We sheared simulated-quartz gouges using a low-velocity rotary shear apparatus and evaluated the relationship between electron spin resonance (ESR) intensity and displacement quantitatively considering problems of contaminants. ESR intensity of E1’ centre increased while OHC and peroxy centre kept constant with the increasing displacement up to 1.4 m. Microstructural analysis showed grain size reduction and fracture of starting gouges; hence, the fracture can affect the change in ESR intensity. ESR measurements were also conducted for starting gouges with variable amounts of contaminants, and it was confirmed that the effect of contaminants on the change in ESR intensity was negligible. Moreover, we estimated the temperature rise by the frictional heating on the surface and between particles, and it was shown that the effect of frictional heating on ESR intensity was also negligible in our experimental condition. Therefore, we could clarify the relationship between ESR intensity and fracturing with various displacements separately from contaminants and frictional heating. The results imply that the zero-setting of ESR signals cannot occur by the fracture with low frictional heating at the shallow depth.
- electron spin resonance
- paramagnetic centre
- simulated-quartz gouge
Fault dating by Electron Spin Resonance (ESR) is a method that estimates a time of fault formation or the last movement of the fault using ESR signals of quartz grains contained in fault gouge proposed by Ikeya
The E1’ centre is one of the fundamental and major paramagnetic lattice defects in crystalline quartz, which is an unpaired electron at an oxygen vacancy (Feigl
The results of Tanaka (1987) and Yang
We used quartz sands bought from the Association of Powder Process Industry and Engineering (APPIE), Japan as starting materials for the simulated-quartz gouges. Quartz sands are called JIS test powder1-1 (Lot number. P060425) or Tohoku quartz sand No. 5 in Yamagata, Japan. According to the APPIE instruction, the manufacturing process is that ore mixed clay having strong fire resistance was repeatedly washed and separated with water. Subsequently, collected sands were processed by dry sieving, and then these sands were sieved with milling to fit a standard. The shape and overview colour of this powder are irregular and white, respectively. The grain size is 45–300 μm. The particle density and apparent densities of particles placed loosely and tightly in the containers are 2.6–2.7, 1.503 and 1.612 g/cm3, respectively. The powders consist of 95% SiO2 and 5% trace amount of impurities (Fe2O3, Al2O3, TiO2, MgO and ignition loss) and we regarded the quartz sands as “pure quartz gouges”. We used the quartz sands in all experiments without any processing.
Low-velocity shear tests were performed for quartz gouges to mimic fault motion at the shallow depth using a low-velocity rotary shear apparatus in the Research Centre for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University (
ESR measurements were performed using a JEOL RE-2X X-band ESR spectrometer in the Department of Chemistry, Graduate School of Science, Tohoku University. Here, to clarify the effects of fracture with increasing fault displacement at the shallow depth, and brass and carborundum contaminants on ESR intensity, we performed the following experiments. To reveal the relationship between ESR intensity and fault displacement, we performed ESR measurements for starting and sheared gouges (Measurement 1). In order to verify the uniformity of quartz sands, we have confirmed the reproducibility of ESR characteristics. In particular, we paid special attention to the characterisation of the starting materials. Therefore, we had created ten different batches of samples and carried out ESR measurements for them. ESR measurements for sheared gouges were conducted for three samples because the shear tests were performed three times in each condition. In order to confirm the effect of contaminants originated from the forcing blocks on ESR intensity, first, ESR measurements were performed for starting gouges with different weights from 180 to 200 mg (quartz only sample). Then, we measured the ESR intensities of starting gouges with various amounts of contaminants, adjusting the total weight of 200 mg. Mixed samples contain brass debris (mixed sample-1) or #320 grit SiC powder carborundum (mixed sample-2). Measurement 2 is the investigation of the effect of brass contaminants on ESR spectra, and Measurement 3 is that of #320 carborundum. Samples were put into quartz tubes, and they were set into an ESR spectrometer. ESR measurements for E1’ centre were conducted under room temperature with a microwave power of 0.01 mW, a sweep width of ±5 mT, a modulation frequency of 100 kHz, a modulation amplitude of 0.079 mT, a sweep time of 60 s, a time constant of 0.1 s and a repetition of 40 times. The instrumental settings of oxygen hole centre (OHC, O3−, e.g., Ikeya, 1993) and peroxy centre (≡Si–O–O·, e.g., Ikeya, 1993) were the same as that of E1’ centre except for a microwave power of 1 mW and a repetition of 10 times. ESR measurements were repeated five times under the same condition for each sample. ESR intensity and g-values were those averages obtained from five ESR spectra.
The XRD measurements were conducted to identify contaminants in the sheared samples for starting and sheared gouges, #320 carborundum and brass powder with a Philips X’pert-MPD PW3050 diffractometer set with CuKα, radiation (λ = 1.5418 nm) in the Department of Earth Science, Graduate School of Science, Tohoku University. The samples were scanned from 25° to 45° with a step size of 0.02° and an acquisition time per step of 2.0 s. XRD peaks in spectra were identified using the ICDD (International Centre for Diffraction Data) database. In addition, the approximate weight of brass debris contained in sheared samples was calculated from the peak area ratio of a maximum peak of brass at 42.2–42.3° to that of quartz at 26.7° using software Origin.
Microstructures of starting and sheared gouges were observed using an FE-SEM (JEOL JSM 7001F) in the Department of Earth Science, Graduate School of Science, Tohoku University. Element composition analysis was also performed with energy dispersive spectroscopy (EDS) detectors (Oxford, INCA), at 15 kV accelerating voltage and 10 mm working distance.
In Measurements 1–3, Each ESR spectrum for samples showed anisotropic peaks at g2 = 2.0006, which corresponds to E1’ centre (g1 = 2.00179, g2 = 2.00053 and g3 = 2.00030, e.g., Ikeya, 1993) (
ESR intensity of E1’ centre increased with displacements and reached about 120% at a displacement of 1.4 m. That of OHC kept constant with increasing displacement. Those of peroxy centre-A, B and C kept constant, increased by about 30%, increased slightly by about 10% at the maximum displacement, respectively (
The powder XRD analysis shows that sheared gouge contains brass debris (
ESR intensity of E1’ centre in simulated-quartz gouge (φ = 45 – 300 μm) changed with increasing displacement by low-velocity shear tests, while those of OHC and peroxy centre (-A) did not change (Section 3 – ESR measurement). Especially, E1’ centre increased with increasing displacement. From SEM observations (
For frictional heating during shear tests, the temperature rise was estimated (Archard, 1959; Sibson, 1975). Because shear tests were performed at a normal stress of 1.0 MPa, a slip rate of 0.76 mm/s and displacements up to 1.4 m, the maximum temperature rise considering the local temperature rise between gouge particles was estimated to be about 14°C, where a frictional coefficient is 0.90, a slip velocity is 0.76 mm/s, a contact radius is 10 μm, a contact indentation strength is 10.9 GPa and a thermal conductivity is 4.3 W/mK (Table 1, Rempel and Weaver, 2008). Additionally, Yang
In our shear tests, we confirmed the blackening of starting gouge. The blackening was likely to occur due to brass contaminants mixed from the forcing blocks during shear tests (
In addition, we used #30 carborundum during the preparation for rough surfaces of the forcing blocks. After the polishing, we observed the surfaces using an optical microscope, and the small amount of carborundum remained in the grooves of the surfaces. Therefore, although we could not find carborundum in sheared gouges by XRD analysis or SEM observations, the ESR intensity change is also possible due to carborundum. From the results of Measurement 3, ESR intensities of E1’ centre, OHC and peroxy-A decreased with the decreasing weight of starting gouge and the increasing weight percentage of contaminants. Intensities for those ESR signals in quartz only sample and mixed sample-2 do not greatly deviate from the weight dependency of ESR intensity (
Tanaka (1987) and Yang
The sensitivity of ESR measurements decreases by microwave loss. Microwave loss in ESR measurements is dominant by electric conductivity, dielectric loss, magnetic loss because ESR measurements are performed at a constant frequency and intensities of electric and magnetic fields of the microwave. When the target sample contains materials with high microwave loss, the sensitivity of the ESR spectra of the sample decreases. Therefore, the intensity of ESR signals of target signals (to that of Mn marker) is likely to decrease with increasing the weight of materials with high microwave loss. Tanaka (1987), Yang
Previous shear tests by Tanaka (1987), Hataya and Tanaka (1993) and Yang
We performed low-velocity shear tests for simulated-quartz gouges, and the results show the relationship between ESR intensity for E1’ centre and displacement following a linear straight line or a saturating exponential curve (
If the E1’ centre is produced on the newly formed surfaces, there should be a strong correlation between ESR intensity for E1’ centre and grain size reduction. Coop
We performed low-velocity shear tests mimicking fault motions at the shallow depth for simulated-quartz gouges and clarify the following:
The change in ESR intensity during shear tests were not disturbed by either brass nor carborundum contaminants originated from the forcing blocks. ESR intensity of E1’ centre increased and OHC and peroxy centre kept constant by fracture with the increasing displacement. From comparison with previous studies, we revealed the relationship between ESR intensity for E1’ centre and fracture with various displacements separately from contaminants and frictional heating.
The change in ESR intensity during shear tests were not disturbed by either brass nor carborundum contaminants originated from the forcing blocks.
ESR intensity of E1’ centre increased and OHC and peroxy centre kept constant by fracture with the increasing displacement. From comparison with previous studies, we revealed the relationship between ESR intensity for E1’ centre and fracture with various displacements separately from contaminants and frictional heating.
Supplementary material, containing additional figures S1–S5 is available online at