The Problem of Real and Unreal Tsunamis

Indonesia was beset by disasters in 2018, including two high casualty local tsunamis: in coastal western Sulawesi – impacting the city of Palu, on September 28, and around the Sunda Strait, between Java and Sumatra, on December 22. These events may have appeared unusual, but the great subduction zone tsunamis, like those in the Indian Ocean in 2004 and Japan in 2011, have reset our imagination. Before 2004, forty years had passed without any transoceanic tsunamis. Overall, local tsunamis are more common, presenting many challenges in how they can be anticipated.

The Palu tsunami reminds us how “strike-slip” faults, involving only horizontal displacement can still generate tsunamis, first as a result of vertical displacement at “jogs”, where the fault rupture jumps alignment, as well as from triggered submarine landslides. It seems both factors were important in driving the Sulawesi tsunami that became amplified to more than four meters (13 feet) in the funnel-shaped Palu embayment.

The December 22 Sunda Strait tsunami was caused by a submarine landslide on the erupting Anak Krakatoa volcano and arrived without warning, in the dark of mid-evening. More than 400 people drowned mainly around a series of beach resorts in Banten and Lampung provinces, although water levels in the tsunami only reached a meter or two above sea level. An audience of 200 enjoying a concert at the Tanjung Lesung Beach Resort, staged directly on the beach by Indonesian rock band Seventeen were caught unaware. 29 concertgoers were killed together with four people associated with the band.

Tanjung Lesung beach the day after the tsunami. Image credit: Indonesian Kostrad, part of the Indonesian National Armed Forces [Public domain], via Wikimedia Commons

For both these tsunamis the impact highlighted how, close to the equator where there are no tropical cyclones and hence no tropical cyclone storm surges, people live right down to sea level. In higher latitudes a two meter (six feet) tsunami would not impact coastal buildings.

The disaster management community is challenged how to issue effective warnings for such local tsunami, including how to monitor landslides on an erupting volcano. It would be impractical to place tsunami-monitoring buoys in front of every coastal town. The BBC reported that none of Indonesia’s forty tsunami warning-buoys (gifts from the U.S., Germany and Malaysia after the 2004 tsunami), are currently working with a number vandalized or stolen.

However, one lesson we have learnt in 2018 is that tsunamis are “infectious”. The publicity around a tsunami in one region stimulates local tsunami stories all across the globe.

In Wales, the Palu disaster triggered the repeat of stories of a “1607 tsunami” that flooded low areas around the Bristol Channel, and whether Wales was at a greater risk of tsunami as a result of climate change.

In Italy, the Indonesian tsunami led to an article commemorating a tsunami claimed to have destroyed most of the coastal city of Amalfi south of Naples.

There are problems with both these stories. Neither of these catastrophes was a tsunami.

Plaque inside Goldcliff Parish Church, near Newport, Wales, commemorating lives lost in the 1607 Bristol Channel flood. Because of differences in the New Year start date, the plaque references the flood as 1606. Photo © Robin Drayton (cc-by-sa/2.0)

For the 1607 floods in Wales, RMS published a report in 2007 showing all the evidence why the floods were the product of a wind-driven storm surge on top of an astronomical “king tide”. There really is no doubt about it, but the “tsunami” story continues to hold the headlines.

As for the Italian “tsunami”, from the tenth to thirteenth Centuries, Amalfi was a leading Mediterranean port, a rival to Genoa and Venice. The town was founded at the base of a steep ravine where a delta, a few hundred meters across, had opened out into the sea. On the night of November 24, 1343, much of Amalfi was said to have disappeared as a result of a “seaquake”. It turns out that in the late Middle Ages, the terms “seaquake” or “earthquake” were attached as the ultimate cause of a disaster to indicate it was of supernatural origin. The real Amalfi disaster involved the confluence of three natural hazards, none of which was a tsunami.

When Mt. Vesuvius erupted in AD 79 the wind was blowing towards the southwest, and the ash was at its thickest over the mountains above Amalfi. (In nearby Positano, one villa was buried under eight meters or 26 feet of ash). Over the centuries, intense rainfall events – for which this mountainous coast was notorious, brought the ash down the stream in debris flows and formed a delta, at the base of the slope. The delta was fragile and geologically temporary. During the night of November 24, 1343, in the midst of an epic windstorm, much of the port succumbed to a combination of southerly storm waves and river flooding, which eroded the port’s fortifications and quays, accompanied by slumping of the underlying delta. (We know about the storm because the prolific writer and poet Petrarch was in nearby Naples at the time). Losing the harbor ended Amalfi’s glory days.

Over the centuries the mythic “seaquake” has become today’s mythic “tsunami”.

There is a lesson to be drawn from these fabrications. Local tsunamis present enough of a problem without adding fake entries to the historical catalog.

Chief Research Officer, RMS

Robert Muir-Wood works to enhance approaches to natural catastrophe modeling, identify models for new areas of risk, and explore expanded applications for catastrophe modeling. Robert has more than 25 years of experience developing probabilistic catastrophe models. He was lead author for the 2007 IPCC Fourth Assessment Report and 2011 IPCC Special Report on Extremes, and is Chair of the OECD panel on the Financial Consequences of Large Scale Catastrophes.

He is the author of seven books, most recently: ‘The Cure for Catastrophe: How we can Stop Manufacturing Natural Disasters’. He has also written numerous research papers and articles in scientific and industry publications as well as frequent blogs. He holds a degree in natural sciences and a PhD both from Cambridge University and is a Visiting Professor at the Institute for Risk and Disaster Reduction at University College London.

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