A study published Feb. 11 in the journal Science Advances undermines the long-held belief that large earthquakes follow predictable cycles. Instead, the findings show that these earthquakes occur at irregular intervals, coming in bursts and disappearing with long lulls.
“The ‘overdue’ myth is just that – a myth,” lead author Zakaria Ghazou-Shouse, a paleontologist with the British Antarctic Survey, said in a statement. “Our research shows that large earthquakes are just as random and unpredictable as smaller earthquakes. The science is clear: large earthquakes do not follow any timetable.”
Earthquakes are random, not cyclical
This “myth” has led scientists to believe that the San Andreas Fault and the Cascadia Subduction Zone – two of the West Coast’s most dangerous tectonic boundaries – are overdue for megaquakes of magnitude 8.0 or greater. As years go by without disaster, the inevitability of “big” has become simultaneously more fearful and more uncertain.
The Western US is not the only part of the world experiencing massive earthquakes. Scientists have long thought that the Himalayas were also latecomers, as the Central Himalayan fault segment in India and Nepal last produced a violent earthquake in 1505. Previous research has suggested that megaquakes occur here every 500 years.
Ghazou-Shaus and his colleagues decided to take another look at the seismic history of the region. They conducted their study on Rara Lake, a high mountain lake in western Nepal that serves as a natural earthquake record. Strong ground shocks disturb its underwater slopes, leaving distinctive layers in the lake’s sediment.
The researchers identified 50 of these layers dating back 6,000 years and combined this geological record with modern instrumental earthquake data to statistically test the timing of the earthquakes. Then, they compared their findings to long-term seismic records from Indonesia, New Zealand, Chile, and the US Pacific Northwest.
The analysis revealed similar patterns in all of these high-risk locations – or rather, similar lack of patterns. Researchers found that earthquakes occur unpredictably, with active periods followed by long periods of calm. Neither sector exhibited the regular cycle that is the basis of many risk models.
Need for constant preparation
Before you breathe a sigh of relief, let’s clarify what this means. The findings may contradict the idea of ”overdue” megaquakes, but it does mean that predicting these disasters is far more difficult than experts realize.
“Six thousand years of data shows us that large earthquakes can occur at any time,” Ghazou-Shous said. “This significantly enhances seismic hazard estimates – the risk models that shape government policies in earthquake zones, and prioritize public investment and aid.”
In light of this new understanding, he and his team recommend that the public, politicians, and policy makers consider earthquake hazards as an ongoing threat. Their findings underline the need for strong preparedness to avoid the worst damage and casualties in the event or whenever the next “major” attack occurs.
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