A new NASA study (published in Journal of Geophysical Research -- Oceans) is challenging a long-held theory that tsunamis form
and acquire their energy mostly from vertical movement of the seafloor.
An undisputed fact was that most tsunamis result from a massive
shifting of the seafloor -- usually from the subduction, or sliding, of
one tectonic plate under another during an earthquake. Experiments
conducted in wave tanks in the 1970s demonstrated that vertical uplift
of the tank bottom could generate tsunami-like waves.
In the following
decade, Japanese scientists simulated horizontal seafloor displacements
in a wave tank and observed that the resulting energy was negligible.
This led to the current widely held view that vertical movement of the
seafloor is the primary factor in tsunami generation.
The animation shows how waves of energy from the Tohoku-Oki earthquake and tsunami of March 11, 2011, pierced through into Earth's upper atmosphere in the vicinity of Japan, disturbing the density of electrons in the ionosphere. These disturbances were monitored by tracking GPS signals between satellites and ground receivers. A model of ocean tsunami wavefronts [Song, 2007] is overlaid in blue to show the correlation between variations in the ionosphere above and ocean surface below.
Note that traveling ionospheric disturbances (TIDs), visible throughout the animation, are correlated with the position of the tsunami.
In 2007, Tony Song, an oceanographer at NASA’s Jet Propulsion
Laboratory in Pasadena, California, cast doubt on that theory after
analyzing the powerful 2004 Sumatra earthquake in the Indian Ocean.
Seismograph and GPS data showed that the vertical uplift of the seafloor
did not produce enough energy to create a tsunami that powerful.
But
formulations by Song and his colleagues showed that once energy from the
horizontal movement of the seafloor was factored in, all of the
tsunami’s energy was accounted for.
Those results matched tsunami data
collected from a trio of satellites –the NASA/Centre National d’Etudes
Spatiales (CNES) Jason, the U.S. Navy’s Geosat Follow-on and the
European Space Agency’s Environmental Satellite.
Further research by Song on the 2004 Sumatra earthquake, using
satellite data from the NASA/German Aerospace Center Gravity Recovery
and Climate Experiment (GRACE) mission, also backed up his claim that
the amount of energy created by the vertical uplift of the seafloor
alone was insufficient for a tsunami of that size.
“I had all this evidence that contradicted the conventional theory, but I needed more proof,” Song said.
Animation of ‘The Earth’ - modelling tsunami waves propagating over the globe.
With a resolution of 3km and a time step of 1 min, we brought together 10 previous tsunami events in 24 hours - in less than 5 hrs on 36 cores.
The model shows (in order of appearance and triggered every 2 hours): Solomon Islands 2007, Tohuku 2011, Sumatra 2004, Makran/Balochistan 1945, Greece 1956, Lisbon 1755, Dominican Republic 1946, Ecuador/Colombia 2016, then Tohuku 2011 and Valvidia 1960 (simultaneously) and Kamchatka 1952.
His search for more proof rested on physics -- namely, the fact that
horizontal seafloor movement creates kinetic energy, which is
proportional to the depth of the ocean and the speed of the seafloor's
movement.
After critically evaluating the wave tank experiments of the
1980s, Song found that the tanks used did not accurately represent
either of these two variables.
They were too shallow to reproduce the
actual ratio between ocean depth and seafloor movement that exists in a
tsunami, and the wall in the tank that simulated the horizontal seafloor
movement moved too slowly to replicate the actual speed at which a
tectonic plate moves during an earthquake.
“I began to consider that those two misrepresentations were
responsible for the long-accepted but misleading conclusion that
horizontal movement produces only a small amount of kinetic energy,”
Song said.
Building a Better Wave Tank
To put his theory to the test, Song and researchers from Oregon State
University in Corvallis simulated the 2004 Sumatra and 2011 Tohoku
earthquakes at the university’s Wave Research Laboratory by using both
directly measured and satellite observations as reference.
Like the
experiments of the 1980s, they mimicked horizontal land displacement in
two different tanks by moving a vertical wall in the tank against water,
but they used a piston-powered wave maker capable of generating faster
speeds.
They also better accounted for the ratio of how deep the water
is to the amount of horizontal displacement in actual tsunamis.
The new experiments illustrated that horizontal seafloor displacement
contributed more than half the energy that generated the 2004 and 2011
tsunamis.
“From this study, we’ve demonstrated that we need to look at not only
the vertical but also the horizontal movement of the seafloor to derive
the total energy transferred to the ocean and predict a tsunami,” said
Solomon Yim, a professor of civil and construction engineering at Oregon
State University and a co-author on the study.
Photo taken March 11, 2011, by Sadatsugu Tomizawa
and released via Jiji Press on March 21, 2011, showing tsunami waves
hitting the coast of Minamisoma in Fukushima prefecture, Japan.
Credits: Sadatsugu Tomizawa CC BY-NC-ND 2.0
The finding further validates an approach developed by Song and his colleagues that uses GPS technology to detect a tsunami’s size and strength for early warnings.
The JPL-managed Global Differential Global Positioning System (GDGPS)
is a very accurate real-time GPS processing system that can measure
seafloor movement during an earthquake.
As the land shifts, ground
receiver stations nearer to the epicenter also shift.
The stations can
detect their movement every second through real-time communication with a
constellation of satellites to estimate the amount and direction of
horizontal and vertical land displacement that took place in the ocean.
They developed computer models to incorporate that data with ocean floor
topography and other information to calculate the size and direction of
a tsunami.
“By identifying the important role of the horizontal motion of the
seafloor, our GPS approach directly estimates the energy transferred by
an earthquake to the ocean,” Song said.
“Our goal is to detect a
tsunami’s size before it even forms, for early warnings.”
It is as if a child has been doodling with large coloured crayons.
What you see are actually the great gouge marks left on the seafloor when the keel of a giant block of ice has dragged through the sediments.
The arcs and loops record the movement of the berg as it turns about, caught in the wind, currents and tides.
This "ice art" is from a stunning new collection of images that detail how glacial action has shaped the ocean floor in Earth's polar regions.
The atlas is the work of more than 250 scientists from 20 countries and represents our most comprehensive view yet of what the seabed looks like at high latitudes.
"We now have a critical mass of high-resolution imagery, of the imprints left by the action of ice," explained Dr Kelly Hogan, one the collection's editors from the British Antarctic Survey (BAS).
"We can see where the ice has been and what it's done, and this allows us to compare and contrast. Looking at what has happened in the past can help us understand what may happen in the future with modern ice sheets as they respond to climate change."
In the most extreme places in the world, the most remarkable people gather scientific evidence about our changing planet.
From the discovery of the hole in the ozone layer, to the effects of climate change, the polar regions act as early warning systems.
With the help of the British Antarctic Survey, governments and world leaders have been able to use the evidence gathered to make better decisions for the future of the Earth.
The book, published as a "memoir" of the Geological Society of London, has taken four years to put together.
It contains mostly images produced from ship echosounders, which pulse the ocean bed and then build depth maps from the resulting return signal (the "false colours" will usual denote different depths).
A first version of the book came out in 1997, but comprised really only low-resolution data.
The new tome, on the other hand, benefits from 20 years of multibeam swath technology, which is capable of creating spectacular perspectives of the ocean floor, often at sub-metre resolution.
All ships now entering polar waters will routinely map the bed in this way.
An almost circular iceberg plough mark off Svalbard.
(Red is 25m water depth; green is 50m)
"Today's icebreakers and ice-strengthened vessels all carry these high-res systems; they are standard. No longer do we get just single points, but a whole fan of sound goes down to the seafloor to collect 3D data. And we're finding all these weird and wonderful features," Dr Hogan told BBC News.
As well as the iceberg ploughmarks, there are the classic forms left by marine glaciers.
These include long striations, drumlins (egg-shaped mounds) and moraines (bulldozed sediment ridges).
Drumlins (elongate hills aligned with the ice flow direction) from the Gulf of Bothnia in the Baltic Sea
"When the moraines are pushed close together, they're most likely annual features, but move a bigger distance offshore and you might find moraines that are from thousands of years ago and are spread further apart. We could go date the sediments to look at how quickly ice has retreated. So, this gives you an idea of how we can use the imagery to help us understand what is going on."
Another of the more intriguing features is the permafrost polygon.
A great expanse of these shapes is captured on the floor of the Laptev Sea off the Siberian coast in the Arctic.
These were actually created above water in ground that contracted and expanded in multiple cooling and warming cycles - only then to be submerged beneath the waves.
These shapes were made on land and then submerged around 7,000-8,000 years ago
They illustrate how the sea can sometimes preserve a record that might otherwise be lost on land, perhaps covered over by vegetation or human development, or lost entirely through erosion.
And flicking through the book, it is clear that not all ice action has been constrained to today's polar waters.
You will find, for example, images in the atlas from around the UK.
But then 25,000 years ago, Britain was covered by a colossal ice sheet that extended down from the north.
An iceberg ploughmark showing rotation, from the central Barents Sea
(Red is 240m water depth; purple is 252m)
Likewise, Libya makes an entry with glacial lineations in the Murzuq Basin that are several kilometres long.
These ice features are present because 450 million years ago, north Africa was centred over the South Pole, forming a part of the Gondwana supercontinent.
But as impressive as all this imagery is, it still only represents a small fraction of the total seafloor currently found above 60 degrees latitude.
As with the ocean more generally across the globe, our knowledge of the shape of bed is poor.
If you were to divide the polar seas into 500m boxes, more than 85% of them would have no depth soundings.
"That will change in time as all these vessels routinely gather multibeam data," said Dr Hogan.
That part of the berg which lies underwater can be scouring the bed
NASA/ Jane Peterson
This beautiful multibeam data shows bulldozed sediments (moraines)
created as ice retreated and advanced
The unmistakable grooves cut in the seafloor off West Antarctica as ice flowed right to left
Rising ocean temperatures drive more intense and longer lasting toxic outbreaks.
Researchers have long suggested that climate change could mean more damage from algal blooms — runaway growths of algae that can strangle marine ecosystems and devastate coastal economies.
Now, a study has unpicked how warming ocean temperatures have already driven an intensification of blooms around North America — the first time this link has been established at an ocean scale.
Harmful algal blooms can occur when changes in water conditions lead to a huge growth in the number of a particular species of algae.
The blooms can produce toxins, become so large that they kill marine life, and even turn water a different colour.
Research has established that one factor that helps blooms to spread is a sudden increase in nutrients such as nitrogen and phosphorus — often from agricultural fertilizers — and it has also linked warming temperatures to individual events.
But the broader influence of climate change on these outbreaks is less well quantified.
Christopher Gobler, who studies coastal ecosystems at Stony Brook University in Southampton, New York, and his colleagues looked at the relationship between blooms in the North Atlantic and North Pacific oceans and temperature changes in the region to investigate whether the events there were linked to were linked to ocean warming.
Previous research has studied the influence of climate change on algal blooms in individual coastal systems, says Gobler, but this is the first to assess ocean-basin-wide trends.
The researchers looked at two species of algae: Alexandrium fundyense and Dinophysis acuminata. (These species produce toxins that can cause illnesses — sometimes fatal — in humans who eat shellfish contaminated with them).
They used detailed data on sea-surface temperatures to model trends in the growth rates of the algae species and the periods when blooms occurred from the early 1980s into the twenty-first century.
The Treacherous and Productive Seas of Southern Africa
The light blue swirl to the east of Cape Agulhas is a phytoplankton bloom in an area of cool, upwelling water.
At dozens of sites, their model suggests that new blooms could occur where they hadn't happened before.
The researchers found higher potential growth rates and longer bloom seasons for both species in many parts of the Atlantic coast and along Alaska.
In some places, they found the bloom season to now be more than a month longer than it was in 1982.
The team used its model to retrospectively predict what harmful blooms might have occurred in the time period and found good agreement with actual events.
“What’s different about this study is they’re going beyond just saying 'Look, warmer is bad' and trying to do a rigorous quantitative analysis,” says Anna Michalak, who studies climate change and water quality at the Carnegie Institution for Science in Stanford, California.
Gobler points out that that the ocean does not warm uniformly, with some regions warming faster than average and some cooling.
This means blooms will appear in new regions, and may actually reduce in some places they currently occur.
Factors other than temperature are still clearly important, he says.
Many factors
Michalak points out that for events such as major droughts and floods, researchers can now often estimate how much more likely such events will be as a result of climate change.
But there is less research on the relationship between water quality and climate change.
“We don’t have that science on the water quality side,” she says, adding that this paper is one step in that direction.
In 2011, North America’s Lake Erie experienced one of its worst algal blooms in decades. NASA acquired October 5, 2011
Michalak’s team has previously studied a massive 2011 bloom in Lake Erie — one of the largest ever on the Great Lake.
They pinned the event on a combination of weather conditions and agriculture putting huge nutrient loads in the lake.
That interplay of different factors is one reason why not everyone is yet prepared to say that climate change will necessarily lead to more harmful algal blooms.
Mark Wells, who studies harmful algal blooms at the University of Maine in Orono, points out that that rising temperatures alone may not always cause more harmful algal blooms.
In 2010 in the Gulf of Maine, he says, there were very high water temperatures, and many blooms were expected.
But in fact, water temperatures were so high that the layers of sea water became stratified and prevented mixing and the transfer of nutrients, so in the end there were fewer blooms than expected.
Still, he says, the latest study “provides a lot of new evidence” and represents a believable trend.
Panoramic views
photo JC Bourniche
