A very powerful magnitude 8.8 earthquake off Russia's Far Eastern Kamchatka coast on Wednesday triggered tsunami warnings as far away as French Polynesia and Chile, and was followed by an eruption of the most active volcano on the peninsula.
The shallow quake damaged buildings and injured several people in the remote Russian region, while much of Japan's eastern seaboard — devastated by a 9.0 magnitude earthquake and tsunami in 2011 — was ordered to evacuate, as were parts of Hawaii.
By the evening, Japan, Hawaii, and Russia had downgraded most of the tsunami warnings. But authorities in French Polynesia warned residents of several of the remote Marquesas Islands to move to higher ground and expect waves as high as 2.5m.
What is a 'megathrust fault'?
The quake off Russia occurred on what is known as a "megathrust fault", where the denser Pacific Plate is sliding underneath the lighter North American Plate, scientists said.
The Pacific Plate has been on the move, making the Kamchatka Peninsula area off Russia's Far East coast, where it struck, especially vulnerable to such tremors — and bigger aftershocks cannot be ruled out, they said.
Roger Musson, honorary research fellow at the British Geological Survey, said: "The Kamchatka seismic zone is one of the most active subduction zones around the Pacific Ring of Fire, and the Pacific Plate is moving westwards at around 80 mm per year."
"Subduction events," in which one plate pushes under another, are capable of generating far stronger earthquakes than "strike slips", such as the one that hit Myanmar in March, where plates brush horizontally against one another at different speeds.
A 9.5-magnitude earthquake struck in a central region of Chile in 1960. Source: SBS News
Why are tsunamis more likely?
Shallow "megathrust" events are more likely to cause tsunamis because they burst through the sea floor and displace huge volumes of water.
With a relatively shallow depth of 20.7km, Wednesday's earthquake was always going to create such tsunami risks, experts said.
Adam Pascale, chief scientist at Australia's Seismology Research Centre, said: "It is an offshore earthquake and when you have offshore earthquakes there is the potential for tsunamis."
Tsunami waves of around 1.7m reached as far as Hawaii, less high than originally expected, but scientists warned such waves do not have to be especially big to do damage to the relatively low-lying coastlines of Pacific island nations.
The impact of a tsunami depends on its "run-up" as it approaches coastlines, Pascale said.
Are more aftershocks expected?
Wednesday's quake has already triggered at least 10 aftershocks above magnitude 5, and they could continue for months, said Caroline Orchiston, director of the Centre for Sustainability at the University of Otago in New Zealand.
"Earthquakes by their nature are unpredictable, there are no precursors that are scientifically consistent in earthquake sequences. Before this morning, those other ones were the main shocks," she said.
"This demonstrates that large-magnitude earthquakes generate aftershock sequences that start immediately, and some of these can be damaging in their own right."
The 8.8 magnitude event on Wednesday came less than two weeks after a magnitude 7.4 earthquake in the same area, which has now been identified as a "foreshock".
Bigger aftershocks cannot entirely be ruled out, she added, but their magnitude and frequency normally tend to decrease over time.
