Last Friday, an unusual sequence of events unfolded on Sulawesi Island in Indonesia, with catastrophic results.
It began with an earthquake. This wasn't at all out of the ordinary: Indonesia is a nation of islands born from very complex and deadly plate tectonics. Sulawesi is right in the path of a triple junction, where three major plates – the Australian, Philippine, and Sunda – converge. The island is stitched together from island arcs and continental fragments mashed together by colliding plates, plus volcanoes birthed by subduction. The area is a mess of thrust and strike-slip faults all trying to accommodate the motions of the major plates, plus microplates resulting from all the tectonic chaos.
Various blocks of the island's crust rotate as the plates converge. Motion between those blocks is handled by strike-slip fault zones. One of those, the Palu-Koro fault, cuts through the northeastern part of the island. This fault zone is the one that slipped Friday. It began with several jarring foreshocks, one a hefty M 6.1 three hours prior to the mainshock. And then, during rush hour local time, the mainshock hit: M 7.5. Buildings collapsed. People died. Tragic, but not at all out of the ordinary in this part of the world.
Tsunamis aren't unusual here, either. We all remember the 2004 Boxing Day tsunami that devastated the region after an M 9.1 megathrust earthquake. Tsunamis have also been caused by volcanic eruptions, most memorably the 1883 eruption of Krakatau. What is unusual is the fact this one resulted from an earthquake on a strike-slip fault.
Strike-slip faults typically displace land horizontally, not vertically. You can get an idea of the result by sliding your hands together underwater: the surface ripples, but there's no large displacement of water. Horizontal seabed movement doesn't fuss the sea too much. Besides, the epicenter for this earthquake was on land. Shaking spread beyond the peninsula, but you wouldn't generally think of tsunamis happening because of it, much less the massive twenty foot monster that swept up Palu Bay and devastated the coastal regions.
But while strike-slip faults aren't quite as capable of tsunami generation as the vertical displacement caused by megathrust monsters, they have their ways. The most long-recognized one is landslides. All that shaking can cause unstable slopes to come loose and fall. This is what happened in Alaska's Lituya Bay in 1958, when a much larger quake on a strike-slip fault caused 90 million tons of rock and debris to fall from the steep slopes into the bay, sending water sweeping nearly 2,000 feet high along the shores. Whether those landsides begin on land and drop into the sea, or happen entirely beneath the waves, they can displace massive amounts of water, sending seismic sea waves barreling into coastal areas and wreaking havoc.
We don't yet know if landslides caused this tsunami. They aren't the only way strike-slip faults can create tsunamis. Researchers have modeled these quakes, and found that parts of the slipping fault can actually displace the seabed. Strike-slip fault zones don't run in straight lines with a single fracture: they're often curvy, and they're comprised of numerous breaks. While most of the ground motion is horizontal when the fault slips, some movement will end up being accommodated by vertical "pop-up" structures. And those structures can displace enough water to cause a respectable tsunami.
Combine those potential causes with a long, narrow bay that channels the water, and you end up with massive waves that lay waste to everything in their path, no megathrust necessary.
Making matters worse, the earthquake happened during the Palu Nomoni Festival, which is held on the beach every September, placing countless festival-goers directly in the tsunami's path.
We don't yet know the full scale of the devastation. Almost a thousand people are already confirmed dead, and with the amount of destruction, it's inevitable that those numbers will climb. This is already the worst seismic disaster Indonesia has suffered since the 2004 megathrust earthquake. It's a grim and stark reminder of the myriad ways plate tectonics can reduce human civilization to rubble in seconds.