Insights into the tectonics in Myanmar from new high-resolution and integrated focal mechanism catalogs

• Main Author: Wardah Shafiqah Binti Mohammad Fadil
• Other Authors: Wei Shengji
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Geology::Volcanoes and earthquakes
• Online Access: https://hdl.handle.net/10356/164105
• Institution: Nanyang Technological University

We investigate the seismically active and tectonically complex oblique subduction system in Myanmar, using data from new regional seismic networks (2016-2021) and available focal mechanisms in the past ~60 years (1963-2021), compiled from published studies and the Global Centroid Moment Tensor catalog. We ask how the NE-SW oriented motion of the Indian Plate relative to the Sunda Plate is partitioned, which structures are accommodating slip seismically and what are their corresponding seismogenic zones. Our study shows that the oblique India-Sunda motion is distributed along seismically active structures within the overriding Myanmar Plate i.e., the right-lateral Sagaing Fault, thrust and strike-slip faults in the upper crust beneath the Central Myanmar Basin (CMB) (e.g., the 2018 Mw 6.0 Bago-Yoma earthquake sequence), a backstop fault system that separates the eastern edge of the accretionary wedge (Indo-Myanmar Range; IMR) from the forearc crust and strike-slip faults within the deep accretionary wedge. The subducting Indian Plate is seismically active down to ~160 km beneath the Myanmar Plate with a seismogenic depth down to ~25 km from the top of the slab. The trench-parallel trend of the intraslab earthquake P-axes indicate a background stress field influenced by the trench-parallel mantle flow. More strike-slip events to the north of 22 ̊N suggest more dominant lateral shear within the slab while more normal-faulting events to the south of ~22 ̊N suggest more dominant plate-bending processes. The nodal planes of the intraslab earthquakes viewed in a locally unfolded Indian slab reference frame highlight the potential re-activation of subducted lithospheric fabrics that are similarly oriented to the normal faults and fracture zones in the Bay of Bengal and the Wharton Basin and are optimally oriented to the background stress field. Although there is a lack of earthquakes along the Rakhine- Bangladesh megathrust, shallow thrust-faulting earthquakes within the fold-and-thrust belt in the outer wedge of the IMR along with geological, geodetic and paleo-seismological x studies support ~E-W convergence and active subduction. In addition, our study shows a ~220 km long seismic gap along the locked central segment of the Sagaing Fault that is also adjacent to a gap in seismicity beneath the CMB. Our findings emphasize the need for a re-assessment of the seismic hazard in Myanmar given the potential for seismic gaps along the Sagaing Fault, the CMB and the megathrust to rupture as large earthquakes, the active seismicity along pre-existing faults (some of which are unmapped/blind) beneath the CMB, the seismically active subducting Indian Plate that is capable of producing moderate to large intraslab earthquakes and the close proximity of these active structures to populated cities (e.g., Mandalay, Nay Pyi Taw, Bago, Yangon), thick (6-15 km) sedimentary basins and other infrastructure (dams, highways and railway tracks).