Bats love sea caves, which provide a cool and moist hideaway that is perfect for their roosts. For thousands of years, bats have therefore been visiting this particular cave.
Rubin, the study’s lead author and a professor at EOS, Horton, and their team discovered that organic debris from the copious quantities of guano is present above each of the eleven identified historic tsunami beds. It neatly marks each off, like lines of icing in a layer cake.
The scientists knew what a tsunami bed looks like in the area because they extensively studied the one left behind after the 2004 disaster. It, and the eleven ancient beds, all consist of fine-grained sand, pieces of shale, and mudstone known as “rip-up clasts,” weathered cave chalk, and abundant numbers of preserved tiny marine animals, mostly originating from the ocean depths.
“We were able to refine the timing of past tsunamis with radiocarbon dating,” Rubin said, adding that a statistical model further “allowed us to understand the uncertainties of timing between events, and we were able to make a comparison between our record of past tsunamis to other sites around the Indian Ocean.”
He and his team believe that the Sunda — also called Sumatra — Megathrust is the most likely source for triggering both earthquakes and tsunamis, at least in this region. In other areas, volcanic eruptions and underwater landslides may also lead to similar events.
As for why smaller tsunamis sometimes occur relatively close together, Rubin said, “The closely spaced tsunamis perhaps represent temporal clustering of earthquakes that produced tsunamis. It seems that earthquakes during this period are separated by just a few decades.”
RELATED: A Big Earthquake Can Trigger Others in Minutes
The take-away messages from the research for those hoping to prepare for future tsunamis are twofold. First, the scientists have shown via their newly discovered sedimentary record that earthquakes tend to cycle from larger to smaller events. They were not surprised, for example, that some 2,000 years went by without a single major tsunami. The problem is that pressure can build up over such time, leading to even bigger future natural disasters.
Second, the remarkable variability of tsunami recurrence suggests that regional hazard mitigation plans should be based on the high likelihood of future destructive tsunamis, rather than on estimates of recurrence intervals. As the new paper reports, big tsunamis vary in recurrence time from only 60 years to about 2,000 years.
“Although our study is the first to use a new depositional environment — the sea cave — for reconstructing tsunami records, it will take many more high quality records to understand how a single offshore fault can produce such a varied history of tsunamis,” Rubin said.
“We continue to work on the cave to estimate the power of the tsunami waves that flooded the cave over the last 8,000 years,” he added.
WATCH: How Well Can We Predict Earthquakes?