New underwater mapping technique boosts understanding of quake and tsunami triggers

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Scientists have developed a new method for mapping underwater rock formations to help them better understand what triggers earthquakes and tsunamis. 

The method combines a traditional technique – ‘acoustic mapping’ – with a newer method called ‘full waveform inversion’. Acoustic mapping involves sending soundwaves from the ocean surface down to the seabed and into Earth’s crust beneath. The time it takes for the soundwaves to bounce back indicates the depth and structure of the sea bed, as well as the likely composition of the rocks that make it up. 

Full wave inversion is a data matching procedure that converts the soundwaves into higher resolution. The resulting maps of the seabed are thus far more intricately detailed.

“We can now scan underwater rocks to see their properties in greater detail,” said the report’s author, Melissa Gray, of Imperial College London’s Department of Earth Science and Engineering. “Hopefully this will help us to better work out how earthquakes and tsunamis happen.”

Gray’s team of geologists used the new technique to map rocks along a fault-line off the east coast of New Zealand’s North Island. The rocks were in an area known as the Hikurangi subduction zone, a fault-line where the edge of the Pacific tectonic plate tucks under the edge of the Australian plate.

A subduction zone is an area prone to seismic events because of the presence of two plates pushing against one another. This pressure eventually causes one of the plates to ‘slip’ suddenly under the other one triggering a quake and – if the slipping happens underwater – a tsunami. 

But the slipping can also take place slowly over time, an event which, while releasing the same amount of energy, usually doesn’t lead to an earthquake. Gray and her colleagues were particularly interested in studying these so-called ‘slow slips’, which they believe could “unveil a treasure trove of clues about how larger, more devastating quakes happen”.

Their new method has allowed them to map the Hikurangi fault zone in unprecedented detail capturing crucial new data, such as the shallow faults responsible for the Gisborne tsunami, a 10m-high (32ft) wave that struck New Zealand’s North Island in 1947. (That tsunami was caused by a relatively slow slip.) 

The researchers want to use their method to map other earthquake-prone zones around the world. “Ultimately, we want to understand why some slips cause devastating earthquakes, while others do not,” said Gray’s colleague Dr Rebecca Bell.

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