Determination of the youngest active domain in major fault 1 zones using medical X-ray computed tomography-derived 2 density estimations

Determination of the youngest active domains in fault zones that are not overlain by Quaternary

) that does not disrupt lower river terrace deposits, which means 175 that there has been no known activity during the late Quaternary (Okada, 1992). The Hatai 176 tonalite is distributed within 2 km of the northern side of the MTL in this area, and consists 177 mainly of plagioclase, hornblende, and chloritized biotite. The Hatai tonalite is affected by 178 mylonitization along the MTL, and contains foliations that are almost parallel to the MTL 179 (Takagi, 1985).  (Figs 3c and 2b). ATS-2 is a solid pelitic schist with well-developed schistosity, 211 whereas ATS-1 contains 2-cm-diameter quartz crystals. The cataclasite samples from the Hatai 212 tonalite were analyzed using X-ray CT imaging (ATR-2) and density and X-ray fluorescence 213 (XRF) analyses (ATR-3). 214 Photomicrographs of the fault rocks are shown in Fig. 3g   The density, ρt, porosity, ϕ, and Zet measurement results are shown in Table 2 and Fig. 5, and  338   Table 3 contains the XRF analysis results, which were used to calculate Zet. 339 There is a decrease in ρt as the youngest fault plane Y of every analyzed fault is approached 340 (Fig. 5a). There is an ~24% increase in ϕ as ρt decreases by 1 g/cm 3 , regardless of rock type 341 (fault rock or protolith; Fig. 5b). The mean ϕ values are 1.5% (standard deviation (SD) = 1.0%) 342 for the protolith, 12.6% (SD = 6.9%) for the cataclasite, 12.0% (SD = 4.8%) for the fault gouge 343 19 along the inactive faults, 17.4% (SD = 4.6%) for the fault gouge along the active faults, and 344 32.2% (no SD calculated since there were only two samples) for the fault breccia (Table 2). 345 However, every rock type yielded a positive correlation between ρt and Zet (Fig. 5c), even 346 though the ρt and Zet are expected to vary among different rock types. Fault breccia YDA-1 and 347 YDA-2 of the Yamada Fault both exhibit maximum ρt decreases of ~40%, whereas the Zet values 348 are almost the same as that for protolith YK-1, which means that the relationship between ρt 349 and Zet for the fault breccias is quite different from those of the other samples. This is because 350 YDA-1 and YDA-2 have been strongly affected by weathering, as evidenced by their dark-351 brown color at outcrop and an Fe2O3 content of 4.28 wt%, which is much higher than those of 352 the fault gouge, cataclasite, and protolith samples (Table 3) (2) 371 where I0 is the initial intensity of the incident X-ray beam, I is its emergent intensity, S is the 372 sample thickness, and μ is the linear X-ray attenuation coefficient (LAC) of the sample. The 373 LAC depends on both bulk density, ρ, and the atomic number, Z (Wellington and Vinegar, 374 1987): 375 where E is the X-ray energy (keV), a is a nearly energy-independent coefficient this is termed 377 the Klein-Nishina coefficient, and b is a constant. Equation (3) is applicable for monochromatic 378 X-rays, such as those from a synchrotron radiation facility. However, this equation does not 379 hold for most commercial X-ray CT scanners, which use polychromatic X-ray beams, because 380 μ depends on the X-ray energy (e.g., Nakano et al., 2000;Tsuchiyama et al., 2000). The 381 photoelectric absorption of a compound consisting of multiple types of atoms is proportional to 382 the effective atomic number calculated via equation (1) (Wellington and Vinegar, 1987). 383 The CT number, NCT, which determines the contrast of a CT image, is defined as: 384 where μw is the X-ray attenuation coefficient of pure water. A polychromatic X-ray CT scanner 386 will allow μ to vary depending on the X-ray energy (effective energy), as described above. The 387 influence of the variations in μ with the energy differences in an X-ray energy distribution is 388 reduced by calculating NCT, which is standardized using the μ ratio in equation (4)

CT image analysis methods 393
A CT image is essentially a bitmap of each pixel's CT number; however, it also contains various 394 artifacts due to the X-ray photography and image reconstruction. Therefore, the effects of these 395 artifacts, especially BH, must be eliminated or reduced to ensure the accuracy of the CT 396 numbers and therefore provide an accurate quantitative analysis. 397 22 BH artifacts cause the edges of a CT image to appear brighter than the center, such that the CT 398 numbers along the edges of a sample are greater than those in the center. This occurs because 399 the lower-energy X-rays are absorbed more readily than the higher-energy ones when 400 polychromatic X-rays pass through a sample near its center, where the transmission thickness 401 is large. and rock type in this study (Fig. 5c). Therefore, we investigated the relationship among NCT, ρ, 424 and Ze using the recorded CT images taken for a single tube voltage (140 kV). 425 We used a third-generation medical X-ray scanner (Aquilion Precision TSX-304A 160-row 426 multi-slice CT; Canon Medical Systems Co., Ltd.; Otawara, Tochigi, Japan) at CRIEPI. The 427 scanner has a 0.25-mm slice thickness and 0.098-0.313-mm pixel size. The X-ray tube has a W 428 target and a 0.4 mm × 0.5 mm focal size. Three-dimensional CT images were acquired using a  Table 4 shows the X-ray CT image, density, and XRF analysis results. The CT images for sample MZ-5, a pelitic schist protolith in the Sanbagawa Belt, exhibits a 452 striped pattern corresponding to planar schistosity (Fig. 6b) and possesses a NCTM value of 2056. 453 A narrow band (≤1 mm wide) that is brighter than the rest of the image is inferred to be a 454 phengite vein, which has a greater effective atomic number than either quartz or albite. 455 Example histograms of the CT values in each of the sampled zones are shown in Fig. 6c-e.  456 Approximately 40,000-130,000 pixels are analyzed in each region, with the NCTM values 457 generally following a normal distribution and possessing a standard deviation of 112-312 458 (Table 4). There may either be an increase in the frequency to values lower than NCTM, or a 459 small side peak that is lower than NCTM if the sample contains many cracks; however, NCTM 460 corresponds to the CT value of the matrix, with the influence of cracks excluded. 461 The NCTM-ρt relationship for Sanbagawa pelitic schist possesses a high positive correlation (ρt 465 = 9.54 × 10 -4 NCTM + 0.76, γ = 0.958; Fig. 10a). The calculated density from this equation, ρc, 466 is consistent with the real value, ρt, and possesses an error of <9.5% (Table 4). 467 The NCTM-Zet relationship (Zet = 2.67 × 10 -4 NCTM + 11.8) can be derived from the 468 abovementioned NCTM-ρt relationship; the ρt-Zet relationship is shown in Fig. 5c (Zet = 0.28ρt + 469 26 11.6, γ = 0.847). The effective atomic number calculated from this equation, Zec, is consistent 470 with the real value, Zet, and possesses an error of <1.4% (Table 4). 471 <Figure 6 472 473

The MTL at the Awano-Tabiki outcrop 474
The imaging results for samples AT, HA-1, and ATS-2 (Fig. 7a-c)  mottled white regions throughout the CT images (Fig. 7b), with a NCTM value of 1908. A 1-2-485 mm-thick band that appears brighter than the rest of the image is inferred to be hornblende and 486 chlorite, both of which have larger effective atomic numbers than quartz and plagioclase. 487 27 Sample ATS-2, a pelitic schist protolith from the Sanbagawa Belt, possesses striped patterns 488 corresponding to planar schistosity in the CT images (Fig. 7c), with a NCTM value of 1961. A 489 narrow band (at most ~1 mm wide) that appears brighter than the rest of the image is inferred 490 to be a thin layer containing phengite and calcite, both of which have larger effective atomic 491 numbers than quartz. 492 Example CT value histograms for each zone are shown in Fig. 7d-h. Approximately 12,000-493 200,000 pixels are analyzed in each zone, with the NCTM values generally following a normal 494 distribution and possessing a standard deviation of 90-210 (Table 4). There may be a slight 495 increase in the frequency to values higher than NCTM due to the influence of minerals with a 496 large effective atomic number in some instances; however, NCTM corresponds to the CT value 497 of the matrix, with the influence of these minerals excluded. 498 The NCTM-ρt relationships for the Sanbagawa pelitic schist and Ryoke tonalite possess high 502 positive correlations (pelitic schist: ρt = 1.08 × 10 -3 NCTM + 0.56, γ = 0.857; tonalite: ρt = 1.19 503 × 10 -3 NCTM + 0.40, γ = 0.813; Fig. 10b, c). The ρc values are consistent with the ρt values, and 504 possess errors of <10.7% (Table 4). 505

The Tsuruga Fault at the Oritodani outcrop 513
The CT results for samples T-3, K1, and T-5 (Fig. 8a-c) are representative of the five samples 514 (T-3, C-2, K-1, C-1, and T-5) collected from the Oritodani outcrop. 515 Sample T-3, which was taken from the rocks in the fault fracture zone that formed during the 516 most recent fault activity, appears dark in the fault gouge (T-3-1, T-3-2 and T-3-3) around the 517 main fault plane Y in the CT image (Fig. 8a), with NCTM values in the 1185-1492 range. The 518 smallest NCTM value is in T-3-3, which is in contact with the main fault plane Y. We consider T-519 3-2 to be possibly affected by the most recent fault activity based on our abovementioned 520 analysis, but its NCTM value is 1428, which is about the same as that in T-3-1 and exceeds that 521 in AT-2, a fault gouge along an inactive fault. Furthermore, the observed microstructures in T-522 3 suggest that repetitive fault activity, which is indicative of an active fault, is limited to fault 523 29 gouge T-3-3. Therefore, we classify T-3-1 and T-3-2 as inactive fault gouge, and T-3-3 as active 524 fault gouge in this analysis. The cataclasite (T-3-4) outside of the fault gouge appears brighter 525 than the fault gouge, with a NCTM value of 1622. 526 Sample K-1, a Koujaku granite protolith, possesses dark-gray and fine-grained white areas 527 throughout the CT images (Fig. 8b), with a NCTM value of 1656. The small white areas (≤2-mm 528 diameter) in the image are inferred to be biotite, which has a larger effective atomic number 529 than either quartz or plagioclase. 530 Sample T-5, which is a metabasalt protolith, is largely gray in the CT image, with the exception 531 of a white area at the upper right of the sample (Fig. 8c) and has a NCTM value of 2590. 532 Example CT value histograms for each zone are shown in Fig. 8d-h. Approximately 20,000-533 310,000 pixels are analyzed in each region, with the NCTM values generally following a normal 534 distribution and possessing a standard deviation of 70-206 (Table 4). There may be a slight 535 increase in the frequency to values above NCTM due to the influence of minerals with a large 536 effective atomic number in some instances, but NCTM corresponds to the CT value of the matrix, 537 with the influence of these minerals excluded. 538  Fig. 10d, e). The ρc values are the ρt values, and possess errors of <3.7% (Table 4). 544 The NCTM-Zet relationships (granite: Zet = 2.83 × 10 -4 NCTM + 11.6; metabasalt: Zet = 7.66 × 10 -545 4 NCTM + 12.1) can be derived from the abovementioned NCTM-ρt relationships; the ρt-Zet 546 relationships are shown in Fig. 5c  Sample YK-1, a Miyazu granite protolith, possesses dark-gray and fine-grained white areas 558 throughout the CT images (Fig. 9b), with a NCTM value of 1730. The white area (≤2-mm 559 31 diameter) in the image is inferred to be biotite, which has a larger effective atomic number than 560 both quartz and plagioclase. 561 Example CT value histograms for each zone are shown in Fig. 9c-e. Approximately 50,000-562 330,000 pixels were analyzed in each region, with the NCTM values generally following a normal 563 distribution and possessing a standard deviation of 119-224 (Table 4). There may be a slight 564 increase in the frequency to values above NCTM due to the influence of minerals with a large 565 effective atomic number in some instances; however, NCTM corresponds to the CT value of the 566 matrix, with the influence of these minerals excluded. 567 the NCTM values that were calculated from the 2D CT images. 570 The NCTM-ρt relationship for Miyazu Granite has a high positive correlation (ρt = 9.79 × 10 −4 571 NCTM + 0.85, γ = 0.893; Fig. 10f). The ρc value is consistent with ρt, and possesses an the error 572 of <9.1% (Table 4). 573 The NCTM-Zet relationship (Zet = 6.95 × 10 -4 NCTM + 11.2) can be derived from the 574 abovementioned NCTM-ρt relationship; the ρt-Zet relationship is shown in Fig. 5c (Zet = 0.71ρt + 575 10.6, γ = 0.975). The Zec value is consistent with Zet, and possesses an error of <0.7% (Table 4).

ρt-ρc and Zet-Zec relationships 585
There is no significant difference in the ρ-ϕ relationship for various fault rock and protolith 586 types (Fig. 5b), whereas the trend of the ρt-Zet relationship appears to be dependent on the 587 analyzed fault rock/protolith type (Fig. 5c). This indicates that NCTM, which is a function of ρ 588 and Ze, must be treated as an effective parameter for examining fault rock and protolith 589 characteristics by fault rock/protolith type. We observe strong correlations between ρc and ρt, 590 and Zec and Zet for each fault rock and protolith type, as shown in Fig. 11 (ρ: γ = 0.944, Ze: γ = 591 0.895). Therefore, NCTM, which is calculated by fault rock/protolith type, should be a reliable 592 parameter for calculating the ρt and Zet values of a given rock sample and determining its fault 593 rock/protolith characteristics. 594 33 596

Fault rock characteristics based on the NCTM-rock/protolith ratio (ρt and Zet) 597
relationship 598 We have demonstrated that ρt, Zet, and NCTM all decrease as the main fault plane is approached 599 Furthermore, ρt is affected by Zet, as shown in Fig. 5c, with a distinct ρt-Zet relationship for each 600 fault and protolith type. Therefore, the effect of Zet on ρt is suppressed by using the 601 rock/protolith density ratio of each fault and protolith type. 602 Table 5 shows the results of the analyzed fault rock characteristics based on the relationships 603 between NCTM and the ρt and Zet rock/protolith ratios. The statistics of the determined NCTM 604 values and the ρt and Zet rock/protolith ratios are provided in Table 6 and Fig. 12a-c. 605 <Table 5 <Table 6 <Figure 12 606 The NCTM values (taken at 140 kV) were ~1900 ± 300 for the protoliths, ~1650 ± 250 for 607 cataclasite, ~1450 ± 200 for the fault gouge along inactive faults, and ~1100 ± 100 for the fault 608 gouge along active faults, as shown in Fig. 12a. Both the NCTM values and NCTM variations 609 decrease as the fault rock becomes more heavily deformed and the main fault plane is 610 The rock/protolith ρt ratio was ~0.8 ± 0.15 for cataclasite and the fault gouge along inactive 612 faults, and ~0.7 ± 0.1 for the fault gouge along active faults, as shown in Fig. 12b. The ρt ratio 613  Table 1. Locations and fault/protolith details of the analyzed samples. See Figs 1 and 2