According to Indonesia's geological agency, Anak Krakatoa had been showing signs of heightened activity for days, spewing plumes of ash thousands of metres into the air.
A man sits on a piece of debris at a tsunami-ravaged village in Sumur, Indonesia, Monday, Dec. 24, 2018.
The Meteorology, Climatology and Geophysics Agency (BMKG) tweeted on Saturday night that the waves which hit coastal areas on both sides of the strait were high tidal waves caused by the full moon.
"They suspect it was related with the Anak [Krakatau] volcano off the coast and possibly an underwater landslide but what we hear from sources down there is that a wave about three feet in height came up without warning and actually travelled up inland for about 100 feet", he said. These volcanoes could cause a tsunami on eruption, meaning that such a system was needed, he said.
Aftermath of the tsunami RIAU IMAGES Barcroft Media via Getty Images
Ifan said: "Most of us, I and our manager are on the shore, including myself, when I was thrown into the middle of the sea, there were probably around 20 to 24 corpses around me".
Simon Boxall of Southampton University said the region was in spring tide, "and it would appear that the wave hit some of the coastal areas at the highest point of this high tide, exacerbating the damage done".
The disaster came just days before the 14th anniversary of the devastating Indian Ocean quake and tsunami on December 26, 2004 that claimed more than 230,000 lives across the region - with Indonesia the hardest hit.
More than 120 people are missing, and another 1,400 were injured when the wave bulled through the islands of Java and Sumatra.
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Sutopo Purwo Nugroho, head of public relations at Indonesia's National Disaster Mitigation Agency, said 843 people were injured and 28 were missing, as of 4pm local time.
This weekend's tsunami was the second deadly one to hit the island nation in 2018.
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Gegar Prasetya, co-founder of the Tsunami Research Center Indonesia, said Saturday's tsunami was likely caused by a flank collapse - when a big section of a volcano's slope gives way.
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"There was no natural disaster, and the Anak Krakatau eruption also wasn't that big", Nugroho told Metro TV, noting there were no "significant" seismic tremors to indicate a tsunami was coming. It continues to grow each year and erupts periodically. Although we are in a much better position to warn inhabitants of nearby islands about such events today, the population of the region has risen so dramatically since 1883 that the effects of an explosion of similar size could be nearly unimaginable.
Indonesia is prone to tsunamis because it lies on the Ring of Fire - the line of frequent earthquakes and volcanic eruptions that circles virtually the entire Pacific Rim.
Indonesia has been hit by a series of natural disasters in recent months, including a powerful quake that hit the island of Sulawesi on September 28. Roads and infrastructure are poor in many areas, making access hard in the best of conditions. It spawned a giant tsunami off Sumatra island, killing more than 230,000 people in a dozen countries - the majority in Indonesia.
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