3:30 PM - 5:00 PM
[SCG54-P02] Feldspar luminescence signal changes during an earthquake estimated from surface samples and friction experiments: example of the Futagawa fault
Keywords:luminescence, fault
[Introduction]
It has been clarified in the laboratory that luminescence signals are reset by frictional heat associated with seismic fault slips (e.g., Oohashi et al., 2020), and the possibility of using them to identify the latest activity age of active faults has been proposed. ing. In order to clarify the relationship between seismic fault slip and luminescence signal reset in a natural fault zone, this study focuses on (1) the time of the latest activity, (2) the location of the latest activity surface, and (3) the slip parameters. (4) There are no thermal events unrelated to seismic fault slip. Luminescence dating of the specimens after the high-speed friction experiment was carried out, and the conditions for disappearance of the feldspar luminescence signal were estimated.
[Futagawa fault surface samples age]
Sub-sampling was performed by covering the outcrop with a blackout curtain at the Kamikin outcrop on the Futagawa fault, where about 2 m of displacement was observed in the 2016 Kumamoto earthquake. The sample was sieved under running water to obtain particles of 73-120 μm. Next, 2.58 ~ 2.63 g/cm3mineral was obtained using a heavy liquid. Finally, feldspar was extracted by separating magnetic minerals with a magnetic separator.
Luminescence was measured by the pIRIR225 method using the Riso TL/OSL reader of the National Institute of Advanced Industrial Science and Technology. Equivalent dose values were obtained in the range of approximately 170 - 185 Gy for pIRIR225. After fading correction based on Kars et al. (2008), ages of 77-96 ka were obtained for all samples. The obtained age agrees with the age of the Aso-4 pyroclastic flow deposit of 89 ka by the K-Ar method (Matsumoto, 1996) without any significant difference regardless of the distance from the slip surface. It is assumed that there was no event for the signal to reset.
[Signal change due to high velocity frictional experiments]
In order to clarify changes in feldspar luminescence signals during earthquakes in the subsurface, friction tests were carried out on mineral samples of 73-150 μm, and luminescence dating of recovered samples was performed. The experiment was carried out at an equivalent displacement velocity of 1 m/s, a displacement time of 2 s, a displacement of 2 m, and vertical stresses of 0.5 and 3 MPa. After the experiment, the sample from the outer circumference to 7.5 mm was collected, and then wet sieved to remove the sample that was crushed to less than 72 μm by the experiment. Mineral particles in the range of 2.40 to 2.81 g/cm3 were then extracted by heavy liquid separation.
As a result of luminescence measurement, the equivalent dose value of the post-experimental sample with a normal stress of 0.5 MPa was approximately 192.61 Gy/ka, which was not significantly different from the starting sample (approximately 185.68 Gy). On the other hand, in the experiment with a normal stress of 3 MPa, the equivalent dose value was 56.28 Gy and the age value was 33 ka, which were significantly decreased. The reason for the rejuvenation of the age value in the 3 MPa normal stress experiment is considered to be the thermal effect (maximum about 108 ℃ at 0.5 MPa, maximum about 812 ℃ at 3 MPa) during the high-speed friction experiment.
[Estimation of reset depth]
Based on the dry density of the Aso-4 pyroclastic flow of 1.23 g/cm3 (Nakazawa et al., 2017), the complete reset depth is considered to be about 352 m from the linear approximation of the vertical stress applied in the friction experiment and the equivalent dose value.
[References]
Durcan et al., 2015. Quaternary Geochronology. 28, 54 - 61.
Matsumoto., 1996./ Geological Survey of Japan, 501, 12-17.
Nakazawa et al., 2017. Journal of Geography, 124, 347 - 359.
Okumura and Shimooka., 2011.Engineering Geology of Japan, 1, 5-17.
Oohashi and Tamura., 2016. Journal of Geography, 122.
Oohashi et al., 2020. J. Geophys. Res. Solid Earth 125 (10) e2020JB019900.
Kars et al., 2008. Radiation Measrements, 43, 786 - 790.
It has been clarified in the laboratory that luminescence signals are reset by frictional heat associated with seismic fault slips (e.g., Oohashi et al., 2020), and the possibility of using them to identify the latest activity age of active faults has been proposed. ing. In order to clarify the relationship between seismic fault slip and luminescence signal reset in a natural fault zone, this study focuses on (1) the time of the latest activity, (2) the location of the latest activity surface, and (3) the slip parameters. (4) There are no thermal events unrelated to seismic fault slip. Luminescence dating of the specimens after the high-speed friction experiment was carried out, and the conditions for disappearance of the feldspar luminescence signal were estimated.
[Futagawa fault surface samples age]
Sub-sampling was performed by covering the outcrop with a blackout curtain at the Kamikin outcrop on the Futagawa fault, where about 2 m of displacement was observed in the 2016 Kumamoto earthquake. The sample was sieved under running water to obtain particles of 73-120 μm. Next, 2.58 ~ 2.63 g/cm3mineral was obtained using a heavy liquid. Finally, feldspar was extracted by separating magnetic minerals with a magnetic separator.
Luminescence was measured by the pIRIR225 method using the Riso TL/OSL reader of the National Institute of Advanced Industrial Science and Technology. Equivalent dose values were obtained in the range of approximately 170 - 185 Gy for pIRIR225. After fading correction based on Kars et al. (2008), ages of 77-96 ka were obtained for all samples. The obtained age agrees with the age of the Aso-4 pyroclastic flow deposit of 89 ka by the K-Ar method (Matsumoto, 1996) without any significant difference regardless of the distance from the slip surface. It is assumed that there was no event for the signal to reset.
[Signal change due to high velocity frictional experiments]
In order to clarify changes in feldspar luminescence signals during earthquakes in the subsurface, friction tests were carried out on mineral samples of 73-150 μm, and luminescence dating of recovered samples was performed. The experiment was carried out at an equivalent displacement velocity of 1 m/s, a displacement time of 2 s, a displacement of 2 m, and vertical stresses of 0.5 and 3 MPa. After the experiment, the sample from the outer circumference to 7.5 mm was collected, and then wet sieved to remove the sample that was crushed to less than 72 μm by the experiment. Mineral particles in the range of 2.40 to 2.81 g/cm3 were then extracted by heavy liquid separation.
As a result of luminescence measurement, the equivalent dose value of the post-experimental sample with a normal stress of 0.5 MPa was approximately 192.61 Gy/ka, which was not significantly different from the starting sample (approximately 185.68 Gy). On the other hand, in the experiment with a normal stress of 3 MPa, the equivalent dose value was 56.28 Gy and the age value was 33 ka, which were significantly decreased. The reason for the rejuvenation of the age value in the 3 MPa normal stress experiment is considered to be the thermal effect (maximum about 108 ℃ at 0.5 MPa, maximum about 812 ℃ at 3 MPa) during the high-speed friction experiment.
[Estimation of reset depth]
Based on the dry density of the Aso-4 pyroclastic flow of 1.23 g/cm3 (Nakazawa et al., 2017), the complete reset depth is considered to be about 352 m from the linear approximation of the vertical stress applied in the friction experiment and the equivalent dose value.
[References]
Durcan et al., 2015. Quaternary Geochronology. 28, 54 - 61.
Matsumoto., 1996./ Geological Survey of Japan, 501, 12-17.
Nakazawa et al., 2017. Journal of Geography, 124, 347 - 359.
Okumura and Shimooka., 2011.Engineering Geology of Japan, 1, 5-17.
Oohashi and Tamura., 2016. Journal of Geography, 122.
Oohashi et al., 2020. J. Geophys. Res. Solid Earth 125 (10) e2020JB019900.
Kars et al., 2008. Radiation Measrements, 43, 786 - 790.