The Palu tsunami: time to reevaluate tsunami hazard assessment?

by Amar Rahman (Principal Risk Engineer / Global Practice Leader Natural Hazards at Zurich Insurance Company Ltd)

In the desperate aftermath of the earthquake and tsunami that centered on Palu in the Indonesian island of Sulawesi, questions are being asked.

A lot of focus is on the effectiveness of the tsunami monitoring and warning system. Some are suggesting there has been a lack of maintenance and investment in the tsunami warning system in this region. Some have questioned why the nearest tsunami sensor was located 200km away from Palu. Others have scrutinized the type of tsunami system believing the GPS devices and tide gauges around Sulawesi would have been of limited effectiveness; sensors on the ocean bottom, as is the case in many other tsunami-exposed regions, would have provided a better warning.

But maybe the central question is, are we correctly assessing the risk of tsunamis? Or perhaps we are being blinded by conventional wisdom and historical facts that cause us to ignore other evidence?

Don’t rely on conventional thinking

Conventional wisdom says that large-magnitude tsunamis, like the one at Palu that reportedly reached up to 6 meters, are mainly triggered by offshore subduction faults. This is where one tectonic plate slides under another to cause a massive displacement of water.

But the earthquake near Palu was an offshore strike-slip event, where one plate slides across another horizontally. You would not expect that type event to create a devastating tsunami.

So we need to look at other factors – and not just the type of fault – that created the Palu tsunami. For instance, did the fault rupture zone pass through an underwater landscape or topographic feature that amplified the waves? Or was it due to unique topographic features of the seafloor or the geographic features of the bay near Palu; or perhaps a massive underwater landslide?

Palu should be a wakeup call – we shouldn’t let our perception of risk affect our risk (resilience) management planning.

Managing the risk

The unconventional formation of the Palu tsunami should bring a greater focus on risk management. This means we need to look at scenarios and exposures (population at risk, etc.). From a risk (resilience) management perspective here are some of the main issues to consider regarding any type of with natural catastrophe:

• A lack of historic evidence does not preclude occurrence of a natural catastrophe. “It hasn’t happened in the five or 10 years that I’ve been here”, is not a sound risk management approach. Consider all perils that may impact your business and community including, but not limited to, supply chains, infrastructure, and so on.

• In risk management planning consider the exposures that may be impacted by the event and not only the hazard level. In this case of Palu tsunami, what are the occupancies along the coast, and how many people would potentially be impacted by a natural catastrophe?

• In the absence of design tools (e.g. hazard maps or building codes) use a scenario-based approach in preparing risk controls. For example, what historical events have impacted the region (not only the specific location)? How did the infrastructure perform? How did the critical facilities and buildings perform? and so on. In the absence of hazard maps, this approach does not provide a probability of occurrence of the event, but can give a very good indication of potential damage levels, including business interruption.

• As we learned from Palu, just because a protection system is in place doesn’t mean it will work when needed. An effective and reliable protection system must be designed, detailed and installed to meet local or, if these are not available, international standards and state-of-the-art. And it must also be continuously maintained to ensure it will operate as designed.