Difference in Gas Emission Pattern under Different Compression Rates

\hspace*{5mm}In order to explain precursory changes observed in radon, hydrogen and methane concentrations in groundwater or soil-gas immediately before earthquake occurrences, experimental works of gas emission prior to and associate with rock fracture events have been carried out since 1970' s. A pioneering work by Giardini et al. (1976) showed that H2, CH4, H2O, N2, CO, O2 and CO2 were emitted from granite and gneiss under uniaxial compression to the rock fracture. In this study, we particularly focused on methane, investigating the characteristic features of methane emission with varying the compression rate and the distribution of methane of emitted gas in the granite. \hspace*{5mm}In last AGU fall meeting, we reported methane emission behavior under two different compression rates. In case of loading rate=11.1 kN/m2sec along the long-axis, the concentration of methane in the vacuum chamber began to increase from the background level when the pressure exceeded roughly half of the failure pressure (28.39±2.25 MN/m2). Then, it was increasing in an accelerative manner toward the maximum at the failure point of the sample. In contrast, under the 2-fold compression rate (22.2kN/m2sec), the methane concentration increased very slowly or negligibly before the failure and suddenly increased at the time of the failure. From this result, relationship of loading rate and temporal variations of the methane concentration expects to indicate the relationship between the concentration of emitted methane and both state of the rock and crack generation. We carried out a continuous loading experiment for detailed discussion on gas emission before and after the rock fracture. Sample used in this study were cylindrical Inada granite (d=50 mm, l=100 mm) were placed in a vacuum chamber equipped with a uniaxial compression device and were compressed continuously with different loading rate. The chemical composition of emitted gases in the chamber during a course of uniaxial compression were analyzed by a quadrupole mass spectrometer every 7 seconds (Hal-201, HIDEN, UK). \hspace*{5mm}We reported the result of temporal variations of the methane concentration in wide compression rate.