Indonesian tsunami likely caused by ‘flank collapse’ on Anak Krakatau with no earthquake
Scientists, including those from Indonesia’s Meteorology and Geophysics agency, have said the tsunami that killed at least 281 people could have been caused by landslides.
- Anak Krakatau has been erupting since June
- If the landslide was underwater, it’s possible there was no earthquake felt by people
- The tsunami was exacerbated by abnormally high tides
They said the landslides likely happened on the steep slope of the erupting Anak Krakatau volcano.
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.
It’s possible for an eruption to trigger a landslide above ground or beneath the ocean, both capable of producing waves, he said.
“Actually, the tsunami was not really big, only 1 metre.
“The problem is people always tend to build everything close to the shoreline.”
Ben van der Pluijm, an earthquake geologist and a professor in the University of Michigan, agreed the tsunami may have been caused by a “partial collapse” of Anak Krakatau.
“Instability of the slope of an active volcano can create a rock slide that moves a large volume of water, creating local tsunami waves that can be very powerful,” he said.
“This is like suddenly dropping a bag of sand in a tub filled with water.”
Sutopo Purwo Nugroho, spokesman for the disaster mitigation agency said the tsunami waves were exacerbated by abnormally high tides because of the full moon.
Why wasn’t there an earthquake?
Rahmat Triyono, earthquake and tsunami chief at Indonesia’s Meteorology, Climatology and Geophysics Agency told the New York Times that there was “no tsunami warning” before the waves hit.
“There was no earthquake,” he explained.
Jackie Caplan-Auerbach, a geophysicist at Western Washington University told The Verge that there was probably some shaking, but not enough to be felt by people nearby.
“Anything that rapidly displaces water can cause a tsunami, whether or not it also shakes the ground,” she said.
“The fact that there was no major earthquake means that whatever caused the tsunami near Krakatau put most of its energy into the water rather than into the ground.”
The 305-metre-high Anak Krakatau, whose name means “Child of Krakatoa,” lies on an island in the Sunda Strait between Java and Sumatra islands, linking the Indian Ocean and the Java Sea.
It has been erupting since June and did so again about 24 minutes before the tsunami, Indonesia’s Meteorology and Geophysics agency said.
Anak Krakatau formed over years after the 1883 eruption of the Krakatoa volcano, one of the largest, most devastating in recorded history.
That disaster killed more than 30,000 people, launched far-reaching tsunamis and created so much ash that day was turned to night in the area and a global temperature drop was recorded.
Most of the island sank into a volcanic crater under the sea, and the area remained calm until the 1920s, when Anak Krakatau began to rise from the site.
It continues to grow each year and erupts periodically although it is much smaller than Krakatoa.