Persaud Lab

Bangladesh-India-Myanmar Array (BIMA)

**FIG. 1 Location map of the BIMA seismic array** showing deployed locations of 29 broadband seismometers in Bangladesh (red) and 31 in Myanmar (blue). Examples of teleseismic recordings in Bangladesh and Myanmar are shown in Figures 2 and 3, respectively. Figure 4 shows stacks of computed receiver functions at each station that are projected onto a profile at latitude 23.5˚N. Large red triangles represent the locations of the major volcanoes in Myanmar from Lee et al. (2016). Black lines are major faults. Pink lines represent the plate boundary faults. Source: Ajala et al. (2018).

The Indo-Burma subduction system comprises the Indian plate subducting beneath the nonrigid Burma microplate. In the process, the largest river delta (~19 km sedimentary layer thickness) in the world, the Ganges-Brahmaputra Delta located on the Indian Plate is being accreted onto the Burma plate resulting in a broad (~200 km wide) subaerial accretionary prism. A GPS study indicates that this extensive subduction system is locked and is therefore capable of a large Mw>8.2 earthquake that could have devastating effects fo rthe world's most densely populated region.

**FIG. 2 (a) Locations of deployed broadband seismometers (29 stations) in Bangladesh.** (b) Epicentral locations of earthquakes (blue circles) in the range of 30 to 90 degrees used in the preliminary analysis. Black circles represent distance in 30˚ increments from the center of the BIMA array (red triangle). Red lines show major plate boundaries. (c) Example record section of vertical component seismograms recorded by the broadband seismometers.

**FIG. 3 (a) Locations of deployed broadband seismometers (31 stations) in Myanmar.** (b) Epicentral location of the earthquake (blue circle) plotted in the record section in c). Black circles represent distance in 30˚ increments from the center of the BIMA array (red triangle). Red lines show major plate boundaries. (c) Record section of vertical component seismograms from the event plotted in b), recorded by the Myanmar broadband seismometers.

As part of an effort to improve our understanding of the structure of this extreme subduction system and its related earthquake hazard, the collaborative NSF-funded Tripartite Project is has deployed the Bangladesh-India-Myanmar Array (BIMA). Analysis of large magnitude (>5.5) teleseismic (epicentral distance range of ~30º - 90º) earthquakes recorded across the seismic array using the receiver function technique will help illuminate crustal and upper mantle structures of the Indo-Burma subduction system.

Receiver Function Analyses
The 3-component broadband data from the BIMA Array are being used to construct seismic images of the crustal and lithospheric structure across the Indo-Burma Subduction system.

Examples of our teleseimic recordings in Bangladesh and Myanmar are shown in Figures 2 and 3, respectively. Our preliminary receiver function results are shown in Figure 4.

For our preliminary study, we have analyzed 30 teleseismic events with mostly easterly back azimuths.

We use the frequency-domain deconvolution method by Di Bona (1998) to compute our receiver functions.

Depth migration is performed using the time-depth relationship for P-to-S converted waves and velocities from the IASP91 reference model (Kennett & Engdahl, 1991).

Each back-projected receiver function is stacked and smoothed to generate common conversion point (CCP) images that better delineate the geologic features present.

While the receiver functions are projected to a common vertical plane, we note that they sample different portions of the subduction system.

We image first-order structures such as the sediment-basement boundaries, Moho, and the subducting Indian plate.

Publications page

ACKNOWLEDGMENTS
The Bangladesh-India-Myanmar (BIMA) project is funded by NSF Integrated Earth Systems Award 1714651. We thank students and professors from universities in the U.S. and Bangladesh, DMH staff in Myanmar, and PASSCAL personnel involved in deploying and servicing the broadband seismometers. We are grateful to the people at the seismic sites who permitted us to install our instruments; many assisted us in installing the seismic stations. All figures are plotted using the Generic Mapping Tools (Wessel et al., 2019).

FIG. 4 Preliminary 1 Hz radial receiver functions for all events projected onto a 2D plane at 23.5˚N latitude. (A) Stacked moveout-corrected receiver function for select traces in time plotted vertically beneath the stations. In addition to the number of good-quality receiver functions compared to the total number of recorded events at each station, station names are plotted at the top. (B) Stacked receiver functions as in A, but in depth overlain on top of the common conversion point (CCP) image generated from all the array stations. Positive (black) amplitude indicates increasing impedance with depth. In the CCP image, the red colors indicate positive amplitudes, while the blue colors indicate negative amplitudes. Yellow circles in the bottom profile show projected seismicity associated with the downgoing slab (Figure 1). Other filled circles in the profile are interpretations of primary phases in the image and are described in the legend. (NOTE. Not all receiver function traces are shown in the profiles to make the results easier to visualize. Source: Ajala et al. (2020).