Scientists say these waves were tame by comparison to tsunamis of the past

NZ Herald

4 days ago

'It definitely created a Pacific-wide tsunami, which in the context of tsunamis is quite large,' said Tina Dura, a tsunami researcher at Virginia Tech in the US.
'But it's a little bit smaller than could be possible in that magnitude of earthquake.'
The quake occurred near the Kamchatka Peninsula, where the Pacific tectonic plate is sliding underneath the North American plate.
This seismically active 'subduction' zone has produced two of the world's top 10 earthquakes.
In 1952, a 9.0-magnitude earthquake hit less than 32km away from the epicentre of yesterday's quake; that temblor also triggered a Pacific-wide tsunami.
The two plates slipped past one another at the relatively shallow depth of 20km under the ocean, which caused part of the seafloor to thrust upward and displace the water - creating a tsunami.
Wave heights reached much higher than normal near the Kamchatka Peninsula - more than 4.5m, said Alexander Rabinovich, a physical oceanographer and part of the IUGG International Tsunami Commission.
He said local teams will be surveying damage to the low-population area along the southern coast of Kamchatka, where he said wave heights could have reached 15m.
Around Hawaii, wave heights hit 1.5 to 1.8m. Most places around California saw just a foot (30cm) or so in increased wave height - though Crescent City, where tsunamis often get amplified because of the shape its shelf, saw wave heights of almost 1.2m.
Why wave heights were relatively low farther away from the quake 'is the biggest question at the moment', said Viacheslav Gusiakov, a tsunami expert in the Siberian branch of the Russian Academy of Sciences.
As the wave spreads out, it weakens. But the similarly powerful 1952 earthquake in the same region caused bigger waves and more damage in Hawaii than yesterday's quake so far.
One possible explanation, Gusiakov said, is a potential absence of a large landslide in the ocean that could have exacerbated the tsunami.
Underwater movements of sediments or rocks can add to the energy of a tsunami by up to 90%, although this specific case will need to be studied more.
It's also possible that the earthquake itself, although powerful, may have contributed to a milder tsunami.
US Geological Survey modelling suggests the land shifted by 6-9m along a roughly 480km stretch of fault, said Diego Melgar, director of the Cascadia Region Earthquake Science Centre at the University of Oregon.
These sorts of variations in fault movement can be the difference between relatively small tsunamis and disastrous ones, he said.
For instance, the 2011 earthquake that triggered a nuclear crisis in Japan shifted the land by as much as 45m across a similarly long stretch of fault line, creating tsunami waves that were as high as 30m locally.
It also caused millions of dollars in damage in Crescent City and swept one person away.
'Earthquakes have a personality,' Melgar said. 'Those kinds of details really affect the tsunami.'
Part of the answer is also better warnings, experts said.
Tsunami warnings were issued in a timely manner by the Kamchatka and Sakhalin tsunami warning centres, Gusiakov said. So far, no tsunami fatalities have been reported.
The surge of water did cause a deck to break off in Crescent City, but no injuries have been reported so far.
Rabinovich said the tsunami warnings were quite effective this time around, allowing people time to evacuate from coastal areas, take boats out of harbours and prepare.
'When you're all the way across the Pacific Ocean, you do have a little bit more time to get everyone aware and prepared,' Dura said.
That's not always the case.
For example, she said an event could unfold in minutes to hours in the western United States from a rupture in the Cascadia subduction zone.
It's too early to say that the earthquake did not cause any disastrous tsunami damage, Melgar said.
People have become used to seeing disaster impacts broadcast live on social media, but it will take careful analysis of satellite data as well as boots-on-the-ground surveys to know the height of waves that hit the Russian coast, particularly in the sparsely populated Kamchatka Peninsula.
Even though any tsunami impacts were minimal in Hawaii and on the US West Coast, Melgar called it 'a story of triumph' that those areas received warnings and acted quickly.
Some warning systems have been implemented in response to deadlier and more damaging earthquakes.
The 1952 earthquake near the Kamchatka Peninsula caused significant damage in Hawaii. In 1946, a 8.6-magnitude quake in Alaska's Aleutian Islands triggered a tsunami that killed 159 people in Hawaii.
Those disasters were the driving force behind the creation of the US Tsunami Warning Centres, which are part of the National Oceanic and Atmospheric Administration.
'Any warning at all is a huge success,' Melgar said.