[STT60-P10] Evaluation of relationship between fault displacement and ESR intensity using low-speed ring shear apparatus
Keywords:Fault, ESR: Electron Spin Resonance, Displacement, Heat, Temperature Rise, ESR Intensity
There exist several studies on a relationship between fault displacement and ESR Intensity (Tanaka, 1987; Hataya and Tanaka, 1993; Fukuchi, 2004). However, they have the following issues: (1) Conditions of ESR measurement were inappropriate. (2) Specific ESR signal was only measured and other signals were not. (3) Relationship between displacement and ESR intensity was not clearly represented by either a table or a plot. (4) Effect of heat on ESR intensity was not considered. Therefore, we conducted shear tests with silica sand (JIS test powder) under the conditions of shallow depths where frictional heat is negligible to clarify a relationship between displacement and ESR intensity. The inner diameter and outer diameter and thickness of sample are 20 mm, 30 mm and 1.5 mm. We used low-speed ring shear apparatus in Tohoku University. Sliding rate and the normal stress were kept constant to 0.72 mm/s and 0.981 MPa, respectively. Final displacements were varied among test runs up to 1.7 m.
Temperature rise δT on the shear surface was estimated to be a few degrees from Sibson (1975) under the conditions of this experiment. So, an effect of heating is negligible in this study. ESR measurement was conducted under appropriate conditions in which E1’ center representing ≡Si⋅ (⋅ means a lone electron and - means sheared electron pair, and ≡ means one Si atom combined with three Si atoms by each electron pair) does not overlap with R signal (structure corresponding to this signal is not clarified yet) and the other ESR signals can be detected. The relationship between displacement and ESR intensity was investigated by ESR measurements before and after the shear displacement. Many ESR signals such as E1’ center, OHC (O3-), Peroxy center (≡Si-O-O⋅) and so on decreased as displacement increases. This means that accumulation of displacement contributes to zeroing of ESR intensity under a low confining pressure corresponding to a sub-surface. Therefore, it is possible to reset the ESR intensity by fault activities at shallow depths.
Sibson, R. H., 1975, Generation of pseudotachylyte by ancient seismic faulting, Geophysical Journal International, 43(3), 775–794.
Tanaka, K., 1987, Investigations on the Ages of Fault Movements by Election Spin Resonance (ESR) Dating Method (Experiments on the Zero-Setting of the Paramagnetic Defects at the Time of Faulting), CRIEPI Res. Rep., U87039, Central Research Institute of Electric Power Industry, 22p.
Hataya, R. and Tanaka, K., 1993, Studies on Applicability of Fault Dating by ESR Method (1) (Experiments on Zero-setting of ESR Signals in Quartz), CRIEPI Res. Rep., U93019, Central Research Institute of Electric Power Industry, 26p.
Fukuchi, T., 2004, ESR Dating of Fault movements-Its principle and practice, Fukadaken Library, 63, Fukada Geological Institute, 45p.