Java Under Threat of Major Earthquake, Surrounded by Active Faults and Megathrust
Efforts to reduce earthquake disaster risk on Java require a more comprehensive understanding of the presence and characteristics of active faults. Professor Danny Hilman Natawidjaja, Senior Researcher at the Geological Disaster Research Centre of the National Research and Innovation Agency (BRIN), stated that many uncertainties remain regarding earthquake sources on Java, which need further investigation to improve the accuracy of hazard and risk assessments.
This was conveyed by Danny during the Workshop Advancing Multi-hazard Exposure Information in the Java Trench Region, Indonesia: For Enhanced Risk Assessment and Resilience, organised by Geoscience Australia together with several Indonesian government agencies at the B.J. Habibie Building in Jakarta.
In his presentation titled Tectonic Deformation in the Java Trench Region, Danny explained that Java 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. There are a number of known faults, but important characteristics such as slip rates, segmentation, and maximum magnitude are still not fully understood,” he said.
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 presence of this fault contributes to the earthquake hazard level in northern Java, which has often been considered relatively safer compared to the southern region influenced by the subduction zone.
He explained that the earthquake hazard maps currently in use are the result of interpretations from various geological and seismological data that are continuously evolving. Therefore, active fault maps and earthquake hazard maps are not static but are constantly updated as new findings emerge from research.
The BRIN team, for example, recently conducted detailed mapping in the area around Mount Ciremai, which yielded new information on the presence of active faults and changes in the segmentation of several previously known fault structures. These findings have the potential to influence 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 clarified.
Furthermore, Danny emphasised that the threat of earthquakes is not limited to ground shaking. Active faults can also trigger various secondary hazards such as surface rupture, landslides, liquefaction, and even local tsunamis.
According to Danny, the aspect of surface rupture often receives less attention in development planning. In fact, critical infrastructure such as toll roads, railway lines, dams, energy pipelines, and other public facilities risk serious damage if built directly on top of active fault lines.
“Buildings can be designed to be more resistant to earthquake shaking, but it is very difficult to design a structure capable of withstanding ground surface displacement of up to several metres due to fault movement,” he said.
Danny added that several countries such as Japan, New Zealand, the United States, and Taiwan have implemented policies restricting construction in active fault zones. Meanwhile, in Indonesia, the implementation of similar policies still faces challenges due to limited detailed data on the location and characteristics of active faults.
On the occasion, 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 on hazard sources still has 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 an important foundation for producing more accurate risk assessments,” he stated.
Through various ongoing geoscience studies, BRIN is working to strengthen the database and knowledge of earthquake sources in Indonesia. This information is expected to support the formulation of more effective disaster mitigation policies, while enhancing the resilience of communities and infrastructure in facing future earthquake threats.