10:00 AM - 10:15 AM
[SSS10-05] Probing shallow region structure of Atotsugawa fault fracture zone with cosmic-ray muon detector
Keywords:Muography, Fault, Borehole, Atotsugawa
In this talk, I will introduce the method and the observation equipment of the subsurface density structure measurement using the cosmic ray muon observation system in the borehole, and report the measurement result in the borehole in the Atotsugawa fault zone in Hida city, Gifu prefecture in 2016 and the parameter estimation of the Atotsugawa fault.
Parameters such as fault strike, dip, width and density of fracture zones are important basic data for predicting seismic intensity and disaster scale of earthquakes. However, in order to obtain detailed information on these parameters using conventional methods, it is necessary to conduct extensive and spatially dense surveys. In this study, we have developed a method to survey the density structure of the subsurface including faults extensively by observation from a single point using a structure-viewing method using cosmic-ray muons, which has been actively studied in recent years.
In this method, the average density along the path is measured from the attenuation of cosmic-ray muons, which have high penetrating power. The muon attenuation does not depend on any properties other than the density of the penetrating material, and can be measured even in the surface layer of the earth's crust, which is highly heterogeneous. The penetrating power of cosmic-ray muons is so high that they can penetrate several kilometers underground, and can see through large structures such as faults and volcanoes. However, cosmic ray muons fly only from the sky, so until now, only structures that rise above the ground, such as volcanoes, could be observed.
In this study, we developed a compact instrument that can be installed in a borehole with a diameter of 15 cm in order to overcome this problem and to observe underground structures. The instrument has a limited directional resolution due to its small size. The measurement direction of the zenith angle is limited to one direction by a statistical method, and the azimuthal direction is resolved into eight directions. The instrument is installed in a borehole and the subsurface density structure is explored by moving the instrument in the depth direction, and the surrounding density structure including faults is measured extensively from one point to several hundred meters around.
Since 2016, we have measured the density structure up to 100 m underground using a borehole through the Atotsugawa fault in Hida City, Gifu Prefecture. As a result, we detected a low-density region in a direction consistent with past drilling survey records. Furthermore, we estimated the parameters such as the strike and dip of the fault using the measurement results.
Parameters such as fault strike, dip, width and density of fracture zones are important basic data for predicting seismic intensity and disaster scale of earthquakes. However, in order to obtain detailed information on these parameters using conventional methods, it is necessary to conduct extensive and spatially dense surveys. In this study, we have developed a method to survey the density structure of the subsurface including faults extensively by observation from a single point using a structure-viewing method using cosmic-ray muons, which has been actively studied in recent years.
In this method, the average density along the path is measured from the attenuation of cosmic-ray muons, which have high penetrating power. The muon attenuation does not depend on any properties other than the density of the penetrating material, and can be measured even in the surface layer of the earth's crust, which is highly heterogeneous. The penetrating power of cosmic-ray muons is so high that they can penetrate several kilometers underground, and can see through large structures such as faults and volcanoes. However, cosmic ray muons fly only from the sky, so until now, only structures that rise above the ground, such as volcanoes, could be observed.
In this study, we developed a compact instrument that can be installed in a borehole with a diameter of 15 cm in order to overcome this problem and to observe underground structures. The instrument has a limited directional resolution due to its small size. The measurement direction of the zenith angle is limited to one direction by a statistical method, and the azimuthal direction is resolved into eight directions. The instrument is installed in a borehole and the subsurface density structure is explored by moving the instrument in the depth direction, and the surrounding density structure including faults is measured extensively from one point to several hundred meters around.
Since 2016, we have measured the density structure up to 100 m underground using a borehole through the Atotsugawa fault in Hida City, Gifu Prefecture. As a result, we detected a low-density region in a direction consistent with past drilling survey records. Furthermore, we estimated the parameters such as the strike and dip of the fault using the measurement results.