Megathrust Rupture: Pangandaran Coast Swallowed by Tsunami Exceeding 8 Metres

A study conducted by the National Research and Innovation Agency (BRIN) indicates that the formation mechanism of the 2006 Pangandaran tsunami may be more complex than previously understood. In addition to being triggered by an earthquake, the tsunami is suspected to have been influenced by underwater mass movements, which amplified wave formation and accelerated the arrival time of the tsunami at the coast.

“The Pangandaran tsunami is an example of an ‘earthquake tsunami,’ a condition where the resulting tsunami waves are significantly larger than estimates based on recorded earthquake parameters,” stated Wiko Setyonegoro, a researcher at BRIN’s Deep Sea Research Centre (PRLD), during the ‘Deep-Sea Science Forum: Toward Deep Sea Mission 2045’ webinar held recently.

According to him, several studies show that wave heights in certain locations reached over 8 metres, with survivors testifying that waves exceeded the height of coconut trees in various coastal areas.

“After several investigations and analyses, there is a potential possibility that non-seismic factors contributed to the increase of the tsunami on land,” Wо said while presenting research findings regarding the potential for short-range tsunamis triggered by megathrust earthquakes south of Java, using the 2006 Pangandaran tsunami as a case study.

According to Wiko, the Pangandaran tsunami has long been known as an ‘earthquake tsunami,’ a phenomenon where the generated waves are far larger than the measured earthquake source parameters. This condition indicates that the tsunami generation mechanism likely stems not only from earthquake-induced deformation but also involves non-seismic factors that require further study.

“After testing various scenarios, there is a possibility that the earthquake and underwater mass movements worked together to generate the tsunami. This model provides an estimate that more closely matches the actual tsunami conditions,” Wiko explained.

To test this hypothesis, the research team assimilated bathymetry data, topography, historical earthquake catalogues, and various field observation data. Simulations were conducted using a nesting grid system, enabling high-resolution tsunami modelling that depicts inundation areas down to the building scale. Various scenarios were then tested through an iterative calibration process to obtain the model that best fits the 2006 tsunami conditions.

Evaluations of 14 scenarios showed that the model combining the earthquake source with indications of underwater mass movement had the highest level of consistency with field observations. This consistency was evident in wave height, tsunami arrival time, and the level of exposure to infrastructure and casualties.

The study also shows that the presence of underwater mass movements has the potential to accelerate the arrival of a tsunami. In simulations using only the earthquake source, waves were estimated to reach the coast in approximately 40-50 minutes. However, in the model incorporating underwater mass movement, the tsunami was estimated to arrive within 20-30 minutes, aligning with empirical data from 2006 survivors.

High-resolution simulations also revealed that the tsunami did not only hit coastal areas but entered the mainland through the Serayu River estuary in Cilacap, thereby expanding the affected area. Conversely, the model was also able to identify certain areas that were relatively unaffected by the tsunami, which could serve as considerations for determining evacuation zones in the future.

In addition to evaluating the tsunami generation mechanism, the research team also tested earthquake source scaling approaches. According to Wiko, the Takemura formulation produced estimates closer to observational data than previously used formulations.

Wiko noted that these findings are crucial for improving community preparedness and supporting the development of more accurate disaster mitigation systems, including high-resolution tsunami risk modelling and more affordable tsunami monitoring technology. Nevertheless, he emphasised that the suspected involvement of underwater mass movements still requires sensitivity analysis and further research to obtain stronger scientific confirmation.

Moving forward, BRIN plans to continue ocean hazard research through the development of a prototype for a more affordable tsunami monitoring system. This technology is designed to monitor sea-level changes in real-time as part of efforts to strengthen the tsunami early warning system in Indonesia.

“We have initiated collaboration with colleagues from the engineering field to produce more affordable tsunami monitoring tools that can be implemented to support sea wave monitoring,” he concluded.