S U M M A R YIn this paper, we investigate the crustal and uppermost mantle structure beneath Toba caldera, which is known as the location of one of the largest Cenozoic eruptions on Earth. The most recent event occurred 74 000 yr BP, and had a significant global impact on climate and the biosphere. In this study, we revise data on local seismicity in the Toba area recorded by a temporary PASSCAL network in 1995. We applied the newest version of the LOTOS-07 algorithm, which includes absolute source location, optimization of the starting 1-D velocity model, and iterative tomographic inversion for 3-D seismic P, S (or the V P /V S ratio) and source parameters. Special attention is paid to verification of the obtained results. Beneath the Toba caldera and other volcanoes of the arc, we observe relatively moderate (for volcanic areas) negative P-and S-velocity anomalies that reach 18 per cent in the uppermost layer, 10-12 per cent in the lower crust and about 7 per cent in the uppermost mantle. Much stronger contrasts are observed for the V P /V S ratio that is a possible indicator of dominant effect of melting in origin of seismic anomalies. At a depth of 5 km beneath active volcanoes, we observe small patterns (7-15 km size) with a high V P /V S ratio that might be an image of actual magmatic chambers filled with partially molten material feeding the volcanoes. In the mantle wedge, we observe a vertical anomaly with low P and S velocities and a high V P /V S ratio that link the cluster of events at 120-140 km depth with Toba caldera. This may be an image of ascending fluids and melts released from the subducted slab due to phase transitions. However, taking into account poor vertical resolution, these results should be interpreted with prudence. Although the results show clear signatures that are quite typical for volcanic areas (low velocity and high V P /V S ratio beneath volcanoes), we do not observe any specific features in seismic structure that could characterize Toba as a super volcano.

[1] The 26 December 2004 Sumatra earthquake (M w = 9.1) initiated around 30 km depth and ruptured 1300 km of the Indo-Australian -Sunda plate boundary. During the Sumatra-OBS (ocean bottom seismometer) survey, a wide-angle seismic profile was acquired across the epicentral region. A seismic velocity model was obtained from combined travel time tomography and forward modeling. Together with reflection seismic data from the SeaCause II cruise, the deep structure of the source region of the great earthquake is revealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, and the subducting slab can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km from the trench axis, below the fore-arc basin. A high-velocity zone at the lower landward limit of the ray-covered domain, at 22 km depth, marks a shallow continental Moho, 170 km from the trench. The deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault. Assuming 100°-150°C as its updip limit, the seismogenic zone is predicted to begin 5-30 km from the trench. The downdip limit of the 2004 rupture as inferred from aftershocks is within the 350°-450°C temperature range, but this limit is 210-250 km from the trench axis and is much deeper than the fore-arc Moho. The deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate.

Significant earthquakes on the island of Sumatra, Indonesia, have predominantly been earthquakes with a thrust mechanism that occurred due to the subduction process and seismotectonics near coastal cities of West and South Sumatra, which could be affected by earthquakes triggered by these seismic sources. We compared the Seismic Hazard Function (SHF) of two coastal cities of Sumatra: Bengkulu and Padang. The results showed that the SHF of Bengkulu is higher than that of Padang. Estimated earthquake hazards are presented in the form of seismic hazard maps expressed as the PGA of 10% rate of exceedance probability in 50 years. In estimating the seismic potential in Sumatra, the seismic moment rate was jointly estimated from the smoothed mean seismicity rate and the pre-seismic subduction surface strain rate model. In this study, the island of Sumatra was chosen as a master model for Seismic Hazard Analysis (SHA). The motivation for choosing Sumatra for the SHA was because of the large body of complete historical earthquake data of the North Western Sunda Arc. The SHF is calculated based on a magnitude range of 6.0 to 9.0 during 50 years with the radius distance from the source less than or equal to 100 km.

Summary We investigated the seismic shear wave velocity structure of the upper crust beneath the Bandung area in West Java, Indonesia, using ambient seismic noise tomography. We installed 60 seismographs to record ambient seismic noise continuously in the city of Bandung and its surrounding area for 8 months. After inter-station cross-correlation of recordings of ambient seismic noise, we obtained empirical Green's functions for Rayleigh waves. Group velocity dispersion curves for Rayleigh waves between periods of 1 s and 8 s were measured on each inter-station path by applying the multiple filter analysis method with phase-matched processing. The spatial variation of group velocities shows a good correlation with the geological structure of the Bandung Basin. The Rayleigh wave dispersion maps were inverted to obtain the 1D shear wave velocity profiles beneath each station, which were interpolated to infer a pseudo-3D structure under the study region. The results show that the Bandung Basin has a thick layer of sediment. Along the northern, eastern and southern mountains surrounding the Bandung Basin there is high-velocity structure, except to the west of the Tangkuban Parahu volcano, where a massive low-velocity structure extending throughout the upper crust might indicate the presence of fluids or partial melts.

Summary In this paper, we compare two different methods for group velocity inversion: iterative, least-squares subspace optimization, and probabilistic sampling based on the Trans-dimensional Bayesian method with tree-based wavelet parameterization. The wavelet parameterization used a hierarchical prior for wavelet coefficients which could adapt to the data. We applied these inversion methods for ambient noise tomography of the western part of Java, Indonesia. This area is an area prone to multiple geological hazards due to its proximity to the subduction of the Australia Plate beneath Eurasia. It is therefore important to have a better understanding of upper crustal structure to support seismic hazard and disaster mitigation efforts in this area. We utilized a new waveform dataset collected from 85 temporary seismometers deployed during 2016–2018. Cross-correlation of the waveform data was applied to retrieve empirical Rayleigh wave Green's functions between station pairs, and the spatial distribution of group velocity was obtained by inverting dispersion curves. Our results show that, although computationally expensive, the Trans-dimensional Bayesian approach offered important advantages over optimization, including more effective explorative of the model space and more robust characterization of the spatial pattern of Rayleigh wave group velocity. Meanwhile, the iterative, least-square subspace optimization suffered from the subjectivity of choice for reference velocity model and regularization parameter values. Our Rayleigh wave group velocity results show that for short (1–10 s) periods group velocity correlates well with surface geology, and for longer periods (13–25 s) it correlates with centers of volcanic activity.