On 30 July at 12:25am BST (11:25am local time), a massive earthquake struck just off the coast of the Kamchatka Peninsula, Russia. With a magnitude of 8.8, the event was the sixth-largest earthquake in recorded history, raising the spectre of a tsunami on the scale of the 2004 Indian Ocean disaster.
Within hours, more than two million people across the Pacific were ordered to evacuate, as warnings rippled out to coastlines from China and New Zealand to Peru and Mexico.
But barring some damage in nearby regions of Russia, for the most part, the world was spared from catastrophe on this occasion. As people heeded warnings and headed for higher ground, slowly but surely, most tsunami warnings were downgraded and rescinded.
The wave wasn’t coming. But why?
How tsunami warnings work
Tsunami warning systems have come a long way since the 2004 Indian Ocean Tsunami claimed the lives of more than 200,000 people.
“There are lots of tsunami warning centres around the globe,” says Prof Alison Raby, an expert in environmental fluid mechanics at the University of Plymouth.
“One or more of these centres will be alerted to an earthquake event, and then find out its location, magnitude and depth, because that’s an important parameter for predicting tsunamis, and then on that basis issue some sort of wider alert.”
Because seismic waves travel around 100 times faster than tsunamis, information about an earthquake arrives well before the first wave does. But waiting to confirm the tsunami itself is rarely an option. By the time ocean-floor pressure gauges or satellites detect abnormal sea-level changes, it may already be too late.
Detection speed depends on how close the source is to the nearest detection system or coastal depth gauge. In practice, that can mean anything from five minutes to two hours.
Using data from past quakes and complex computer models, scientists at warning centres often have only minutes to decide whether to sound the alarm. In many cases, the first alerts go out just five minutes after the ground stops shaking.
The final step – reaching people in harm’s way – has also improved since 2004. Back then, many coastal communities had little or no warning. Now, emergency alerts can be sent directly to mobile phones, giving people precious time to reach higher ground before a wave arrives.
Erring on the side of caution
This year’s earthquake in Russia was what’s known as a megathrust quake. These occur in subduction zones where one tectonic plate is forced under the other, producing the most powerful earthquakes we know of.
As one plate slips under, the other is forced up, causing the seafloor to abruptly rise and displace vast volumes of water. This sudden uplift sets off waves that can race across entire ocean basins, growing in height as they approach shallow coastlines.
Megathrust earthquakes were also responsible for the 2004 Indian Ocean earthquake and the 2011 Tōhoku earthquake in Japan, both of which produced towering tsunamis with waves more than 30m (100ft) high. It’s little wonder, then, that warnings were issued across the Pacific this time too.
The trouble is that while the earthquakes themselves may be similar, many other factors go into producing a major tsunami.
“It’s not as simple as detecting an earthquake and then looking at a simulation to work out what size of tsunami will occur,” Raby says. “You don’t know whether it’s also been affected by underwater landslides or whether other mechanisms are at play.”
How much data we have from a specific location matters too. While the same region in Russia was hit by a magnitude 9 quake back in 1952, it remains sparsely populated and efforts to model the region may not be as thorough as other hotspots.
Even globally, records are thin. Precise earthquake measurements go back little more than a century, and only a fraction of quakes generate tsunamis. That means the sample size is too small to make ironclad predictions.
“For each of these events that comes along, we feel relatively confident we understand what’s going on, but it always brings fresh understanding and fresh questions,” Raby says. “I’m sure seismologists and earthquake engineers will learn something from this most recent event that they haven’t appreciated beforehand.”
Tsunami warning systems have clearly come a long way, and it’s safer to trigger an evacuation for a tsunami that never comes than to miss one that does. Still, the balance is delicate.
“The issue is that people get a bit blasé,” Raby says. “When people evacuate, they might lose revenue, or there could be traffic accidents, and then they’re going to be more cynical about the next one. So there is a danger of too many false warnings.”
Even so, she remains hopeful: “I’m relatively optimistic that we’re getting better at this, but we’re not in a perfect prediction situation at all.”
Read more:
- A 100 ft 'mega tsunami' could hit the US at any moment. And that’s only the beginning
- A massive earthquake could be brewing beneath North America, study suggests
- Something very weird is happening to the planet's earthquakes. Here's why
About our expert
Alison Raby is a professor in environmental fluid mechanics at the University of Plymouth, UK. Her research on tsunamis has been published in peer-reviewed journals, including the International Journal of Disaster Risk Reduction and Marine Geology.