Refined Seismicity Patterns in Sumatra from High-Resolution Earthquake Relocation Enabled by an Expanded Seismic Network

The recent expansion of the broadband seismic network operated by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG) across Sumatra provides improved spatial coverage and an opportunity to characterize regional seismicity in greater detail. To improve earthquake locations, we applied manual phase repicking, waveform cross-correlation, and double-difference relocation to a dataset of 7,497 events recorded between 2020 and 2024, resulting in the Sumatra Optimized and Updated Relocation Catalog from Expanded Seismic Network (SOURCE). Of these, 7,100 events were successfully relocated. The relocation reduces differential-time residual scatter and sharpens seismicity patterns across the region. Bootstrap resampling with 100 realizations per relocation region indicates stable relative locations, with horizontal median semi-axis lengths of the per-event 95% confidence error ellipsoids generally below 1 km. At the regional scale, the relocated seismicity delineates the Sunda subduction system to depths of about 220 km and reveals a southward increase in slab dip along the Sumatra margin. In central Sumatra, local density maxima within the intermediate-depth seismicity distribution coincide with the projected trace of the subducted Investigator Fracture Zone (IFZ), providing clearer seismological support for segmented intraslab deformation beneath this part of the margin. In the overriding plate, SOURCE also sharpens several shallow crustal clusters, including Djabo, Kabanjahe, Toba, and Lahat, and helps clarify localized upper-plate deformation. Some of these clusters may reflect active local faults that remain poorly resolved in existing surface mapping. These results provide an updated framework for interpreting seismotectonic complexity in Sumatra and a refined earthquake catalog for future tectonic and hazard-related studies.