People survey damage outside the shopping mall following earthquakes and tsunami in Palu, Central Sulawesi, Indonesia. (AP Photo/Tatan Syuflana)
The confirmed death toll from the earthquake and tsunami that destroyed the Indonesian city of Palu reached 844 Monday evening. Waves rose 20 feet, which has surprised some scientists who said they wouldn’t normally associate such high levels of destruction with the kind of earthquake that preceded the tsunami.
‘Vertical’ earthquakes
Catastrophic tsunamis are often triggered by ‘megathrust earthquakes’, which occur at subduction zones when one tectonic plate is forced under another, causing massive chunks of the earth’s crust to move vertically. Such movements on the ocean’s floor cause huge volumes of water to be displaced suddenly, and throw up giant waves that can travel very fast across great distances. The December 2004 Indian Ocean tsunami, which had waves up to 100 ft high and killed nearly a quarter million people, was triggered by a megathrust earthquake of 9.1-magnitude in Sumatra.
‘Horizontal’ earthquakes
Friday’s 7.5-magnitude quake was triggered by what is called a ‘strike-slip fault’, in which the earth’s movement is largely horizontal. This would not normally trigger a tsunami — however, it is possible for a strike-slip fault to also have some amount of vertical motion that could displace water. Or the fault’s rupture zone, which in this case was estimated to be about 70 miles long, may pass through an area where the seafloor rises or drops off, so that when the fault moves during the quake, it pushes seawater in front of it, explained a report in The New York Times.
Landslide, topography
There is yet another possibility — that the quake set off a landslide on the ocean floor that displaced a lot of water and created waves. Several such events occurred during the 9.2-magnitude earthquake in Alaska in 1964. Again, The NYT’s explainer said, the tsunami could have been impacted by Palu’s location at the end of a narrow bay. The coastline and the contours of the bottom of the bay could have focused the wave energy and guided it up the bay, increasing the wave height as it approached the shore.
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