Five earthquakes struck Indonesia’s southern waters in the span of just five hours on Jan. 22, recalling the horror of a Christmas Day tsunami that pounded the country’s shores and left hundreds dead.
There was no immediate tsunami warning or reports of damage or casualties from the quakes, the worst of which registered 6.4 on the Richter scale. It hit at a depth of 22 miles, and at a distance of about 143 miles south of Raba in the east of the island of Sumbawa, which forms part of West Nusa Tenggara province, according to the U.S. Geological Survey.
The 6.4 quake hit at about 1 p.m. local time on Tuesday and was the last of the five to strike. Its epicenter was located south of the Indonesian town of Raba, the U.S. Geological Survey (USGS) said, with a radius that reached nearly as far as Australia.
Before the final quake struck, another four were recorded in the same area, two of which were less than 10 minutes apart.
Five Quakes in Five Hours
The first earthquake struck at 7.59 a.m. local time, and registered at a magnitude 6.0, the USGS said, with an epicenter below the earth’s surface at a depth of 10.4 miles. It was located 40 miles south-southwest of Bogorawatu, Indonesia.
Less than 10 minutes later, a magnitude 4.9 tremor was recorded 45 miles south-southwest of Bogorawatu, Indonesia. It struck at a depth of 6.2 miles below the surface.
A third earthquake, measured at 4.5, struck about an hour later 45 miles south-southwest of Kahalu, Indonesia. The epicenter of that quake, exactly the same as with the previous one, was 6.2 miles deep.
The fourth quake registered 4.8 and hit 50 miles south-southwest of Bogorawatu, Indonesia, nearly two hours after the first earthquake and at a depth of 21 miles.
The fifth and final tremor was the strongest of the lot, with seismologists cited by the Daily Mail saying that it may have been felt up to 400 miles away.
Indonesia suffered its deadliest year in more than a decade in 2018 as a series of earthquakes and tsunamis killed more than 3,000 people.
About 430 people were killed, with at least 159 missing, after a tsunami off the west coast of Java during the Christmas season evoked memories of the Indian Ocean tsunami triggered by a massive earthquake on Dec. 26, 2004.
That disaster killed 226,000 people in 14 countries, including more than 120,000 in Indonesia.
Deadly Ring of Fire
Indonesia is a disaster-prone archipelago that straddles the most seismically active region in the world—the Pacific Ring of Fire. About 90 percent of the world’s earthquakes are registered in the region, the USGS said. The area is also home to 75 percent of the world’s active volcanoes.
“The next most seismic region (5-6 percent of earthquakes) is the Alpide belt, according to the USGS. The belt extends from the Mediterranean region, eastward through Turkey, Iran, and northern India.
“The Ring of Fire isn’t quite a circular ring. It is shaped more like a 40,000-kilometer (25,000-mile) horseshoe. A string of 452 volcanoes stretches from the southern tip of South America, up along the coast of North America, across the Bering Strait, down through Japan, and into New Zealand. Several active and dormant volcanoes in Antarctica, however, ‘close’ the ring,” according to National Geographic’s summary of the area.
Professor Dougal Jerram from the University of Oslo explained in the wake of the deadly December 2018 earthquake off the coast of Indonesia that tsunamis caused by volcanic activity might not trigger a warning and can strike unexpectedly.
“Tsunamis can be caused by volcanically induced landslides above or below water, and by volcanic eruptions themselves,” Jerram told The Guardian.
“Unlike tsunamis caused by earthquakes, such volcanically induced tsunamis may not trigger warning systems that are designed to alarm after large quakes, and thus may provide little warning.”
Dr. Simon Boxall from the National Oceanography Centre and University of Southampton spoke in the aftermath of the same devastating tsunami saying, “There will be an outcry as to why an early warning system didn’t kick in.”
Boxall added, “The same criticism was leveled after the September Palu tsunami, which killed 2,000 people.”
He further explained, “These tsunamis are very localized and to cover the Indian Ocean with sufficient sensors to warn against all such eventualities would require many thousand buoys on the network.”
But professor David Rothery, who teaches Planetary Geosciences at the Open University, told The Guardian that even if warning buoys had been placed right next to the volcano that triggered the landslide, it might not have done much good.
“Tsunami warning buoys are positioned to warn of tsunamis originated by earthquakes at underwater tectonic plate boundaries,” Rothery said. “Even if there had been such right next to Anak Krakatau, this is so close to the affected shorelines that warning time would have been minimal given the high speeds at which tsunami waves travel.”
Reuters contributed to this report.