<![CDATA[North Sumatra is characterized by complex interactions among the Sumatran Fault System, regional subduction, and the volcanotectonic setting of the Toba Caldera, producing heterogeneous shallow crustal seismicity and structurally controlled subsurface environments. This study investigates the seismotectonic and subsurface structural characteristics of North Sumatra using double-difference earthquake relocation, with particular emphasis on the Toba–Samosir region. Earthquake phase data recorded during 2021–2025 were processed using the HypoDD method. A total of 834 events were included in the relocation analysis, of which 411 yielded stable relocation solutions. Event pairing generated 8,084 earthquake pairs and 65,581 differential-time observations from P- and S-wave arrivals. The relocated catalog shows a more compact spatial distribution than the initial locations, particularly around Tarutung and Lake Toba–Samosir. In Samosir, 46 relocated earthquakes occur at depths of 0.39–14.15 km, with a median depth of 5.60 km, indicating predominantly shallow crustal seismicity. Comparison with mapped tectonic structures shows that several relocated clusters occur near the Renun and Toru fault zones, whereas the principal Samosir cluster is spatially offset from the mapped Samosir Fault. This pattern may indicate a previously unresolved subsurface seismogenic structure that could influence fracture-controlled permeability and subsurface fluid pathways within the Toba–Samosir environmental system, although this interpretation requires further geological and geophysical verification. Relocation quality is supported by a marked shift in RMS residuals toward lower values, with the dominant post-relocation interval occurring at 0–100 ms compared with approximately 400–600 ms before relocation. These results demonstrate that double-difference relocation can improve the characterization of shallow subsurface structures and provide a structural basis for understanding tectonic controls on environmental and potentially hydrogeological processes in the Toba–Samosir region.]]>
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