We estimate the one-and three-dimensional P wave velocity structure beneath the Toba caldera complex, a 30 Â 100 km topographic depression in North Sumatra, using arrival time data of local earthquakes recorded by a 40-station seismic network that operated for 4 months. Inversions reveal the presence of P velocities up to 37% below normal that likely map the distribution of magma within this subduction-related volcanic system, considered the world's largest. In the upper 10 km of crust the largest low-velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome. A smaller volume of low velocities is observed in the upper crust under the north end of the depression. Separating the two regions is a zone of locally high velocities, indicating that the shallow, subcaldera magma system is composed of two separate reservoirs, not a single one that extends the entire length of the caldera complex. Above each lowvelocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse $74 kyr ago. A low-velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low-frequency earthquakes in the 20-to 40-km-depth range. The low-frequency earthquakes apparently record the migration of melt in the mafic roots of this large-volume, crustal magma system.

In order to characterize the subsurface structure of the Jakarta Basin, Indonesia, a dense portable seismic broad-band network was operated by The Australian National University (ANU) and the Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) between October 2013 and February 2014. Overall 96 locations were sampled through successive deployments of 52 seismic broad-band sensors at different parts of the city. Oceanic and anthropogenic noises were recorded as well as regional and teleseismic earthquakes. We apply regularized deconvolution to the recorded ambient noise of the vertical components of available station pairs, and over 3000 Green's functions were retrieved in total. Waveforms from interstation deconvolutions show clear arrivals of Rayleigh fundamental and higher order modes. The traveltimes that were extracted from group velocity filtering of fundamental mode Rayleigh wave arrivals, are used in a 2-stage Transdimensional Bayesian method to map shear wave structure of subsurface. The images of S wave speed show very low velocities and a thick basin covering most of the city with depths up to 1.5 km. These low seismic velocities and the thick basin beneath the city potentially cause seismic amplification during a subduction megathrust or other large earthquake close to the city of Jakarta.

The complex geological structures in East Java and Bali provide important opportunities for natural resource exploitation, but also harbor perils associated with natural disasters. Such a condition makes the East Java region an important area for exploration of the subsurface seismic wave velocity structure, especially in its upper crust. We employed the ambient noise tomography method to image the upper crustal structure under this study area. We used seismic data recorded at 24 seismographs of BMKG spread over East Java and Bali. In addition, we installed 28 portable seismographs in East Java from April 2013 to January 2014 for 2-8 weeks, and we installed an additional 28 seismographs simultaneously throughout East Java from August 2015 to April 2016. We constructed inter-station Rayleigh wave Green's functions through cross-correlations of the vertical component of seismic noise recordings at 1500 pairs of stations. We used the Neighborhood Algorithm to construct depth profiles of shear wave velocity (Vs). The main result obtained from this study is the thickness of sediment cover. East Java's southern mountain zone is dominated by higher Vs, the Kendeng basin in the center is dominated by very low Vs, and the Rembang zone (to the North of Kendeng zone) is associated with medium Vs. The existence of structures with oil and gas potential in the Kendeng and Rembang zones can be identified by low Vs.

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