Near regional geology of Pantai Gosong NPP Site, West Kalimantan, Indonesia: Capable fault and geotechnical hazards assessment

The Pantai Gosong in West Kalimantan has been selected for the NPP site and requires an evaluation of external hazards potential that could affect the safety of the plants. The capable fault and geotechnical hazards are primary siting safety-related criteria and require a thorough assessment. The methods applied in this research are to analyze the Pliocene to Holocene tectonic, collect and analyze earthquake events from BMKG and USGS, calculate the b-value, and identify structural lineament from satellite imagery. All of the results of the desk study were then brought to the field for near-regional geological mapping. The first stage in field mapping is constructing the geological map, determining the Pleistocene layer, and evaluating all fault capabilities recognized in the field. The second stage is to conduct a site area geotechnical mapping using the RMR method.In Pliocene, the near-regional area is part of a stable Sundaland. A volcanic activity occurred east of Niut volcano, 80 km from the site. Essential sedimentation for capable fault analysis is identified in the Early Pleistocene. Based on the earthquake catalog, West Kalimantan shows a low seismicity region. In the 500 km radius, no b-value is recorded, which could mean that there are data limitations have occurred in the region to calculate a statistically significant b-value. In the near 1000 km radius, the b-value of 0.8 to 1.2 is related to thrust faults, and the higher value of 1.2 to 1.7 is related to the thrust mechanism with a strike-slip component. Lineament analysis suggested that the approximated fault and approximated concealed fault from the regional geological map were relatively old, buried under the younger sediments, and showed no sign of reactivation as lineament with the direction of these faults was minor. There were no structure-related features detected on the traces of these faults.Near regional mapping confirms the division of rock units of Andesite Unit, Granodiorite Unit, Quartz Diorite Unit, Sandstone Unit, Dissected Alluvial Deposit Unit, River and Swamp Deposit Unit, and Littoral Deposit Unit. The latter three are used as field indicators to determine fault capability in the principle of crosscut. The most essential rock unit for capable fault analysis is the Dissected Alluvial Deposit Unit. It is a Late Pleistocene sediment, observed ideally in the area of the Placer gold mine. Based on field surface investigation, some faults were identified. Nevertheless, it could not be considered a capable fault since none crosscut the sediment of Late Pleistocene to Holocene. From field surface mapping, none of this layer is crosscut by fault. Using the RMR method, the geotechnical study classified rocks in the site area into a very good and good rock class. The site area is an ideal rock site for NPP construction. There is no geotechnical hazard identified in the site area.