Long-dead marine organisms could influence the next big earthquake

Tiny creatures tens of millions of years old could influence the size of the next destructive earthquake in the Hikurangi subduction zone.

The Subduction Zone, New Zealand’s largest fault, marks the boundary where the Pacific Plate plunges under the Australian Plate. Huge “megathrust” earthquakes with magnitudes greater than 8 can be generated in the region.

dr Carolyn Boulton of Te Herenga Waka – Victoria University of Wellington led a team of earthquake scientists studying a rocky cliff on the Hungaroa Fault, located on the edges of the Hikurangi subduction zone.

Layers of limestone, mudstone and siltstone on the bluff near Tora, about 35km south-east of Martinborough, are a handy indicator of what’s happening in the offshore subduction zone, says Dr. Boulton.

Rocks similar to those on the cliff were deposited on the sea floor between 35 and 65 million years ago, but their location makes them difficult to study. Instead, scientists can look at the rocks on land to learn more about what’s happening beneath the sea.

“The rocks all contain calcite from ancient unicellular marine organisms, mostly foraminifera, like plankton. We found that calcite from these tiny organisms can affect movement in the subduction zone. Just imagine, these tiny, long-dead organisms can affect how two giant tectonic plates mechanically interact.”

dr Boulton says if the calcite in the rocks can dissolve — like sugar in tea — the fault can be weak and slide easily without an earthquake. However, if the calcite is unable to dissolve, the fault can “block” and store energy that could be released in a large tremor.

“Calcite dissolves faster when it’s subjected to heavy use and when temperatures are cooler. It dissolves more easily at low temperatures – let’s say room temperature. But it gets harder to dissipate as the temperature rises — say, deeper in the earth.

“In the subduction zone, the temperature rises more slowly than on land – only by about 10ºC/km. So the fault is very sensitive to what calcite, those shells of ancient dead marine organisms, is doing.

“The amount and behavior of calcite from these organisms is a big piece of the puzzle of how big the next earthquake might be.”

The Hikurangi subduction zone still holds many mysteries for scientists to uncover, says Dr. Boulton.

“Geologists studying the Alpine Fault and other faults on land can study them up close. But looking into the Hikurangi subduction zone requires expensive drill rigs. This means that our record of previous large earthquakes in the region is not that good.

“Our observations of Tora tell us that the shallow part of the subduction zone can accommodate plate motion by slowly shedding — or rapidly shedding in large and damaging earthquakes.

“What we really want to know is: are there any slow-slip events out there that we haven’t spotted? Are the rocks moving without earthquakes or are they really blocked? That will help us tell what might happen in the next earthquake.” says Dr. Boulton.

A megathrust earthquake along the subduction zone could produce a large tsunami, evidence of which has been found in geological excavations and the fossil record along the east coasts of both the North and South Islands and through the Cook Strait.

Scientists predict a 26% chance of a major earthquake on the southern edge of the Hikurangi subduction zone in the next 50 years.

Research results are published in the journal lithograph.