Why in news?
=>The earthquake that preceded the tsunami destroyed the Palu city in Central Sulawesi, Indonesia.
=>Tsunamis typically are the result of the abrupt motion of large submarine earthquakes at tectonic plate boundaries. And earthquakes are not uncommon in Indonesia; the island chain sits within what’s known as the Ring of Fire, a curving horseshoe-shaped chain of tectonic plate boundaries that hugs the Pacific basin. It is home to around 90 percent of the world’s earthquakes.
Why it was a surprise?
=>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.
=>The 7.5-magnitude earthquake appears to be the result of what’s known as a strike-slip fault, which takes place as two blocks of crust grind against one another, largely in a horizontal direction. Tsunamis more commonly follow vertical movement in the crust, which disrupts the overlying water and can generate massive waves crashing onshore.
=>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.
=>The 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.
=>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.
=>Such occurrences have long been known to cause massive tsunamis; a rockslide triggered by a magnitude 7.8 earthquake set off the mega-tsunami in Lituya Bay, Alaska, in 1958. It sent water roaring up mountainsides to reach elevations of 1,700 feet, and remains the largest tsunami recorded in modern times.
=>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.
=>The dangers of the Palu–Koro Fault have been known since the 1970s. Work in the 1990s and 2000s showed the plates on either side of the fault were moving in opposite directions “faster than the San Andreas Fault,” meaning considerable stress was building up along the fault.
=>A 2017 paper identified this fault as the most dangerous in the area, and suggested a future rupture was most likely to occur in the center of the valley where Palu is situated.
=>Being situated in this spot exacerbated the damage in Palu because the valley is filled with “flat, weak, soft, wet sediment.
=>Surveys of the bay floor will need to be done to confirm the cause of the Sulawesi tsunami. They can look for signs of a landslide, and clues as to how much the fault moved.
=>However it was triggered, the event makes it clear that coastal communities in seismically active regions should not only be worried about tsunamis from subduction zones and this need to be better communicated to local communities.
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