Ground Motion Modelling in Bangladesh Using Stochastic Method

--Geological and tectonic framework indicates that Bangladesh is one of the most seismically active regions in the world. The Bengal Basin is at the junction of three major interacting plates: the Indian, Eurasian, and Burma Plates. Besides there are many active faults within the region, e.g. the large Dauki fault in the north. The country has experienced a number of destructive earthquakes due to the movement of these active faults. Current seismic provisions of Bangladesh are mostly based on earthquake data prior to the 1990. Given the record of earthquakes post 1990, there is a need to revisit the design provisions of the code. This paper compares the base shear demand of three major cities in Bangladesh: Dhaka (the capital city), Sylhet, and Chittagong for earthquake scenarios of magnitudes 7.0MW, 7.5MW, 8.0MW, and 8.5MW using a stochastic model. In particular, the stochastic model allows the flexibility to input region specific parameters such as shear wave velocity profile (that were developed from Global Crustal Model CRUST2.0) and include the effects of attenuation as individual components. Effects of soil amplification were analysed using the Extended Component Attenuation Model (ECAM). Results show that the estimated base shear demand is higher in comparison with code provisions leading to the suggestion of additional seismic design consideration in the study regions. Keywords---Attenuation, earthquake, ground motion, stochastic, seismic hazard. I.INTRODUCTION ANGLADESH is one of the most densely populated countries in the world. A large number of these populations are living in cities like Dhaka, Sylhet, and Chittagong. The infrastructure in most of these big cities has not been designed and built to standard practice and hence are vulnerable to natural disasters. Dhaka the capital city has a total population of over 13 million with an average of 45,000 people per square kilometer which makes it particularly vulnerable to earthquake hazard. Given these cities proximity to seismic sources a proper seismic hazard assessment is essential. This assertion is further compounded by the fact that the seismic provisions in the code are based on data prior to the 1990’s. Post the 1990’s there has been a major improvement in the earthquake data collection worldwide. Accordingly there has been major earthquakes of magnitudes >7 in the recent history. In the absence of major broadband ground motion recording stations, the use of semi-empirical or stochastic model is M. Ahmed is graduate Civil engineer. BEng (Honours 1 Class H1) from Victoria University, Melbourne, Australia (Phone: +61431209730; e-mail: [email protected]). S. Venkatesan is lecturer, School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Australia (e-mail: [email protected]). quoted as a viable alternative procedure [1]-[3]. In this paper, the authors have resorted to a component stochastic modelling approach “GENQKE” [1], [2] as it offers the flexibility to include region specific geological and seismological parameters. II.GEOLOGY AND SEISMICITY OF BANGLADESH Geologically, Bangladesh and most of west Bengal of India are occupied by the Bengal basin. Bengal Basin is surrounded by Indian Shield to the west and north, and the Indo-Burman Ranges to the east of Bangladesh (Fig. 1). Therefore, this basin is in close proximity to two subduction zones created by India and Eurasia plates. Continental collision between India and Eurasia plates at the region of Himalayas makes it one of the most seismic hazard areas on earth. The Indian plate is thrusting beneath Eurasian plates, converging at a relative rate of 40-50 mm/yr [4]. The tremors can be felt as far as Bangladesh which is in excess of 1000 km. The recent earthquake on 25 April, 2015 (7.8 Mw) in Nepal and the subsequent aftershocks were felt in Dhaka and many other cities in Bangladesh although no major structural damaged has been reported. Therefore, the country is surrounded by the Himalayan Arc, the Shillong Plateau and the Dauki fault system in the north; the Burmese Arc and Arakan Yoma anticlinorium in the east; the Naga DisangHaflong thrust zone in the northeast; and the recently activated inner fault in Haluaghat of Mymenshingh makes a compelling case for review of seismic hazard assessments. Bangladesh had experienced a number of large magnitudes in these regions e.g. Great Indian earthquake (1897) magnitude (Richter) of 8.7, Assam earthquake (1950) magnitude (Richter) of 8.5. Previous researchers, e.g. [5], identified five major tectonic blocks that can produce damaging earthquakes in the future. These blocks are (Fig 1): 1) Bogra fault Zone, 2) Tripura fault zone, 3) Assam fault zone, 4) Shillong plateau, and 5) Dhubri fault zone. Of all these blocks, 1and 2 are less active and 3, 5 are far away from the major cities listed in the study. This leaves the Shillong plateau as the nearest site source for stochastic modelling. Ground Motion Modelling in Bangladesh Using Stochastic Method Mizan Ahmed, Srikanth Venkatesan B World Academy of Science, Engineering and Technology International Journal of Civil, Environmental, Structural, Construction and Architectural Engineering Vol:9, No:12, 2015 1526 International Scholarly and Scientific Research & Innovation 9(12) 2015 scholar.waset.org/1999.3/10003039 In te rn at io na l S ci en ce I nd ex , C iv il an d E nv ir on m en ta l E ng in ee ri ng V ol :9 , N o: 12 , 2 01 5 w as et .o rg /P ub lic at io n/ 10 00 30 39