Seismic Vp and Vp/Vs structure of the geothermal area around Tarutung (North Sumatra, Indonesia) derived from local earthquake tomography

A passive seismic experiment with 42 short-period stations was setup for 10 months from May 2011 within a 20-30 km radius covering the Tarutung basin and the northern part of the Sarulla graben along the seismically active Sumatran fault (North Sumatra, Indonesia). The objective of the project is to investigate the geothermal setting based on the Vp and Vp/Vs structures and the distribution of seismicity. We recorded 2856 local earthquakes by at least 8 stations which were reduced to 809 events having gap angle less than 180°. The initial earthquake locations were determined using simultaneous inversion for hypocenters and 1D velocity structure followed by 3D tomographic inversion. The resulting seismicity distribution, Vp and Vp/Vs structure reveal systematic differences between the Tarutung basin and the Sarulla graben. The Tarutung pull-apart basin is characterized by a complex structure reflected by a broader distribution of seismicity compared with the very narrow, rather vertical hypocenter distribution in the Sarulla graben, which was formed by fault-perpendicular extension and not by pull-apart tectonics. High Vp/Vs values near to the surface are imaged within the Sarulla graben and northeast of the Tarutung basin. These features are interpreted as fluid bearing sediments with fracturing indicating potentially favorable conditions for geothermal exploitation. The localized fluid flow northeast of Tarutung could be explained by the releasing step-over structure of the pull-apart basin and resulting dilatational stress regime in this part of the basin. Heat sources are assumed to be located at greater depth in association with magmatic activities accompanying the subduction process, and the heat is transported along the Sumatran fault to shallow depth.