Noise-Free Estimation of Temporal Change in Seismic Wave Attenuation Using High-Stable Vibration Sources.
Abstract We developed a method to detect changes in attenuation in transfer functions obtained by precisely controlled artificial seismic sources, namely Accurately Controlled Routinely Operated Signal System (ACROSS), and applied it to monitor the temporal changes for in-situ data collected by previous studies. Our method, together with the usage of the ACROSS sources, is less susceptible to change in noise level to which conventional methods such as envelope calculation suffer. The method utilizes the noise level that is independently estimated in the frequency domain. Thus, we can eliminate the influence of the noise by subtracting it from the observed signal. To test the performance, we applied this method to a dataset obtained at Awaji Island, Japan from 2000 to 2001. We detected the changes in amplitude with several causes including ground motion in an earthquake and water injection experiment. At the 2000 Western Tottori earthquake (MW = 6.6, Epicenter distance of 165 km), a sudden decrease in amplitude up to 5% followed by gradual recovery are clearly observed. These coseismic changes in amplitude are consistent with the opening of fluid-filled cracks as proposed by previous studies. We convert the amplitude change into ΔQ-1, which gives similar values as reported by previous studies using natural earthquakes. Increases in amplitude up to 5% associated with water injection experiments are also observed. During these experiments, the amplitude increased several days after the beginning of the injection and recovered to the previous level. This may be the result of a stress increase caused by the injection followed by a saturation increase by water diffusion.