5:15 PM - 6:30 PM
[SSS11-P08] Dense Microtremor Surveys on the Eastern Part of the Nara Basin, Japan
Keywords:H/V spectra, active fault
We are conducting research on the Nara-bonchi-toen Fault Zone (Eastern Marginal Nara Basin Fault Zone). Since the location of these faults have been mainly determined by the topography, it is very important to clarify the depth distribution of the basement rocks to see if there is any depth difference at the fault locations. In this study, we conducted microtremor array surveys and single-point microtremor observations in the Nara Basin to estimate the location of the fault by clarifying depth of 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 spectrum has been used to develop a 3D velocity structure model of the Nara Basin (Sekiguchi et al., 2019).
Microtremor arrays with a maximum radius of 400 m were observed at five locations on the Narayama hill, two locations in the northern part of Nara City, three locations in the southern part of Nara City, and three locations in the central part of Tenri City. In addition, single-point microtremor observations were conducted at about 300 locations, mainly in the eastern Nara Basin. Six to eight seismometers were used for the 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. The instruments used are all the same, consisting of a three-component velocty-type seismometer (Tokyo Sokushin SE-321, 5V/kine, natural-period 10 seconds) and a data logger (Hakusan LS-8800, GPS time calibration, 24-bit AD conversion). The X direction of the microtremor was set to magnetic north. In addition to the 105 microtremor array stations reported in Yoshimi et al. (2020), we have accumulated more than 600 microtremor data points in total.
In this presentation, we explain the basement depth distribution estimated from the peak period of H/V spectra, along with the basement depth and velocity structure model estimated from the microtremor array. We also show the distribution of the eastern margin fault zone of the Nara Basin illustrated by the basement depth distribution.
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 express our gratitude to Nara City, Tenri City, Yamato Koriyama City and local residents for their cooperation in the microtremor array observation.
References
Morikawa, H., Toki, K., Onoe, K., Akamatsu, J., and Takeuchi, T., Estimation of Deep Ground Structure in Northern Nara Basin using Microseism Array Record, 3rd Symposium on Urban Earthquake Hazards, a-26, 143-144, 1998.
Okumura, A., A. Samukawa, T. Sugai, M. Takada, and H. Soma, Comprehensive survey of the fault system at the eastern margin of the Nara Basin, 1996 Summary Report of the Research on Active Faults, Geological Survey of Japan, 51-62, 1997.
Haruko Sekiguchi, K. Asano, and T. Iwata, Three-dimensional velocity structure model of Nara Basin, Jour. Geol. Soc. Japan 125, 715-730, 2019.
M. Yoshimi, K. Asano, T. Iwata, K. Ninomiya, and L. Sugiyama, Microtremor array survey at the eastern margin of the Nara Basin, JpGU, 2020.
Microtremor arrays with a maximum radius of 400 m were observed at five locations on the Narayama hill, two locations in the northern part of Nara City, three locations in the southern part of Nara City, and three locations in the central part of Tenri City. In addition, single-point microtremor observations were conducted at about 300 locations, mainly in the eastern Nara Basin. Six to eight seismometers were used for the 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. The instruments used are all the same, consisting of a three-component velocty-type seismometer (Tokyo Sokushin SE-321, 5V/kine, natural-period 10 seconds) and a data logger (Hakusan LS-8800, GPS time calibration, 24-bit AD conversion). The X direction of the microtremor was set to magnetic north. In addition to the 105 microtremor array stations reported in Yoshimi et al. (2020), we have accumulated more than 600 microtremor data points in total.
In this presentation, we explain the basement depth distribution estimated from the peak period of H/V spectra, along with the basement depth and velocity structure model estimated from the microtremor array. We also show the distribution of the eastern margin fault zone of the Nara Basin illustrated by the basement depth distribution.
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 express our gratitude to Nara City, Tenri City, Yamato Koriyama City and local residents for their cooperation in the microtremor array observation.
References
Morikawa, H., Toki, K., Onoe, K., Akamatsu, J., and Takeuchi, T., Estimation of Deep Ground Structure in Northern Nara Basin using Microseism Array Record, 3rd Symposium on Urban Earthquake Hazards, a-26, 143-144, 1998.
Okumura, A., A. Samukawa, T. Sugai, M. Takada, and H. Soma, Comprehensive survey of the fault system at the eastern margin of the Nara Basin, 1996 Summary Report of the Research on Active Faults, Geological Survey of Japan, 51-62, 1997.
Haruko Sekiguchi, K. Asano, and T. Iwata, Three-dimensional velocity structure model of Nara Basin, Jour. Geol. Soc. Japan 125, 715-730, 2019.
M. Yoshimi, K. Asano, T. Iwata, K. Ninomiya, and L. Sugiyama, Microtremor array survey at the eastern margin of the Nara Basin, JpGU, 2020.