JAKARTA– Over a thousand people have been killed and many remain missing after a tsunami struck the Indonesian island of Sulawesi on September 28, triggered by a powerful 7.5-magnitude earthquake.
The crisis impacted at 18:03 local time (10:03 UTC), triggering dozens of aftershocks and at the same time, Indonesia’s meteorological and geophysics agency BMKG, just after the initial quake, issued an underestimated tsunami warning of potential waves of 0.5 to three meters.
Earthquake, volcanoes, and tsunamis aren’t uncommon in Indonesia, as the world’s most populous Muslim county is located in the Ring of Fire; a major area in the basin of the Pacific Ocean where such natural phenomena occur.
The area of 40,000 km horseshoe shape is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts, and plate movements.
It contains 452 volcanoes (more than 75% of the world’s active and dormant volcanoes). Moreover, at least 81% of the world’s largest earthquakes occur along it.
Failing Warning Systems
The system did, in fact, pick up the tsunami, but it failed to accurately gauge the scale of the tsunami. BMKG revealed that the nearest tidal gauge to Palu city which was swept by the tsunami was well over 200km away.
At that distance, the tidal gauge, which measures changes in sea level, only recorded an insignificant six cm rise. Thus, the tsunami height was estimated to be less than 0.5m, while in fact, it reached six meters high.
Non-working Advanced Buoys
Indonesia actually had a more advanced tsunami warning system that included a network of 21 buoys connected to seafloor sensors which would have transmitted advance warnings.
However, none of these buoys – donated by the US, Germany, and Malaysia after the devastating 2004 Indian Ocean tsunami that killed nearly a quarter of a million people – are working. Some have been damaged by vandals and others have been stolen.
Because of this, the BMKG predicts post-earthquake tsunamis using a modeling system based on the earthquake depth and magnitude.
BPPT, the national Indonesian agency which manages the devastated buoy system, has previously acknowledged that government efforts have mostly focused on post-earthquake relief while paying minimal attention to pre-disaster anticipation.
Whether the recent tsunami was expected or not, professor Philip Li-fan Liu, vice-president of the department of civil and environmental engineering at the National University of Singapore, explained that the recent earthquake which led to the tsunami isn’t the kind of earthquake that typically generates a major tsunami.
Tsunamis are typically generated when there is a large vertical displacement. But in this case, the tectonic plates were rubbing against each other horizontally, and when ruptures it only creates a significant horizontal movement and not much of a vertical movement.
Why such massive waves were unleashed on Palu could perhaps be explained by the nature of the bay itself.
According to Dr. Hamza Latief, an oceanographer at the Bandung Institute of Technology, Palu has witnessed tsunamis in the past and when a tsunami hits its narrow and elongated bay its impact is amplified.