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.
A version of this article was originally published in Insurance Day
The Mw7.5 earthquake in Sulawesi, Indonesia on September 28 reminds us that fourteen years after the terrible Indian Ocean tsunami, and despite significant investment in systems intended to provide tsunami warnings, the risk to life and property is not going away. To understand why the destruction and loss of life in the city of Palu, with a population of 350,000, is so great (1,300 and rising) we need to understand why this location has proved such a nexus of vulnerabilities.
First, Palu is located less than one degree south of the equator. That means it is in the “shadow zone” for tropical cyclones. In most of the world’s oceans, no tropical cyclone can exist within ten degrees of the equator, although in the western Pacific the typhoon exclusion zone can narrow down to six to eight degrees from the equator. The lack of Coriolis force at the equator prevents a collection of thunderstorms gaining a structured rotation (and tropical cyclones spin in opposite directions in the northern and southern hemispheres).
The lack of tropical cyclones means there are no significant storm surges, or even much in the way of significant wind-driven waves, and as a result people build their houses right down to sea level. This means, in comparison even with a coastal city in Philippines or China, there were many more seafront buildings exposed to a tsunami that reached no more than three to five meters above sea level.
The earthquake and subsequent tsunami that struck the Indonesian island of Sulawesi on Friday, September 28, has already claimed the sinister accolade of being the deadliest earthquake in the world this year.
According to local authorities, there have so far been 1,374 reported fatalities, but this figure is set to rise as rescue efforts spread out from the main cities. At this stage, thousands of people are believed to still be trapped under the rubble of collapsed buildings, and at least 60,000 people are displaced with limited food and water supplies.
The 7.5 magnitude earthquake struck the island of Sulawesi on Friday, September 28, approximately 48 miles (78 kilometers) north of Palu, a coastal city with around 330,000 residents. The earthquake triggered a ten foot (three meter) high tsunami, that impacted the coastal areas of western Central Sulawesi, including Palu City and Donggala, a regency with a population of around 275,000.