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The application of magnetic and geoelectrical methods for archaeological study is presented in this paper. The study was intended to map near-surface structures around the Indrapatra Fort, the 16th-century fortification built during the Early Aceh Sultanate in the north of Sumatra. Some of the structures around the fort have been buried after the 2004 Indian Ocean Tsunami or even by previous paleotsunami deposits. The ground magnetic measurements were performed over an area of 6 750 m2 using a high-resolution Proton Precession Magnetometer G-19 Geometric. Total magnetic field data were acquired as a 3-m grid station along survey lines spaced 3-m apart. Total magnetic field anomalies were calculated by diurnal and International Geomagnetic Reference Field corrections. To clarify the position of objects causing the anomalies, analytical signals and tilt derivative filters have been applied to the total magnetic field anomalies. The method found several structures with contrasting physical properties to those of the surrounding material. Both filters were able to identify the presence of circular, elongated, and square anomalies around the Indrapatra site. The archaeological interpretation of such structures is in terms of wells, canals, and road floors. The structures are made of limestone boulders buried within the uppermost sand layer. The burning process to cement the boulders causes their magnetic properties to be different in contrast to the layer of sand that covers them. Based on magnetic interpretation, three geoelectrical profiles were measured crossing the targeted objects using Supersting R8/IP 56 Electrode Electric Resistivity. Wenner configuration with 1-meter electrodes spacing was applied for a better model resolution. The inverted models of apparent resistivity data show an agreement with magnetic interpretation at shallow depth. The models also imaged the depth and geometry of the objects based on the electrical resistivity properties of subsurface materials. It is expected that the structures preserved tsunamigenic deposits, so the discovery of the structures will be able to contribute to further research on paleotsunami in Aceh. [ABSTRACT FROM AUTHOR]

The area surrounding Jaboi volcano, Indonesia, has been planned for geothermal energy development with capacity of ~ 50 Mwe. Geophysical surveys can contribute to the understanding of the hydrogeology of the area because they can provide information without interferences, which could potentially react to the flow of hot water. In many cases, the occurrence of a hot spring is closely related to its local fault system, which permits the flow of thermal fluid to its surface. This research applied the very low-frequency electromagnetic (VLF–EM) method and then compared it with electrical resistivity data to map fractures, faults, and geological aspects in the Jaboi volcano manifestation area. In tropical areas such as Indonesia, the VLF–EM method has been applied widely in studying subsurface structures. However, it is rarely used in volcanic areas with high terrain conditions even though the mobility of VLF methods is simpler than the electrical resistivity method. At the west side of the volcano, the 2D models from both VLF–EM and electrical resistivity showed the conductive zone (log 0.5–1.5 Ωm) found as a response to the manifestation, which has a similar direction with a local fault known as the Ceuneuhot fault. At the east side, the 2D models from both methods showed a similar conductive zone at depths of 15–25 m, which is caused by the existence of bedrock (> log 2.5 Ωm). The bedrock at depths of 0–10 m inhibits the rise of fluid to the surface so that there is no geothermal manifestation in the other local fault known as the Leumomate fault. In addition, the 2D models from the two methods showed the contrast of some fracture zones as entry and exit points of fluid in the Jaboi volcano. In addition, analysis of Landsat 8 optical images also showed high-temperature values and low vegetation indices in several active craters as indicators of volcanic activity. Based on the processed data, it is concluded that VLF–EM can be used as a fast and accurate method to provide information about near-surface structures in a geothermal field, and it can be used safely to suggest a location for drilling programs or to give contributions to protect geothermal springs. [ABSTRACT FROM AUTHOR]

The Great Sumatran Fault (GSF) activity is a severe threat to Banda Aceh development as the capital city of Aceh Province, Indonesia. The earthquakes originating along this fault trace, despite generating low strength, considerably threaten infrastructure and human lives. Therefore, a detailed study of the GSF activity and presence becomes critical. In this paper, we applied the Global Gravity Model plus (GGMPlus) to map the subsurface structure and modeling of two GSF segments with a resolution of 200 m/px, namely the Aceh and Seulimeum segments toward the north of the Sumatran Island. The Bouguer anomaly data are inconsistent with the geology of the study areas, dominated by igneous rocks on the Aceh segment and volcanic rocks on the Seulimeum segment. Further, the contrast between the Seulimeum segment in the northeast and the Aceh segment in the southwest can be demonstrated by high-pass filtering. The GGMPlus data validation results with field measurements using the Scintrex CG-5 Autograv, the root mean square error obtained via data comparison are 12.32% in the Krueng Raya fault zone, and 26.1% at the Seulawah Agam Volcano area, respectively. We also performed 2D gravity data modeling along with the Aceh and Seulimeum segments in the NW–SE direction. This model was then compared with the geological cross section, seismicity, and magnetotelluric data. The results of Singular Value Decomposition and Occam inversion show three vertical blocks of high densities with an interspersion of lower densities, which can be confirmed as the Aceh and Seulimeum segments. Based on data processing, it can be concluded that the GGMPlus satellite can improve the maps and images of the northernmost GSF structure. [ABSTRACT FROM AUTHOR]

Since the 2004 Sumatra-Andaman earthquake ( M 9.2), the northwestern part of the Sumatran island has been a high seismicity region. To evaluate the seismic hazard along the Great Sumatran fault (GSF), we installed the Aceh GNSS network for the Sumatran fault system (AGNeSS) in March 2005. The AGNeSS observed co-seismic offsets due to the April 11, 2012 Indian Ocean earthquake ( M 8.6), which is the largest intraplate earthquake recorded in history. The largest offset at the AGNeSS site was approximately 14.9 cm. Two M 6.1 earthquakes occurred within AGNeSS in 2013, one on January 21 and the other on July 2. We estimated the fault parameters of the two events using a Markov chain Monte Carlo method. The estimated fault parameter of the first event was a right-lateral strike-slip where the strike was oriented in approximately the same direction as the surface trace of the GSF. The estimated peak value of the probability density function for the static stress drop was approximately 0.7 MPa. On the other hand, the co-seismic displacement fields of the second event from nearby GNSS sites clearly showed a left-lateral motion on a northeast-southwest trending fault plane and supported the contention that the July 2 event broke at the conjugate fault of the GSF. We also calculated the Coulomb failure function ΔCFF caused by the first event to evaluate its effect on the second event. The results showed that the July 2 event was likely brought 0.1 MPa closer to failure by the January 21 event. [ABSTRACT FROM AUTHOR]

The devastating 2004 Indian Ocean tsunami caught millions of coastal residents and the scientific community off-guard. Subsequent research in the Indian Ocean basin has identified prehistoric tsunamis, but the timing and recurrence intervals of such events are uncertain. Here we present an extraordinary 7,400 year stratigraphic sequence of prehistoric tsunami deposits from a coastal cave in Aceh, Indonesia. This record demonstrates that at least 11 prehistoric tsunamis struck the Aceh coast between 7,400 and 2,900 years ago. The average time period between tsunamis is about 450 years with intervals ranging from a long, dormant period of over 2,000 years, to multiple tsunamis within the span of a century. Although there is evidence that the likelihood of another tsunamigenic earthquake in Aceh province is high, these variable recurrence intervals suggest that long dormant periods may follow Sunda megathrust ruptures as large as that of the 2004 Indian Ocean tsunami. [ABSTRACT FROM AUTHOR]