Kinematics of faults in Bengal Basin: Constraints from GPS measurements

Abstract

The crustal deformation of Bengal Basin is constrained by motion on faults within the basin and those along the basin boundaries on all sides except the southern margin. Eocene Hinge Zone (EHZ), the most prominent structure within Bengal Basin, running through Kolkata-Ranaghat-Mymensingh, shows present day reactivation by hosting 8 earthquakes of magnitude 5.3 to 7.2 on or near its vicinity since 1906. Analysis of GPS derived velocities of stations around the hinge belt reveals a change in crustal kinematics across the belt. The predicted fault model that best fits the observed GPS station velocities on the surface shows that EHZ is a low angle east dipping fault which is locked to a depth of 10 km below the alluvium cover with a maximum slip rate deficit of 1.6 ± 0.7 mm/yr in its northern part at a shallow depth from surface while the deficit in the southern part of EHZ is 1.0 ± 0.6 mm/yr. The Garh-Moyna Khandaghosh fault running almost parallel to the western basin-shield margin does not show significant slip rate deficit. The basin-bounding faults are more active than the faults within and therefore they are greater sources of seismic hazard in the basin. The model results reveal that the Sylhet trough region in the northeastern corner is most vulnerable to future earthquakes as it undergoes deformation due to a maximum slip rate deficit of 9.5 ± 2.4 mm/yr on the shallowly dipping locked megathrust along the eastern margin of the basin plus that of 7.4 ± 2.6 mm/yr on the over thrusting Dapsi thrust and Dauki fault in the north.