Resolution Limits of Global Seismic Tomography for Imaging the Flores Back-Arc Thrust: A Tectoglob3D-Based Assessment of the 2026 Mw 7.7 Flores Earthquake

Abstract

The 15 August 2026, Mw 7.7 Flores earthquake, which occurred at a depth of ~15 km offshore northeast of Nagekeo, provides an important case for investigating active deformation along the Flores Back-Arc Thrust. In this study, we assess Tectoglob3D's capability and resolution for studying the seismic structure related to the earthquake sequence. We focus on the P-and S-wave TX2019slab models of Lu et al. (2019). Tomographic cross-sections to depths of about 750 km are compared with the spatial and depth distribution of earthquake hypocenters. The Vp and Vs models can successfully describe the first-order regional structure of the Sunda–Banda subduction system and its deep velocity anomalies. However, the tomographic images do not clearly resolve the shallow structures related to the Flores Back-Arc Thrust. Conversely, the earthquake distribution shows a clear concentration in the back-arc region, suggesting that active seismic deformation is more evident in the back-arc than along the main subduction zone to the south. Further, tomographic models do not reliably reconstruct earthquakes deeper than approximately 40 km. The results suggest a mismatch in scale/resolution between global tomography and localized earthquake structures. Therefore, the current tomographic presentation needs further adjustment for local-scale research. Earthquake relocation should be performed to improve hypocenter accuracy, followed by adaptive-grid local earthquake tomography, with mesh resolution optimized according to seismic-ray density. This approach should improve the imaging of shallow and localized Vp–Vs heterogeneities and better constrain the three-dimensional structure of the Flores Back-Arc Thrust.