Active Faults Revealed and New Constraints on Their Seismogenic Depth from a High-Resolution Regional Focal Mechanism Catalog in Myanmar (2016–2021)

Abstract

ABSTRACTWe derive a new earthquake focal mechanism catalog for 86 Mw>4.0 earthquakes that occurred in the Myanmar region from 2016 to 2021. We apply the generalized Cut-and-Paste inversion method to a new set of regional broadband waveform data to obtain the earthquake focal mechanism and centroid depth with uncertainties estimated in a bootstrapping manner. Compared with global earthquake catalogs, our results are better aligned with mapped, active faults and reveal seismic activity along unmapped, blind faults. Our new catalog shows that the Sagaing Fault is more active in its northern segment with deeper seismogenic zone (∼27 km) compared to its southern segment that has a shallower seismogenic zone (∼10 km), sandwiching a seismic gap in its central segment. Earthquakes that occurred on the unmapped, blind faults beneath the Central Myanmar Basin at shallow depths (3–12 km) suggest a dominating northeast–southwest compressional stress field. Shallow earthquakes beneath the Indo-Myanmar Range (IMR) are rare, instead, north–south-oriented strike-slip faults are active within the deep accretionary wedge or lower crust of the Myanmar plate between depths of 20 and 40 km. At the eastern edge of the IMR, earthquakes with high-angle thrust mechanisms occurred between depths of 30 and 48 km, likely along steep faults separating the accretionary wedge from the Myanmar forearc crust. High-resolution intraslab focal mechanisms show that to the north of 22° N, slab deformation is dominated by strike-slip earthquakes with subvertical fault planes down to a depth of ∼25 km beneath the slab, suggesting lateral shear within the slab due to the northward motion of the Indian plate. To the south, more normal-faulting earthquakes suggest a stronger role of plate-bending processes in the slab deformation.