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WITHDRAWN Velocity Structure of the Crust and Uppermost Mantle Beneath Sumatra Derived from Raytracing on Seismic Phase Data

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WITHDRAWN Sumatra is located at a convergent plate boundary that includes a long subduction zone, active volcanoes, and an onland trench-parallel fault, the Sumatran Fault Zone, that extends the entire length of the island. Due its tectonic setting, Sumatra has abundant seismicity both offshore and onshore. Knowledge of the crustal and upper mantle seismic velocity structure of Sumatra is needed for studies of seismic hazards. In recent years, the seismic network operated by the BMKG in Sumatra has expanded greatly. Seismic waves from a crustal earthquake recorded on an array of seismometers contains valuable information about the seismic structure along its path. Here we present a 2-D P-wave and S-wave seismic velocity model of the crust and uppermost mantle beneath the mainland of Sumatra using observations of earthquake-generated seismic phases. Our analysis is based on seismic observation from 94 stations in Sumatra that recorded 12 shallow crustal earthquakes. The first-onset of crustal direct phases (Pg and Sg) and refracted upper mantle phase (Pn) were analyzed using seismic raytracing to obtain 1-D velocity models for five regions of Sumatra. We then interpolated these five models into a 2-D velocity model of the crust and uppermost mantle beneath Sumatra. We find that crustal thickness increases gradually from north to south, ranging from ~35 km in the norther Aceh Province to ~38 km in southern Sumatra. Thinner crust in the north is likely due to a greater amount of crustal extension. Our crustal thicknesses are consistent with previous estimates based on seismic receiver functions. Average crustal velocities are low with respect to the global average, varying from 6.04–6.31 km/s and 3.41–3.55 km/s for P-wave and S-wave velocities, respectively. The uppermost mantle (sub-Moho) P-wave velocity is 7.61–7.85 km/s which is lower than the global average. The seismic phase Sn is not observed in our data. These results illustrate the power of using simple, direct methods which exploit data from a dense seismic network to determine the seismic structure of the crust and upper mantle.

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Presenting Author: Gatut

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