Why Russia's megathrust earthquake was among biggest ever recorded, but damage was minimal
Elissa Steedman
, ABC
Aerial view of Balneario de Huanchaco in Trujillo, Peru.
Photo:
AFP / Steffano Palomino
Explainer
- One of the largest earthquakes in recorded history set off tsunami warnings across the Pacific Ocean on Wednesday.
But it has not proved as destructive as experts feared.
Here are the factors that contributed to that outcome.
It was among the 10 strongest ever recorded.
The magnitude-8.8 earthquake was the largest since 2011, when a magnitude-9.1 tremor and subsequent tsunami struck off the coast of Japan and killed more than 15,000 people.
Wednesday's earthquake, known as the Kamchatka earthquake, is considered a megathrust earthquake - common to each of the planet's largest recorded quakes.
Earth's outer layers are split into 15 slabs known as tectonic plates.
These plates slowly shift around - moving a few centimetres every year - and often get stuck against each other due to friction.
When that happens, stress builds up over time until the plates suddenly shift and cause seismic waves that shake the ground.
Megathrust earthquakes occur when one tectonic plate has been forced underneath another - a phenomenon known as subduction.
In this case, it was the Pacific plate beneath the North American plate.
Stephen Hicks, an environmental seismology expert at University College London, said megathrust earthquakes "cause some of the world's largest ruptures and trans-ocean tsunamis".
Tsunamis occur when earthquakes displace large amounts of ocean water.
Megathrust quakes often cause tsunamis because the faults they cause tend to come to, or even directly penetrate, the sea floor, Dr Hicks said.
How much of the sea floor moves influences whether a tsunami is formed, and how big it might become, added Lisa McNeill, a tectonics professor at the University of Southampton.
"Some fault movements don't move the sea floor much, and so, there is no tsunami," she said.
In the case of the Kamchatka earthquake, a large slip occurred and generated a tsunami, Dr McNeill added.
Despite the earthquake triggering tsunami warnings across the Pacific Ocean, no deaths were recorded, and minimal structural damage was reported.
Maximum wave heights of five metres were observed in Kamchatka yesterday - a tenth of the heights recorded after the Tohoku Earthquake in 2011, which triggered the Fukushima Daiichi nuclear disaster.
Waves in Hawaii reached a maximum height of about 1.7 metres.
Meghan Miller, a professor of seismology at Australian National University, said where earthquakes strike has a major impact on the severity of subsequent tsunamis.
"For the tsunami and earthquake that happened in Japan in 2011, the distances [to land] were much closer," she told ABC Radio
National Breakfast.
"So the distance for the waves to travel to the coast of Japan was much shorter."
Dr McNeill said the height of tsunami waves can also be affected by the shape of the sea floor near the coast.
"[The waves] are small at sea but travel fast, and it is when they reach shallow water that they build in height again," she said.
" tsunami can have varied heights on one coastline, and it could generate surprisingly large wave heights."
She added that wave heights recorded after the Kamchatka earthquake were still "significant", but the potential for destruction was minimised.
The tremor triggered tsunami warnings across the Pacific Ocean, as far away as Hawaii, Chile, and Ecuador.
"This has been established in the Pacific since the 1960s, and people therefore receive warnings about the tsunami and can evacuate," she said.
"It is harder for those living close to the earthquake as they have less time, but the earthquake is their warning of a tsunami and [the] prompt to move to higher ground."
Many nations with sea borders have government-run programs to detect and monitor threats caused by earthquakes and tsunamis that may impact the region, and are charged with alerting communities of incoming threats and issuing evacuation orders.
There were no warning systems in place in the Indian Ocean when the 2004 Boxing Day tsunami occurred.
The enormity of the natural disaster's death toll, estimated to be about 225,000, is attributed to the lack of evacuation alerts.
-ABC
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