Shear wave velocity estimation in the Sylhet Basin, Bangladesh by H/V analysis: implication for geophysical bedrock depth

The densely populated Bangladesh occupies most of the part of the Bengal Basin; the basin is located just above the subduction margin extended with an N-S alignment between the Indian and the Burmese Plates. The subduction tectonics along this Indo-Burmese plate boundary has put this locality to high seismic risk, which is also supported by the historical earthquake records. Moreover, being in the foothill of the Himalayan Mountains and the Indo-Burmese Ranges, respectively, to the North and the East, this country has become extremely riverine to be filled by sediments. Soft sedimentary layers over geophysical bedrock, with shear-wave velocity, V_S > 760 m/s, can significantly amplify earthquake ground motion to cause damage to infrastructure. In addition, bedrock depth significantly controls the phenomena of soil-infrastructure vibration resonance. That is why, for seismic risk evaluation, it is essential to have adequate information on soft sediment thickness or depth to the sediment-bedrock interface.

The continuously subsiding Sylhet Basin (SB, Zone 1), being a sub-basin within the northern limit of the Bengal Basin, is the flexural depocenter in the northeastern Bangladesh with possibly the thickest (~ 25 km) sedimentary successions (Bürgi et. al. 2021). The active Dauki Fault demarcates the northern limit of the Sylhet Basin as well as the Bengal basin, along which the Shillong Plateau has been uplifted.

In this work, we present V_S velocity up to 3000 m beneath three seismic stations in the Sylhet Basin, namely JAML, SUST and JAFL, data of which are available in the Incorporated Research Institutes for Seismology (IRIS) website. Here, subsurface V_S profile is estimated by inversion of single-station horizontal-to-vertical (H/V) spectral ratio curve within 0.2 to 10 Hz. The H/V curves at three stations are obtained from 15 days continuous recordings of seismic ambient noise data. We use HV-Inv software (García-Jerez et. al. 2016) for the inversion, in which the H/V ratio is interpreted under the diffuse filed assumption (Sánchez-Sesma et. al. 2011) for full H/V inversion considering contribution from the full noise wavefield. The inversion process is constrained using the existing general lithological information as well as unpublishable V_P data from active seismic surveys of Bangladesh Petroleum Exploration Company Ltd. (BAPEX).

From this analysis, we find that geophysical bedrock depth is approximately at 180 m, 220 m and 160 m, respectively, below the stations JAML, SUST and JAFL. To the best of our knowledge, neither the current approach of V_S estimation was applied nor such high-resolution V_Sinformation of sedimentary successions was reported in the study area previously. The presented velocity information could be crucial for engineering development, seismic hazard mitigation, and exploration purpose in the Sylhet Basin.