Science Matters: A wave of insight
Researchers have identified a key component in some earthquakes that cause some tsunamis to be larger and more devastating than previous models had forecast. Science columnist Peter Spinks and earth scientist Simon McClusky from the Australian National University discuss the findings.PT5M37S http://www.canberratimes.com.au/action/externalEmbeddedPlayer?id=d-2onq4 620 349 June 21, 2013
Every so often a tsunami wreaks havoc somewhere in the world. Yet, so far, the size and destructive power of these mighty surging floods of water have been difficult to predict.
That may be about to change. Researchers have identified a key component in some earthquakes that cause some tsunamis to be larger and more devastating than previous models had forecast.
If you feel an earthquake, don't run – run like hell!Richard Hagemeyer, who established the Pacific Tsunami Warning Centre in Hawaii
Earthquake motions that generate tsunamis usually involve sudden vertical movements of the sea-floor that displace vast quantities of ocean water. This, in turn, creates individual waves with great distances between their crests.
The tsunami-devastated Natori city in Miyagi prefecture is seen in this image taken on March 11, 2011.
The waves are not very high in deep waters of the open ocean, where they generally originate and travel at speeds of up to 900 kilometres an hour. As waves ripple outwards from an undersea quake or volcanic eruption, they are barely noticeable.
But once tsunamis arrive in shallower waters, they slow down, causing the ocean surface to rise precipitously. The waves, now colossal walls of seawater, crash over coastlines and flood many kilometres inland into coastal plains.
Vital to getting to grips with the path a tsunami takes is a basic understanding of how earthquakes happen. Scientists know that, in general, quakes occur when a section of Earth's upper layer, the lithosphere, fails. This releases something called strain energy that has built up as the lithosphere gets deformed by the crustal plates shifting around. Earthquakes release this energy — which may have taken tens or thousands of years to accumulate — in anything from a few seconds to several minutes.
"It used to be thought that the ground motion resulting from elastic strain release was solely responsible for generating tsunamis," says Simon McClusky, an earth scientist at the Australian National University. "The elastically rebounding Earth was considered responsible for displacing large volumes of water that caused waves to travel for vast distances across global oceans."
Now, it seems, another type of energy release is also involved. This is gravitational potential energy, Dr McClusky explains, referring to the energy stored in an object as a result of a change in its vertical position or height.
"The centuries of tectonic motion that produced the accumulation of strain energy in the Earth's lithosphere also caused the plates to move vertically – causing gravitational potential energy to build up too," he says. "The release of this energy causes an additional, previously unaccounted for, vertical motion of the sea-floor during these mega-thrust earthquakes."
The new research – led by Cambridge University scientists and published in the respected journal Earth and Planetary Science Letters – concludes that an earthquake's release of these two forms of energy is what contributes to the total destructive power of a tsunami.
"This work will allow us to more accurately model tsunamis following large earthquakes in future," Dr McClusky says.
By way of example, the Joint Australian Tsunami Warning Centre runs simulations based on possible mega-thrust earthquake scenarios using many inputs. This lets scientists forecast the effects of such events, along with the size of tsunamis that might ensue.
"The model improvements should make simulations more accurate, thus providing more reliable information to policymakers," he says. "In turn, this will help implement better mitigation strategies on the ground."
Ring of Fire
Forming part of the volcanoes and fault lines that comprise the Pacific Basin's "Ring of Fire", Indonesia is particularly prone to earthquakes, some of which carry more energy than 200,000 tonnes of TNT explosives.
Most tsunamis – sometimes incorrectly labelled tidal waves – are caused by large and powerful earthquakes like these that occur near the boundaries of tectonic plates. But some result from underwater volcanoes or landslides triggered by earthquake-related ground-shaking.
When they hit a coastline, the ocean rushes out seawards and can completely expose the near-shore sea-floor, complete with seaweed and stranded fish. Sometimes the first sign of an incoming tsunami is this unusual recession of the sea; then the surge rushes in, commonly with disastrous consequences. On occasion, the surge has enough momentum to travel some way uphill.
There are often reports of curious folk who ventured down to investigate a suddenly exposed sea-floor, or to pick up stranded fish, getting caught up and drowned by the following surge.
If the sea rushes out like that, the best thing you can do is get to high ground as quickly as you can, experts say. So the message is this: if you feel an earthquake and you happen to be close to the coast, run as fast as possible inland and uphill.
As Richard Hagemeyer, who established the Pacific Tsunami Warning Centre in Hawaii, once advised: "If you feel an earthquake, don't run – run like hell!"
Tsunamis are by no means restricted to countries such as Japan and Indonesia. In Australia the most vulnerable region is the north-west coast, which directly faces an area of intense earthquakes south of the Indonesian archipelago.
The east coast is also susceptible to tsunamis from subduction zones, where the edge of one crustal plate is forced beneath another.
Owing to the relatively shallow waters of the Bass Strait, tsunamis coming from subduction zones are unlikely to impact in a significant way, says Daniel Jaksa, a co-director of the Joint Australian Tsunami Warning Centre, operated by Geoscience Australia and the Bureau of Meteorology.
Yet tsunamis entering Victorian waters might pose marine threats, such as potentially dangerous waves, strong ocean currents and the possibility of some localised overflow on to the immediate foreshore. "As there are no subduction zones near Victoria, it is very unlikely to receive tsunamis in the form generated in Indonesia," Mr Jaksa says.
The Australian plate, the Earth's fastest-moving continental plate, is shifting slowly but surely by about 80 millimetres a year in a north-east direction. Indonesia and much of South-East Asia sit on the Sunda plate, which moves easterly at about 20 millimetres a year.
The collision of these tectonic plates causes the old marine crust on the Australian plate to dive under the Sunda plate. As the Australian plate moves northwards, the Sunda plate grabs it and is dragged down with the subducting marine crust.
When stresses are greater than the crust can hold, the crust breaks, releasing seismic waves in the form of an earthquake. In subduction zones, the over-riding plate usually moves up and thus causes a tsunami.
Earthquakes occur without warning. Therefore, if a big quake occurs nearby when you are on the coast, a tsunami that may have been generated is upon you within minutes – well before sensors record anything.
It generally takes about five minutes for enough seismic measurements to be made to compute the hypocentre – the time, latitude, longitude and depth of the first point of rupture and preliminary magnitude. Consequently, tsunami warnings help those far enough away from the earthquake to receive a warning.
The great northern Sumatran earthquake that generated the Indian Ocean tsunami in December 2004 ruptured for a trifle short of 10 minutes with a break of about 1200 kilometres, from south of Banda Aceh to north of the Nicobar and Andaman islands.
"If a tsunami warning system had been in place for the Indian Ocean basin in 2004, the tens of thousands far enough away from the earthquake that lost their lives in Indonesia, Thailand, Malaysia, Sri Lanka, India, Maldives and Africa may be with us today," Mr Jaksa says.
Since the December 2004 quake and tsunami, some of these nations have installed detection buoys and warning networks. The system includes stations for measuring seismic waves, plus deep-sea sensors in the form of pressure gauges.
The information gathered from these sources is analysed by about 10 information centres that may then issue tsunami warnings to potentially affected regions.
Read the key research paper in the Earth and Planetary Sciences Letters at: www.sciencedirect.com/science/article/pii/S0012821X12003202
For information on the Joint Australian Tsunami Warning Centre visit:
Learn about the geodynamics and geodesy work of Simon McClusky's group at: http://rses.anu.edu.au/research-areas/geodynamics-geodesy
Zoom in on the mechanisms of tsunamis at: www.ga.gov.au/hazards/tsunami/index.jsp and http://www.em.gov.au/sites/schools/Getthefacts/Tsunami/Pages/default.aspx
Check out the causes of tsunamis at:
At 11am on Friday, IMAX Melbourne Museum screens Forces of Nature, as part of its schools education program. It explains more about earthquakes, as well as some of nature's other powerful phenomena. Details: www.imaxmelbourne.com.au/movie/forces-of-nature/education
VCE Environmental Science study pages:
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