Recent research suggests that tiny creatures, millions of years old, could be the cause of the next devastating earthquake in the Hikurangi subduction zone. New Zealand’s greatest fault, the subduction zone, is the boundary where the Pacific Plate dives beneath the Australian Plate. Massive ‘megathrust’ earthquakes of more than 8 magnitudes can occur in this zone.
A team of seismologists is studying a rock on the Hungaroa fault, located on the edge of the Hikurangi subduction zone. Dr. Carolyn Boulton, who leads this team, says that near Torah, about 35 km south-east of Martinborough, there are limestone, mudstone and siltstone on the rocks. ) layers. These layers reveal what is happening in the subduction zone.
Similar rocks were on the ocean floor between 35 and 65 million years ago, but they were in a place where they were difficult to study. However, scientists can find out what is happening under the sea by looking at the rocks made on the land.
Dr. Bolton says that there is calcite in the rocks, which is formed from old one-celled marine organisms, especially foraminifera, such as plankton. The calcite made up of those tiny organisms can affect movement in the subduction zone. These small, long-dead creatures can influence how the two large tectonic plates interact mechanically.
Dr Bolton says that if the calcite present in the rocks dissolves, then the fault can weaken and slide easily without earthquakes. But if the calcite doesn’t dissolve, the vault can lock up by storing energy, causing a major earthquake. Calcite dissolves rapidly when the temperature is low. But as the temperature rises, it becomes difficult to dissolve.
Researchers say that in the subduction zone, temperatures rise more slowly than on land – about 10ºC/km. So the fault is actually sensitive to calcite. The amount and behavior of the calcite made from these organisms determines how big the next earthquake will be.
Dr Bolton says that the study of Tora tells us that the shallow part of the subduction zone can cause large and harmful earthquakes by slowly or rapidly shifting the movement of the plate. A megathrust earthquake near the subduction zone would have caused a major tsunami, evidenced by geological excavations and fossil records near the eastern coasts of both the North and South islands.
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According to research published in the journal Lithos, scientists estimate that the probability of a major earthquake at the southern edge of the Hikurangi subduction zone is 26% in the next 50 years.