[S-SS10] 地震活動とその物理

[SSS10-P09] Site characterisation and a new seismicity catalog (2016-2022) for Bangladesh from a regional array: possible correlation of seismicity and seasonal monsoon

*Syed Idros Bin Abdul Rahman¹、Karen Lythgoe¹、MD. Golam Muktadir^2,3、Syed Humayun Akhter^3,4、Judith Hubbard^1,5、Shengji Wei^1,5 (1.Earth Observatory of Singapore, Nanyang Technological University, Singapore、2.Bangladesh University of Professionals, Bangladesh、3.University of Dhaka, Department of Geology, Bangladesh、4.Bangladesh Open University, Bangladesh、5.Asian School of the Environment, Nanyang Technological University, Singapore)

キーワード：Seasonal seismicity, HVSR, oblique subduction, India-Eurasia collision

With its high population density and location at the eastern border of the India-Asia collision zone, Bangladesh has experienced a long history of natural disasters, which often results in extensive loss of life and property. The eastern half of Bangladesh is seismically active and is underlain by the Chittagong-Myanmar Fold and Thrust Belt which is an accretionary prism associated with the oblique subduction of the Indian Plate beneath the Burmese plate. The Bengal Basin sits on the downgoing plate and is characterised by extremely thick Cenozoic sediment of up to 20 km thick. The subducting plate interface may be capable of producing great earthquakes of magnitude <8.2, which pose a significant hazard to the high population. Questions on slip partitioning between the frontal and interior faults, effects of Bangladesh’s thick sedimentary cover on earthquake shaking and whether faults are locked and eventually will slip at seismic speed makes seismic hazard in this region hard to constrain. In addition, as Bangladesh sits on one of the largest river delta in the world and lie close to sea level, secondary hazards such as liquefaction and flooding are likely to occur due to earthquake shaking.

In 2016, a network of 28 seismometers was installed along the north-eastern (Sylhet region) and south-eastern (Chittagong region) areas of Bangladesh (the TREMBLE network). This network has been actively running for over 6 years and has captured more than 3000 regional earthquakes and 170 tele-seismic events. Here, we present ongoing results from this installation. A refined earthquake catalogue for the region for the years 2016 to 2022 has been obtained by careful refining of earthquake locations using manual picking of arrival times and a three-dimensional grid search technique. Individual seismic stations’ site characteristics were estimated using the horizontal to vertical spectral ratio method to find the site’s resonance frequency and amplitude. We obtained clear HVSR peaks for only 32% of the stations while the rest had either no peak or a broad peak. Flat profiles or broad peaks indicate low impedance contrast beneath the station, possibly due to the presence of a thick, stiff sedimentary column which is consistent with expected geology. Fewer seismic events were detected during the monsoon season, which could be partly due to elevated noise levels or alternatively signify a relationship between seismicity in the region and seasonal monsoon rainfall.