Explained: The destructive power of the Indonesia 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.

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‘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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Telling Numbers | How banks wrote off loans worth over Rs 2 lakh crore in UPA era

Reserve Bank of India data show the country’s 21 state-owned banks wrote off a total Rs 3,16,500 crore worth of loans between April 2014 and April 2018. The loans written off were over 166% of the banks’ writeoffs in the 10 years up to 2014. The table shows year-by-year data on loan writeoffs by Scheduled Commercial Banks (SCBs) during 10 years of the UPA government. SCBs constitute a larger group that includes PSBs. The data were provided by the RBI in response of an application filed by The Indian Express under the RTI Act.

—SHYAMLAL YADAV