On 28 September 2018, a shallow, large earthquake struck Central Sulawesi, Indonesia. The magnitude 7.5 strike-slip earthquake propagated through Palu Bay and across the provincial capital of Palu. This was surprisingly followed by a localised tsunami with heights up to 4 to 7 metres that swept shore-lying houses and buildings on its way. The combined effects of the earthquake and tsunami led to the deaths of at least 2,256 people. Fears arose that other regions, currently not expected to sustain tsunami-triggering ruptures, may also be at risk.
Surprise tsunami following strike-slip earthquake
The tsunami caught geologists and geophysicists by surprise. The earthquake occurred near an active plate boundary, where earthquakes are common. However, the earthquake offset the ground horizontally and such motion is typically not linked to tsunamis, which occur when the ground moves up or down. So, what happened? How was the water pushed up or pulled down to create this tsunami. One typical explanation is that the earthquake triggered underwater landslides, which then trigger or increase the tsunami.
Using physics-based modelling
An international team of researchers, including Dr. Ylona van Dinther at Utrecht University, has recently submitted a paper to the journal Pure and Applied Geophysics that may offer a different explanation to why the tsunami occurred.
The team uses a physics-based, coupled earthquake-tsunami modelling framework that was tightly constrained by observations. The model combines rupture dynamics, seismic wave propagation, tsunami propagation and inundation. It shows that a submarine strike-slip fault can produce a tsunami if a component of dip-slip faulting occurs. That is most of the slip is horizontal, but there is a distinct vertical component of slip along the fault. This vertical component results from, and is consistent with, a 65 degree dipping fault geometry and a combination of lateral and extensional deformation exerted on the region by complex tectonics. This fault slip causes a vertical offset of the seafloor across the fault segment of 1.5 m. This sources a tsunami with wave amplitudes and periods that match those measured, and inundation that reproduces observations from field surveys. They conclude that a source related to earthquake displacements is probable and that landsliding is not needed to achieve the unexpectedly large tsunami amplitudes.