Unmapped Active Faults Persist: BRIN Calls for In-Depth Earthquake Risk Assessment

A researcher from the Geological Disaster Research Centre at the National Research and Innovation Agency (BRIN), Prof. Danny Hilman Natawidjaja, has revealed that many uncertainties remain regarding earthquake sources in Java, which need further investigation to enhance the accuracy of disaster risk assessments. Danny noted that Java Island has a complex earthquake source system. Besides the subduction zone or megathrust south of Java, there are also various active faults on land that have the potential to trigger destructive earthquakes. “Our knowledge of active faults in Java still holds many uncertainties. Several faults are already known, but their important characteristics, such as slip rate, segmentation, and maximum magnitude, are still not fully understood,” he said. Danny stated that one geological structure of concern is the Java Back-Arc Thrust, a large thrust fault stretching from the Jakarta area to Surabaya. According to him, the existence of this fault contributes to the earthquake hazard level in the northern part of Java, which has often been considered relatively safer than the southern region influenced by the subduction zone. Danny explained that the current earthquake hazard map results from the interpretation of various geological and seismicity data that is continuously evolving. Therefore, active fault maps and earthquake hazard maps are not static but are constantly updated as new findings emerge from research. He revealed that the BRIN team recently conducted detailed mapping in the area around Mount Ciremai, which produced new information regarding the presence of active faults and changes in the segmentation of several previously known fault structures. These findings have the potential to affect earthquake hazard estimates on a local scale. “Every new piece of geological evidence can change our understanding of earthquake sources. The impact may not be very large on a regional scale, but it can be significant for hazard assessment at the local level,” he said. Danny further emphasised that the threat of earthquakes is not solely ground shaking. Active faults can also trigger various secondary hazards such as surface ruptures, landslides, liquefaction, and local tsunamis. He considers that the surface rupture aspect often receives less attention in development planning. In fact, vital infrastructure such as toll roads, railway lines, dams, energy pipelines, and other public facilities risk serious damage if built directly on top of an active fault line. “Buildings can be designed to be more resistant to earthquake tremors, but it is very difficult to design a structure capable of withstanding surface ground displacement of up to several metres due to fault movement,” he said. Danny added that countries such as Japan, New Zealand, the United States, and Taiwan have implemented policies restricting construction in active fault zones. In Indonesia, however, implementing similar policies still faces challenges due to a lack of detailed data on the location and characteristics of active faults. Danny also highlighted the importance of integrating hazard, exposure, and vulnerability data in disaster risk studies. According to him, improving the quality of exposure and vulnerability data will not yield optimal risk assessments if information about the hazard sources still carries high uncertainty. “Risk is a function of hazard, exposure, and vulnerability. Therefore, improving the quality of hazard data, including active fault mapping and understanding earthquake and tsunami cycles, is a critical foundation for producing more accurate risk assessments,” concluded Danny Hilman Natawidjaja.