[S-CG40] Active Tectonics and Seismic Hazards in the Himalayan Region

[SCG40-P04] Measuring surface-wave phase velocities (< 0.2 Hz) using teleseismic/ambient noise data in the Kathmandu Valley, Nepal

*林田 拓己¹、横井 俊明¹、Bhattarai Mukunda²、Pokharel Tara²、Shrestha Suresh²、Timsina Chintan²、Bhattarai Sunita²、Sharma Rajesh²、Nepali Dinesh² (1.国立研究開発法人建築研究所 国際地震工学センター、2.ネパール産業省 鉱山地質局)

キーワード：カトマンズ盆地、S波速度構造、表面波、地震波干渉法

The S-wave velocity (Vs) structure of the Kathmandu Valley (KTV) has been investigated using gravity data (Moribayashi and Maruo, 1980; Pradhan et al., 2018), waveforms from local earthquakes (Dhakal et al., 2016; Bijukchhen et al., 2017), microtremor data (e.g., Poovarodom et al., 2017; Hayashida et al., 2019; Yokoi et al., 2021), and reflection survey (Koshika et al., 2019; Kawasaki et al., 2019). These results indicate the substantial seismic velocity contrast between sedimentary layers and bedrock (Vs > 2000 m/s) and the existence of thick sediments (> 600 m) in the central basin. We conducted microtremor array surveys using broadband seismometers (CMG-40T; Guralp Systems Ltd.) and determined the Rayleigh-wave phase velocities in the lower frequency range (< 1 Hz) to estimate the thickness of the sedimentary layers (Bhattarai et al., 2017; Yokoi et al., 2018, 2019), but the dispersion characteristics below 0.2 Hz were unclear due to the low ambient noise level. To clarify the dispersive characteristics of Rayleigh waves below 0.2 Hz and improve the accuracy of depth estimations, we conducted continuous seismic/microtremor at seven sites in the KTV from February 2018 to November 2020. The station-to-station intervals are 4.6 –14.7 km. First, we applied the seismic interferometry to the continuously recorded seismic ambient noise data and estimated phase/group velocities of the Rayleigh wave at the lower frequency side. Seismic signals from several local earthquakes (> ML=4.0) and distant large earthquakes were detected during the observation period, so we also used the surface-wave component of the recordings for the phase velocity estimations. The calculated phase velocities show dispersive characteristics between 0.03 and 0.2 Hz, connected to the dispersion curves (> 0.2 Hz) derived from existing microtremor results. The inverted one-dimensional seismic velocity structure indicates substantially higher S-wave velocities between 10 and 30 km depth compared with the model of Pandey et al. (1995). The model helps constraint the bedrock depths that better explain dominant frequencies of microtremor H/V spectral ratios in the lower frequencies.



Acknowledgments:

This study was conducted as an international research project, 'Integrated research on great earthquakes and disaster mitigation in Nepal Himalaya (FY2016-FY2021)', under the Science and Technology Research Partnership for Sustainable Development (SATREPS) program of the Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA).