Researchers have identified a key component in some earthquakes that cause some tsunamis to be larger and more devastating than previous models had forecast.
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 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.
Now, it seems, another type of energy release is also involved. This is gravitational potential energy, Dr Simon McClusky, an earth scientist at the Australian National University explains, referring to the energy stored in an object as a result of a change in its vertical position or height, Stuff.co.nz reported.
He said that 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 added that 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 – concludes that an earthquake’s release of these two forms of energy is what contributes to the total destructive power of a tsunami.
The study is published in the journal Earth and Planetary Science Letters.