Ambient seismic noise tomography reveals a hidden caldera and its relation to the Tarutung pull-apart basin at the Sumatran Fault Zone, Indonesia

• We derived a 3D seismic velocity model by ambient noise studies at the Sumatra fault.
• Surface wave phases were used to invert for S-wave structure at depth by tomography.
• The model shows a velocity anomaly, interpreted as a previously unknown caldera.
• It is related to numerous volcanic hot springs, potential sources of geothermal energy.
• Hidden volcanic calderas represent a good indicator for geothermal prospection.

We analyzed the noise recordings of a short-period seismic network to derive a shallow crustal S-wave velocity model at the Sumatra Fault in Northern Sumatra, Indonesia. By correlating the noise of 40 seismic stations' recording for 9 months, we could recover Rayleigh waves from vertical component recordings with sufficient signal-to-noise ratio. Group velocities of the Rayleigh waves could be determined in the period range from 0.71 to 4.4 s. These group velocities were used to invert for 2D group velocity maps at specific periods. Finally, the derived group velocity maps were inverted for a 3D S-wave velocity model. This model shows a region of a strong velocity decrease off the Great Sumatran Fault Zone, at the northeastern margin of the young Tarutung pull-apart basin. This observed low velocity block coincides with a caldera-like morphological feature which is interpreted as the surface expression of a hidden volcanic caldera. Considering the surface manifestations of geothermal activity around this anomaly, we conclude that the caldera is still acting as a heat source. On the other hand, the weak morphological expression at the surface indicates a certain age of the caldera which might be older than the Tarutung pull-apart basin. The findings provide important constraints on general concepts for the formation of pull-apart basins along the Sumatran fault and their relation to volcanism.