture and alteration in almost the entire depth, and many prominent shear planes sandwiching fault clay were present, suggesting that drilling went along in the fault fracture zone from the shallow part to the bottom of the hole. Comparing the resistivity cross sections of three survey lines almost perpendicular to the fault strike obtained by electromagnetic exploration, a three-layer structure of high resistivity-low resistivity-high resistivity was evident in the depth direction on each line. The low resistivity area was about 200 m thick and 800-1000 m wide at a depth of 200 m or more, and is thought to correspond to a fault fracture zone where fracture and shear have progressed due to fault movement. In addition, the resistivity values and widths of the low resistivity areas differed from each survey line, which may reflect differences in the strength of the fault fracture zone.
Compared to the resistivity structure of the survey line that crosses near the drilling site, the resistivity distribution of the physical logging was consistent with the contrast between high and low resistivity areas being recognized in both. Resistivity distribution from the physical logging revealed that the fault fracture zone was a low resistivity area roughly below 100 ohm m, but there were also relatively high resistivity area, indicating the fault fracture zone is significantly heterogeneous. In the resistivity structure from the electromagnetic survey, the fault was recognized as low resistivity areas in a global scale, or as the boundary between low resistivity and high resistivity areas. In the fault fracture zone, the low resistivity is due to the presence of pore water and fault clay, and is thought to be correlated with fault activity.
Among the physical properties of faults, resistivity is an important property for exploring the structure of a fault, because underground resistivity is an indicator of the presence of fluids and clay minerals and the distribution of pores, and fault fracture zones are expected to have low resistivity. Therefore, in addition to physical logging, electromagnetic survey was conducted along transects that crosses the fault strike. The position, shape and the characteristics of the fault fracture zone were estimated associated with fault activity, which cannot be understood by drilling alone.
Physical logging revealed resistivity of 100-600 ohm m, density of 2.0-2.5 g/cc, P-wave velocity of 3-4 km/sec, and neutron porosity of 20-40%, which were consistent with the physical properties of fault fracture zones found by other fault drillings. Furthermore, observation of the recovered cores showed fracture and alteration in almost the entire depth, and many prominent shear planes sandwiching fault clay were present, suggesting that drilling went along in the fault fracture zone from the shallow part to the bottom of the hole. Comparing the resistivity cross sections of three survey lines almost perpendicular to the fault strike obtained by electromagnetic exploration, a three-layer structure of high resistivity-low resistivity-high resistivity was evident in the depth direction on each line. The low resistivity area was about 200 m thick and 800-1000 m wide at a depth of 200 m or more, and is thought to correspond to a fault fracture zone where fracture and shear have progressed due to fault movement. In addition, the resistivity values and widths of the low resistivity areas differed from each survey line, which may reflect differences in the strength of the fault fracture zone.
Compared to the resistivity structure of the survey line that crosses near the drilling site, the resistivity distribution of the physical logging was consistent with the contrast between high and low resistivity areas being recognized in both. Resistivity distribution from the physical logging revealed that the fault fracture zone was a low resistivity area roughly below 100 ohm m, but there were also relatively high resistivity area, indicating the fault fracture zone is significantly heterogeneous. In the resistivity structure from the electromagnetic survey, the fault was recognized as low resistivity areas in a global scale, or as the boundary between low resistivity and high resistivity areas. In the fault fracture zone, the low resistivity is due to the presence of pore water and fault clay, and is thought to be correlated with fault activity.