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.