Beyond Megathrust: Experts Warn of Intra-Slab Earthquake Threats to Java
Warnings regarding the potential threat from tectonic plate movements have been repeatedly issued by the government, specifically the Meteorology, Climatology, and Geophysics Agency (BMKG), as well as disaster experts. The intention of these warnings is not to cause panic, but to serve as a reference for disaster mitigation to reduce fatalities and aim for zero victims.
While much attention is focused on the potential for massive megathrust earthquakes accompanied by tsunamis—with Indonesia surrounded by 14 megathrust segments—there is another potential source of disaster threatening the island of Java. Daryono from the Indonesian Disaster Expert Association (IABI) stated that it is vital to be alert to the potential for large earthquakes in Java, which requires a deep understanding of tectonic mechanisms, particularly the threat originating from the subduction zone of the Indo-Aussie Plate diving beneath the Eurasian Plate.
“For a long time, public attention has been focused on the potential for megathrust earthquakes located at shallow plate contact zones that risk triggering large tsunamis. However, there is another threat that is no less destructive: intra-slab earthquakes (within the plate),” he said on Saturday (5/9/2026).
“Intra-slab earthquakes occur due to the process of deformation and faulting occurring within the subducted plate at intermediate depths (60-300 km) to deep levels (above 300 km),” he added.
According to Daryono, intra-slab earthquakes occur due to the breaking, cracking, or faulting of rocks within the oceanic plate that is diving beneath the continental plate. This is triggered by three main factors: gravitational pull (slab pull) that cracks the plate body like a heavily loaded biscuit; intense bending pressure that causes the plate structure to snap like an extremely bent plastic ruler; and rock dehydration, where geothermal heat ‘squeezes’ water out of minerals, creating internal pressure that makes the plate brittle and prone to fracturing.
“The mechanical characteristics of intra-slab earthquakes often trigger high-magnitude ground motion and wave frequencies that produce a very wide shaking spectrum,” he said.
Historical seismicity records in Java prove the massive seismic impact of intra-slab events. Events such as the 1867 Java Earthquake (M7.8), the 1903 West Java/Banten Earthquake (M7.9), and the 1943 Central Java Earthquake (M7.1) serve as empirical evidence that the release of tectonic stress energy within the plate slab can shake multiple provinces and claim many lives.
History records several intra-slab earthquake events in Java, including:
10 June 1867, M7.8: 500 deaths and widespread damage across almost the entire island of Java.
28 March 1875, M7.6: Widespread damage across almost all of Java.
27 February 1903, M7.9: Highly destructive in Banten, West Java, and Lampung.
23 July 1943, M7.1: Damage occurred in Cilacap, Tegal, Purwokerto, Kebumen, Purworejo, Bantul, and Pacitan, with over 213 deaths.
2 September 2009, M7.0: 108 deaths.
10 April 2021, M7.0: 10 deaths, with damage across 16 regencies and cities.
“Even medium-scale events, such as the Cilacap M5.3 earthquake on 4 September 202_6, which was triggered by a normal fault due to slab pull and plate bending and felt as far as East Java, are types of intra-slab earthquakes,” he noted.
“This phenomenon confirms that if an intra-slab earthquake reaches a magnitude of M6.0 or higher, S-waves (shear waves) propagating efficiently through the dense rock structure of the plate slab will multiply the level of infrastructure damage on the surface,” Daryono explained.
Regarding anticipation, Daryono stated that the primary step in mitigating intra-slab earthquakes lies in the integration of strengthened seismological analysis and structural mitigation engineering.
“From a seismological perspective, mapping stress field zones and real-time microseismic monitoring of the subducted lithosphere is crucial to identify areas of rock acceleration that are prone to collapse,” he said. “This seismic information must serve as the primary basis for updating the seismic hazard maps for the Java region. At the site level, the implementation of earthquake-resistant building codes based on Peak Ground Acceleration (PGA) is the most absolute line of defence,” he emphasised.
While intra-slab earthquakes do not trigger tsunamis, Daryono warned they should not be ignored. “Given that intra-slab earthquakes do not trigger tsunamis but dominate high-frequency ground shaking, the resilience of earthquake-resistant physical construction and spatial planning readiness is key. Therefore, remain alert to the potential of intra-slab earthquakes,” Daryono concluded.