Results from the TREMBLE Seismic Network in Bangladesh (2016-2021): Seismicity, COVID-19 Seismic Noise, and Cyclones

Bangladesh, one of the most densely populated countries in the world, is located on the eastern side of the India-Asia collision zone. The eastern half of the country is underlain by the seismically active Chittagong-Myanmar Fold and Thrust Belt, an accretionary prism associated with the eastward-subducting Indian Plate, and the country is bordered to the north by the Dauki thrust fault, which bounds the southern margin of the actively rising Shillong Plateau. The last great earthquake in this region occurred in 1897 (~Mw8.0). Seismic hazard in this region is poorly constrained due to questions about slip partitioning between the frontal and interior faults of the fold and thrust belt, whether faults will eventually slip at seismic speeds, how shaking will be amplified and/or attenuated by the thick sedimentary cover. Bangladesh, having 230 rivers, sits on top of the worlds largest river delta close to sea level, and therefore earthquake shaking is likely to also trigger secondary hazards, like liquefaction and flooding. In 2016, a collaboration between the Earth Observatory of Singapore and Dhaka University led to the installation of a network of 28 seismometers along the north-eastern (Sylhet region) and south-eastern (Chittagong region) parts of Bangladesh (the TREMBLE network). The aim of this network is to monitor seismicity, and to develop a better understanding of the subsurface structure in the region. This network has been actively running for over 5 years. Here, we present the ongoing results from this installation. The TREMBLE network has captured >2000 earthquake events, and also detected >160 tele-seismic events. Earthquake relocation of the events significantly enhances the accuracy of earthquake locations and depths, providing a reliable earthquake catalog for the region for the years 2016 - 2021. In addition, the network has recorded changes in seismic noise associated with the ongoing COVID-19 pandemic and associated lockdowns; here we calculate the change in noise and compare it to both governmental actions (lockdown announcements) and mobility data. The network has also recorded phenomena, including Cyclone Mora, which hit in May 2017, and generated broad seismic signals across the region. Low frequency signals attributed to possible atmospheric activities are also captured by the network.