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[SCG40-09] BROADBAND MICROTREMOR ARRAY EXPLORATION IN THE KATHMANDU VALLEY, NEPAL
Keywords:Microtremor Array Exploration, Irregular Shape Array, Deep Sedimentary layers, Kathmandu Valley
Strong ground motion of long duration and long predominant period around four seconds were observed during the 2015 Gorkha earthquake at the central part of the Kathmandu Valley (KTV), Nepal (e.g., [1], [2]). KTV is a tectonic basin formed during the Himalayan orogeny due to the collision of the Indian plate to the Eurasian plate (e.g., [3]). The research group to which the authors belong (Group3, SATREPS) has conducted the deep exploration using microtremor from Dec. 2016 to May 2019, with the aim to reveal the cause of the mentioned phenomena to make use it for bettering the seismic hazard assessment. In JPGU2019, we presented a partial report of the activities until Dec. 2018 ([4]). We will report the summary of the activities in JPGU2021.
We conducted the field observation of microtremor in KTV using four broadband seismographs to estimate mainly Vs structure of deep sediments. We used also short period seismometers, land geophones and multi-channel data logger for shallow exploration [4]. Due to the difficulty to set up a large regular polygon arrays in the densely built-up city, we had to deploy large irregular quadrilateral ones based on confirmation of the isotropic nature of coherence function of microtremor, whereas equilateral triangular ones for middle and small sizes. The maximum inter-station distances of each sites were about 900m, and at maximum 1,400 m. Recording duration was one night or longer per deployment. Then we applied 2ST-SPAC analysis to the records of these arrays in a combined way to estimate the dispersion curves.
We, however, had to take countermeasures to extend the analyzable frequency range. To fix the problem supposed to be caused by the insufficient power of microtremor in the frequency range lower than 0.2 Hz, we have combined the phase velocity of Rayleigh waves that was determined by [5] using surface waves components of tectonic earthquake records in the frequency range from 0.01 Hz to 0.13 Hz. To cover highest frequency range we have conducted shallow exploration using different methods, MASW, CCA of mini hexagon array, L-Shape SPAC, etc., using short period seismometers and geophones: from about 5 Hz to higher than 10 Hz.
Besides we filled the gap of dispersion curves appeared in some sites between 0.2 Hz and the lowest analyzable frequency of 2ST-SPAC, i.e., around 0.3 Hz, by applying CCA to the records of scalene triangle arrays extracted from the quadrilateral ones. However, the gap from 0.13 Hz to 0.2 Hz remained unfilled for all sites. We also filled other gaps at a few sites in higher frequency ranges, but lower than 1 Hz, using auxiliary the phase velocities calculated from zero crosses of the SPAC coefficient curves.
We, then, applied the inversion of Vs structure from the determined dispersion curves. The estimated Vs structure showed a sharp discontinuity at a depth of about 600 m at the central part of KTV. In addition, another Vs discontinuity was observed at a depth of about 400 m. According to the information from a few deep drilling in KTV, The former can be interpreted as the seismic bedrock, and the latter is speculated to be a buried terrace formed in an era of discharge of the paleo-Kathmandu lake.
We could show a sufficient efficacy and usefulness of irregular shape arrays, and a comparison of various methods for shallow exploration as a show case, based on these observational activities.
Acknowledgement: This study was supported by JST and JICA under SATREPS: “Integrated Research on Great Earthquakes and Disaster Mitigation in Nepal Himalaya (FY2016-2021)”.
REFERENCES: [1] Bhattarai et al. (2015): Seism. Res. Lett., 86, 40-48;
[2] Takai et al. (2016): Earth Planets and Space, 68(10),1-8;
[3] Sakai et al. (2016): Earth Planets and Space, 68, 31, 1-10;
[4] Yokoi et al. (2019): Proceedings of JPGU2019, Tokyo;
[5] Hayashida et al. (2018): Proceedings of SSJ Fall Meeting.
We conducted the field observation of microtremor in KTV using four broadband seismographs to estimate mainly Vs structure of deep sediments. We used also short period seismometers, land geophones and multi-channel data logger for shallow exploration [4]. Due to the difficulty to set up a large regular polygon arrays in the densely built-up city, we had to deploy large irregular quadrilateral ones based on confirmation of the isotropic nature of coherence function of microtremor, whereas equilateral triangular ones for middle and small sizes. The maximum inter-station distances of each sites were about 900m, and at maximum 1,400 m. Recording duration was one night or longer per deployment. Then we applied 2ST-SPAC analysis to the records of these arrays in a combined way to estimate the dispersion curves.
We, however, had to take countermeasures to extend the analyzable frequency range. To fix the problem supposed to be caused by the insufficient power of microtremor in the frequency range lower than 0.2 Hz, we have combined the phase velocity of Rayleigh waves that was determined by [5] using surface waves components of tectonic earthquake records in the frequency range from 0.01 Hz to 0.13 Hz. To cover highest frequency range we have conducted shallow exploration using different methods, MASW, CCA of mini hexagon array, L-Shape SPAC, etc., using short period seismometers and geophones: from about 5 Hz to higher than 10 Hz.
Besides we filled the gap of dispersion curves appeared in some sites between 0.2 Hz and the lowest analyzable frequency of 2ST-SPAC, i.e., around 0.3 Hz, by applying CCA to the records of scalene triangle arrays extracted from the quadrilateral ones. However, the gap from 0.13 Hz to 0.2 Hz remained unfilled for all sites. We also filled other gaps at a few sites in higher frequency ranges, but lower than 1 Hz, using auxiliary the phase velocities calculated from zero crosses of the SPAC coefficient curves.
We, then, applied the inversion of Vs structure from the determined dispersion curves. The estimated Vs structure showed a sharp discontinuity at a depth of about 600 m at the central part of KTV. In addition, another Vs discontinuity was observed at a depth of about 400 m. According to the information from a few deep drilling in KTV, The former can be interpreted as the seismic bedrock, and the latter is speculated to be a buried terrace formed in an era of discharge of the paleo-Kathmandu lake.
We could show a sufficient efficacy and usefulness of irregular shape arrays, and a comparison of various methods for shallow exploration as a show case, based on these observational activities.
Acknowledgement: This study was supported by JST and JICA under SATREPS: “Integrated Research on Great Earthquakes and Disaster Mitigation in Nepal Himalaya (FY2016-2021)”.
REFERENCES: [1] Bhattarai et al. (2015): Seism. Res. Lett., 86, 40-48;
[2] Takai et al. (2016): Earth Planets and Space, 68(10),1-8;
[3] Sakai et al. (2016): Earth Planets and Space, 68, 31, 1-10;
[4] Yokoi et al. (2019): Proceedings of JPGU2019, Tokyo;
[5] Hayashida et al. (2018): Proceedings of SSJ Fall Meeting.