5:15 PM - 6:45 PM
[MGI31-P03] Characteristics of the latest slip plane by core-loggin of the Neodani Fault drill core
Keywords:latest slip plane, Core-logging, Neodani Fault
The fault rocks of the Neodani Fault have not been studied well, although it is well known for the 1891 Nobi earthquake. Recently, the boreholes penetrating to the Neodani Fault Zone were drilled by the Nuclear Regulation Authority. The borehole logging was not performed well. Magnetic susceptivity measurement, X-ray fluorescence analysis (XRF) and so on were performed using core samples to clarify the characteristics of the latest slip zone from the viewpoint of core-logging.
The drilling sites are at Neo-Nagamine and Neo-Midori, Motosu City, Gifu Prefecture, Central Japan. At Neo-Nagamine, 6.2 m left-lateral displacement was occurred in the 1891 earthquake (Awata et al., 1999). The drilled boreholes are R3NDFP-1, R3NDFD-1 and R3NDFD-1-S1, and R3NDFD-1-S1 is a sidetrack hole from R3NDFD-1. R3NDFP-1 is 30 m of length and inclined at 60° to the horizontal. R3NDFD-1 and R3NDFD-1-S1 are 80 m of length and inclined at 82°. At Neo-Midori, 6 m vertical displacement was occurred in the 1891 earthquake (Koto, 1893). The drilled boreholes are NDFP-1 and NDFD-1-S1. NDFP-1 is 140 m length and inclined at 50°. NDFD-1-S1 is 525 m and inclined at 78°. Well developed fault gouge zone is only one in each borehole. The latest slip zone is recognized as continuous and straight zone. Its depth ranges from 15.93 to 16.07 m in R3NDFP-1, 64.73 to 65.96 m in R3NDFD-1, 64.95 to 66.07 m in R3NDFD-1-S1, 110.62 to 110.69 m in NDFP-1 and 387.52 to 387.67 m in NDFD-1-S1. Host rock of fault rocks is mudstone mélange with blocks of sandstone, chert, basalt and limestone in the Mino Belt. The host rock of the latest slip zone is mudstone in R3NDFP-1, R3NDFD-1, R3NDFD-1-S1 and basalt in NDFP-1, NDFD-1-S1.
The 1 cm-interval measurement of magnetic susceptibility using a portable device shows discontinuous higher anomaly along the latest slip zone in R3NDFD-1-S1. Powder X-ray diffraction patterns of the samples in R3NDFD-1 clarify that magnetite and smectite are included in some samples along the latest slip zones. The 1cm-interval XRF result shows that the variation pattern of Fe2O3 along the depth is similar to that of magnetic susceptibility only in the latest slip zone. R3NDFP-1 and R3NDFD-1 also exhibit high anomaly of magnetic susceptibility along the latest slip zone, but NDFP-1 and NDFD-1-S1 do not show it.
Yang et al. (2020) reported that fine-grained magnetite is produced when heating smectite over 250°C. This suggests that high anomaly of magnetic susceptivity was formed at > 250°C due to frictional heating. Similar variation patterns of Fe2O3 and magnetic susceptivity also support this idea. The reason of discontinuous distribution of high anomaly would be discontinuous distribution of shear heating or fragmentation of continuous high anomaly zone. Magnetic susceptivity of basalt is much higher than mudstone. This is the reason why NDFP-1 and NDFD-1-S1 do not show high anomaly.
The drilling sites are at Neo-Nagamine and Neo-Midori, Motosu City, Gifu Prefecture, Central Japan. At Neo-Nagamine, 6.2 m left-lateral displacement was occurred in the 1891 earthquake (Awata et al., 1999). The drilled boreholes are R3NDFP-1, R3NDFD-1 and R3NDFD-1-S1, and R3NDFD-1-S1 is a sidetrack hole from R3NDFD-1. R3NDFP-1 is 30 m of length and inclined at 60° to the horizontal. R3NDFD-1 and R3NDFD-1-S1 are 80 m of length and inclined at 82°. At Neo-Midori, 6 m vertical displacement was occurred in the 1891 earthquake (Koto, 1893). The drilled boreholes are NDFP-1 and NDFD-1-S1. NDFP-1 is 140 m length and inclined at 50°. NDFD-1-S1 is 525 m and inclined at 78°. Well developed fault gouge zone is only one in each borehole. The latest slip zone is recognized as continuous and straight zone. Its depth ranges from 15.93 to 16.07 m in R3NDFP-1, 64.73 to 65.96 m in R3NDFD-1, 64.95 to 66.07 m in R3NDFD-1-S1, 110.62 to 110.69 m in NDFP-1 and 387.52 to 387.67 m in NDFD-1-S1. Host rock of fault rocks is mudstone mélange with blocks of sandstone, chert, basalt and limestone in the Mino Belt. The host rock of the latest slip zone is mudstone in R3NDFP-1, R3NDFD-1, R3NDFD-1-S1 and basalt in NDFP-1, NDFD-1-S1.
The 1 cm-interval measurement of magnetic susceptibility using a portable device shows discontinuous higher anomaly along the latest slip zone in R3NDFD-1-S1. Powder X-ray diffraction patterns of the samples in R3NDFD-1 clarify that magnetite and smectite are included in some samples along the latest slip zones. The 1cm-interval XRF result shows that the variation pattern of Fe2O3 along the depth is similar to that of magnetic susceptibility only in the latest slip zone. R3NDFP-1 and R3NDFD-1 also exhibit high anomaly of magnetic susceptibility along the latest slip zone, but NDFP-1 and NDFD-1-S1 do not show it.
Yang et al. (2020) reported that fine-grained magnetite is produced when heating smectite over 250°C. This suggests that high anomaly of magnetic susceptivity was formed at > 250°C due to frictional heating. Similar variation patterns of Fe2O3 and magnetic susceptivity also support this idea. The reason of discontinuous distribution of high anomaly would be discontinuous distribution of shear heating or fragmentation of continuous high anomaly zone. Magnetic susceptivity of basalt is much higher than mudstone. This is the reason why NDFP-1 and NDFD-1-S1 do not show high anomaly.