New GNSS and Geological Data From the Indo-Burman Subduction Zone Indicate Active Convergence on Both a Locked Megathrust and the Kabaw Fault

The Indo-Burma subduction zone is a highly oblique subduction system where the Indian plate is converging with the Eurasian plate. How strain is partitioned between the Indo-Burma interface and upper plate Kabaw Fault, and whether the megathrust is a locked and active zone of convergence that can generate great earthquakes are ongoing debates. Here, we use data from a total of 68 Global Navigation Satellite System (GNSS) stations, including newly installed stations across the Kabaw Fault and compute an updated horizontal and vertical GNSS velocity field. We correct vertical rates for fluctuating seasonal signals by accounting for the elastic response of monsoon water on the crust. We model the geodetic data by inverting for 11,000 planar and non-planar megathrust fault geometries and two geologically viable structural interpretations of the Kabaw Fault that we construct from field geological data, considering a basin-scale wedge-fault and a crustal-scale reverse fault. We demonstrate that the Indo-Burma megathrust is locked, converging at a rate of 11.6 +/- 5.4 mm/yr, and capable of hosting >8.2 M-w megathrust events. We also show that the Kabaw Fault is locked and accommodating strike-slip motion at a rate of 8.4 +/- 3.0 mm/yr and converging at a rate of 5.7 +/- 4.1 mm/yr. Our interpretation of the geological, geophysical, and geodetic datasets indicates the Kabaw Fault is a crustal-scale structure that actively absorbs a portion of the convergence previously ascribed to the Indo-Burma megathrust. This reveals a previously unrecognized seismic hazard associated with the Kabaw Fault and slightly reduces the estimated hazard posed by megathrust earthquakes in the region. Plain Language Summary Subduction zones are plate boundaries where two tectonic plates converge and can generate large earthquake along its main fault, the megathrust. Earthquakes rupture on faults that remain locked and accumulate strain during the interseismic period, the period of time between great earthquakes. The Indo-Burma subduction zone, where the Indian plate is converging with the Eurasia plate, is home to more than 200 million people. In spite of its enormous population, it is unclear whether the Indo-Burma subduction zone can generate large megathrust earthquakes. To evaluate its potential to host great earthquakes, we combine new GPS datasets that record plate motion during the interseismic period with geological analysis of the Kabaw Fault, one of the secondary faults in the Indo-Burma subduction zone. We demonstrate that Indo-Burma megathrust and Kabaw Fault are locked and can generate significant earthquakes. For the first time, we show that the Kabaw Fault absorbs a portion of the convergence previously ascribed to the Indo-Burma megathrust which could explain the long repeat time between great earthquakes on the megathrust.