2:15 PM - 2:30 PM
[SSS10-09] Basement depth distributiion in Kizugawa lowland and Nara basin estimated from microtremor H/V peak periods and phase velocites from microtremor survey
Keywords:Nara Bonchi Toen Active Fault Zone, velocity structure, microtremor
We have been conducting research on the Nara-bonchi-toen Fault Zone (Eastern Marginal Nara Basin Fault Zone) as a "Comprehensive Research Project for the Nara-bonchi-toen Fault Zone" funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). In this study, we expand our microtremor array surveys and single-point microtremor observations (Yoshimi et al., 2020 and 2021 JpGU) to broader area in the Nara Basin and Kizugawa lowland to estimate the depth to the basement. In the Nara Basin, the H/V spectra shows distinct peaks and the peak period changes in accordance with the depth of the basement (e.g., Morikawa et al., 1998). The peak period of the H/V peak period has been used to develop a 3D velocity structure model of the Nara Basin (Sekiguchi et al., 2019).
The single-point microtremor observations were conducted at a total of about 500 locations, focusing on the western part of the Nara Basin (east of the Ikoma Mountains including hilly areas) and the Kizugawa lowland (north of Kizu and south of Uji). Six to eight seismographs were simultaneously used for single-point microtremor observations, with a spacing of about 100 m to 1 km, and simultaneous observation data of more than 30 minutes were obtained for the application of the two-point SPAC method. Microtremor array observations were conducted at 12 locations in the Kizugawa lowland and 2 locations in the western Nara basin, based on 7 simultaneous observations with a maximum aperture of 400m-800m and 4 simultaneous observations with a smaller aperture. The instruments used were all the same, consisting of a three-component velocity meter (Tokyo Sokushin SE-321, 5V/kine, 10-second natural period) and a data logger (Hakusan LS-8800, GPS time calibration, 24-bit AD conversion). The X direction of the seismometer was set to magnetic north by referring to the compass. In total, more than 1,200 microtremor data points from the central Nara Basin to the northern Kizugawa Lowland have been accumulated.
To calculate the H/V spectra, the data were divided into 16384 data with 50% overlap, Hamming window was applied, and the intervals containing traffic noise were removed based on the variance of the observed data. Then, Konno-Ohmachi filter was applied to the Fourier amplitude spectra to calculate the H/V spectra. The H/V peak period was defined as the peak value with a period of 0.2 to 5 seconds and a maximum value (but not less than 3). The H/V peak period that clearly differed from that of the near observation point (mainly those that shifted to shorter periods) was removed to form the data set. The H/V peak period data of the previous observation data (e.g. Morikawa et al., 1998) were also added to the dataset. From these H/V peak period data, we estimated the base depth from the Nara Basin to the Kizugawa lowland (including the western hills). For the conversion from H/V peak period (Tp) to base depth (D), we used the empirical formula proposed by Iwata et al. (2008) (D=192.15Tp) based on borehole information.
From the basement depth distribution estimated from the H/V peak period, we can recognize a zone of abrupt basement depth change in the eastern margin of the basin, as well as a zone of abrupt change in the western margin of the Nara Basin which corresponds to Ayameike flexure to Akishino flexure. In addition, the sedimentary basin from the Nara Basin ends near the Iioka hill at its northern end. Then, the basement depth increases at the north of the extension of the Katano fault. We will also show in the presentation the comparison of the depth of the basement and the velocity structure model with P-wave seismic reflection surveys and the 3D velocity structure model.
Acknowledgments
This research was conducted as part of the "Comprehensive Research Project for the Nara-bonchi-toen Fault Zone" funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). We would like to thank Yawata City, Uji City, Kyotanabe City, Joyo City, Ide Town, Seika Town, Kizugawa City, Ikoma City, Nara City, and local residents for their cooperation in the microtremor observations.
The single-point microtremor observations were conducted at a total of about 500 locations, focusing on the western part of the Nara Basin (east of the Ikoma Mountains including hilly areas) and the Kizugawa lowland (north of Kizu and south of Uji). Six to eight seismographs were simultaneously used for single-point microtremor observations, with a spacing of about 100 m to 1 km, and simultaneous observation data of more than 30 minutes were obtained for the application of the two-point SPAC method. Microtremor array observations were conducted at 12 locations in the Kizugawa lowland and 2 locations in the western Nara basin, based on 7 simultaneous observations with a maximum aperture of 400m-800m and 4 simultaneous observations with a smaller aperture. The instruments used were all the same, consisting of a three-component velocity meter (Tokyo Sokushin SE-321, 5V/kine, 10-second natural period) and a data logger (Hakusan LS-8800, GPS time calibration, 24-bit AD conversion). The X direction of the seismometer was set to magnetic north by referring to the compass. In total, more than 1,200 microtremor data points from the central Nara Basin to the northern Kizugawa Lowland have been accumulated.
To calculate the H/V spectra, the data were divided into 16384 data with 50% overlap, Hamming window was applied, and the intervals containing traffic noise were removed based on the variance of the observed data. Then, Konno-Ohmachi filter was applied to the Fourier amplitude spectra to calculate the H/V spectra. The H/V peak period was defined as the peak value with a period of 0.2 to 5 seconds and a maximum value (but not less than 3). The H/V peak period that clearly differed from that of the near observation point (mainly those that shifted to shorter periods) was removed to form the data set. The H/V peak period data of the previous observation data (e.g. Morikawa et al., 1998) were also added to the dataset. From these H/V peak period data, we estimated the base depth from the Nara Basin to the Kizugawa lowland (including the western hills). For the conversion from H/V peak period (Tp) to base depth (D), we used the empirical formula proposed by Iwata et al. (2008) (D=192.15Tp) based on borehole information.
From the basement depth distribution estimated from the H/V peak period, we can recognize a zone of abrupt basement depth change in the eastern margin of the basin, as well as a zone of abrupt change in the western margin of the Nara Basin which corresponds to Ayameike flexure to Akishino flexure. In addition, the sedimentary basin from the Nara Basin ends near the Iioka hill at its northern end. Then, the basement depth increases at the north of the extension of the Katano fault. We will also show in the presentation the comparison of the depth of the basement and the velocity structure model with P-wave seismic reflection surveys and the 3D velocity structure model.
Acknowledgments
This research was conducted as part of the "Comprehensive Research Project for the Nara-bonchi-toen Fault Zone" funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). We would like to thank Yawata City, Uji City, Kyotanabe City, Joyo City, Ide Town, Seika Town, Kizugawa City, Ikoma City, Nara City, and local residents for their cooperation in the microtremor observations.