One-dimensional crustal and uppermost mantle seismic velocity structure of Sumatra, Indonesia, from earthquake traveltimes

We use seismic record sections from six shallow crustal earthquakes recorded by the Indonesian national seismic network to investigate the one-dimensional (1D) crustal and uppermost mantle seismic velocity structure of Sumatra. We identify direct crustal phases (P_g, S_g), Moho reflections (P_mP and S_mS), and mantle-refracted head waves (P_n) that propagate through the island-wide network. Iterative 1D forward modeling is used to generate theoretical traveltimes and derive 1D velocity models by minimizing residuals between observed and calculated arrivals. Our results reveal a positive velocity gradient in the crust and modest lateral depth variations (±2 km) to the Moho discontinuity. The regional average crustal thickness of 35 km likely reflects a localized isostatic root beneath the Barisan Mountains of southwestern Sumatra. The 1D model resolves a distinct upper crustal gradient (0–10 km) where v_P increases from 5.0 km/s to 6.1 km/s, exhibiting high lateral variability influenced by the regional volcanic arcs and transtensional basins. The middle crust (10–25 km) has a more homogeneous structure compared to the upper crust, with v_P increasing steadily from 6.1 km/s to 6.5 km/s. v_P in the lower crust (25–35 km) is 6.5 km/s to 6.6 km/s. The sub-Moho mantle seismic velocity (v_P) has a regional average value of ~7.8 km/s, which is 0.3 km/s lower than the global reference (8.1 km/s), indicating high temperatures and/or the presence of partial melts associated with the active magmatic arc. Comparison with reference seismic crustal models shows that our regional average seismic structure is consistent with a continental arc structure rather than island arc models. These findings provide a well-defined 1D seismic velocity model that is essential for improving earthquake locations and assessing seismic hazards in Sumatra.