Extinct undersea volcanoes behind tsunami quakes

Tsunami earthquakes may be caused by extinct undersea volcanoes causing a "sticking point" between tectonic plates when one plate slides under another, says a new research.

The findings have revealed the causes and warning signs of rare tsunami earthquakes. This may lead to improved detection measures.

Tsunami earthquakes happen at relatively shallow depths in the ocean and are small in terms of their magnitude.

However, they create very large tsunamis. Some earthquakes that only measure 5.6 on the Richter scale generate waves that reach up to 10 metres when they hit the shore.

Researchers can detect the smallest of earthquakes, but the challenge has been to determine which small magnitude events can cause large tsunamis.

"Tsunami earthquakes don't create massive tremors like more conventional earthquakes such as the one that hit Japan in 2011, so residents and authorities in the past have not had the same warning signals to evacuate," said Rebecca Bell from Imperial College London in Britain.

"We are beginning to understand for the first time the factors that cause these events. This could ultimately save lives," she said.

The researchers from Imperial College and GNS Science in New Zealand used geophysical data collected for oil and gas exploration and historical accounts from witnesses of two tsunami earthquakes which happened off the coast of New Zealand's north island in 1947.

The team located two extinct volcanoes off the coast of Poverty Bay and Tolaga Bay that have been squashed and sunk beneath the crust off the coast of New Zealand, in a process called subduction.

The researchers suggest that the volcanoes provided a "sticking point" between a part of the earth's crust called the Pacific plate, which was trying to slide underneath the New Zealand plate.

This caused a build-up of energy, which was released in 1947, causing the plates to "unstick" and the Pacific plate to move and the volcanoes to become subsumed under New Zealand.

This release of the energy from both plates was unusually slow and close to the seabed, causing large movements of the sea floor, which led to the formation of very large tsunami waves.

All these factors combined, said the researchers, are factors that contribute to tsunami earthquakes.

The researchers believe the information they've gathered on these events could be used to locate similar zones around the world that could be at risk from tsunami earthquakes.

The study was published in the journal Earth and Planetary Science Letters.