Crustal Velocity Structure of Eastern Bangladesh from Ambient Noise Tomography

Abstract:

Bangladesh is a country affected by the interactions of three tectonic plates: Indian, Eurasian, and Burma, resulting in the formation of complex fault systems and the occurrence of earthquakes. These earthquakes may pose a threat to the most densely populated region in the world, however, much of the area is covered by thick sediments (up to 20 km) and seasonal flooding, and as a result, the system is difficult to study. For this reason, we focus on improving the imaging of shallow crustal structure using seismic methods, with the hope that this will be used to improve our understanding of the shallow tectonic deformation as well as the deeper subduction zone.

We use continuous data from the seismic network “Temporary Receivers for Monitoring Bangladesh Earthquakes” (TREMBLE). The total information for the analysis comes from up to 22 short period seismometers and 6 broadband seismometers deployed by the Earth Observatory of Singapore and Dhaka University from 2016 to the present in eastern Bangladesh. We cross correlate ambient seismic noise recorded on the horizontal and vertical components to obtain the empirical Green’s function and resulting surface-wave phase-velocity measurements. These measurements are inverted to generate phase-velocity maps at different periods, which can be associated to different crustal depths to analyze and interpret the crustal features.

The dominant crustal geologic features in this region include the Bengal Basin, which has been filling with sediments coming from the Himalaya carried by the Ganges and Brahmaputra rivers since the Eocene; the Ganges-Brahmaputra Delta (GBD), formed by the same processes; and convergent features related to the collision of Indian and Burma plates. This oblique convergence results in the formation of an extensive accretionary prism; the Indo-Burma Ranges (IBR), a wide N-S trending fold and thrust belt; and a large detachment fault in the subsurface, which lies parallel to the bedding and transfers shortening horizontally. We look for lateral and vertical changes in seismic velocities to infer the locations and depths of these features throughout the study region.