The 2025 Mw 7.7 Myanmar Earthquake: Finite Fault Versus Point Source in Modeling Coseismic Ionospheric Disturbances

Coseismic ionospheric disturbances (CIDs), observed in the total electron content (TEC) measured by Global Navigation Satellite System (GNSS) receivers, are often interpreted with a point source at the epicenter. We present a three-dimensional forward model that propagates the acoustic pulse from an extended rupture to the ionosphere and returns synthetic slant TEC along each observed line of sight. The source is a moment-weighted distribution of sub-sources along the fault, each radiating at its own rupture time. Rays are traced through a moving, stratified atmosphere into the far field, with absorption and ray-tube spreading; the neutral motion is projected onto the geomagnetic field, and the electron-density perturbation follows from the three-dimensional continuity equation. We apply the model to the 28 March 2025 Mw 7.7 Myanmar earthquake, which ruptured more than 400 km south of the epicenter along an almost straight fault, ideal for isolating the effect of rupture geometry. We compare finite-fault and point-source predictions with CID arrivals identified on 249 satellite-receiver arcs. The model reproduces the shape of the disturbance and its broadening with distance. The finite fault almost halves the median timing error of the point source (202 s against 383 s) and moves the strongest response toward the southern arcs, where observed amplitudes peak. Arrival times and amplitudes are ordered by distance from the fault rather than from the epicenter, which biases epicentral travel-time analyses. Misfits that grow with distance, common to both sources, point to the modeled propagation and to Rayleigh-wave-induced arrivals in the far field.