Development of 2-Dimensional ESR thermal imaging apparatus and its application to drill core samples collected from a subduction seismogenic zone

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We have developed a 2-Dimensional electron spin resonance (ESR) imaging apparatus to nondestructively analyze heating events in fault rocks. The 2-D ESR imaging apparatus is composed of a slit cavity (slit width 3mmx10 mm), an external coil for 100kHz modulation field and an X-Y stage, in addition to an ordinary ESR spectrometer. To improve the resolution of 2-D ESR images, we remodeled the slit cavity and stuck a thin brass plate with a pinhole of Ф 2.6mm on the slit cavity. Consequently, the resolution is considerably improved from 2~3mm to 0.25mm. We have carried out 1-D continuous ESR measurement and 2-D ESR imaging of Nojima fault rocks using the 2-D ESR imaging apparatus. The target ESR signal for the 1-D and 2-D ESR measurements is a ferrimagnetic resonance (FMR) signal of iron oxides, which are generated and increased by heating. As a result, we have succeeded in detecting with precision of 0.25mm frictional heating events during ancient repeated earthquakes. The detection of f … More rictional heating events with such a high precision is the first in the world. Furthermore, we have studied a new method to estimate frictional heat temperature from 1-D ESR continuous data and developed an analytical computer program for it. In the next step, we plan to carry out 2-D ESR imaging of Taiwan Chelungpu Fault Drilling Project (TCDP) Hole B core samples and estimate frictional heat temperature in the 1999 ChiChi Earthquake, because this drilling project was delayed and consequently we could not measure 2-D ESR images of the core samples in the term of our scientific research project. On the other hand, we have considered how to convert a 2-D ESR image into a 2-D magnetic susceptibility map, and actually made 2-D magnetic susceptibility maps of natural rocks such as magnetite ore, serpentinite, gabbro, Nojima fault rocks and Chelungpu fault rocks by comparison with a standard Nickel sample (54.56 emu/g). In addition, we carried out basic studies on FMR signals of iron oxides and vitrinite reflectances of brown coal and mudstone, applied these indicators for frictional heating to Nojima fault and Chelungpu fault drill core samples, and estimated frictional heat temperatures in the 1995 Kobe and 1999 ChiChi Earthquakes, respectively. Less