Million-years-old dead sea creatures could play a role in the next big earthquake

Researchers in New Zealand have uncovered an intriguing factor that could affect the size of the next destructive earthquake in the Hikurangi subduction zone: fossils of tiny marine organisms that lived tens of millions of years ago.

The Hikurangi subduction zone is New Zealand’s largest plate boundary fault, running off the east coast of the north island, where two plates meet. It is where the Pacific Plate dives beneath the Australian Plate. The region can generate massive earthquakes, with events stronger than magnitude eight on record thought possible.

A close examination of the subduction zone is necessary to accurately predict earthquakes, but its offshore location and depth make it difficult to study. In the new study, a team of researchers led by Dr. Carolyn Boulton from Te Herenga Waka—Victoria University of Wellington investigated a rocky bluff on the Hungaroa fault, located on the margins of the Hikurangi subduction zone.

Layers of limestone, mudstone, and siltstone on a bluff near Tora, about 21 miles (35 km) southeast of Martinborough, provided a convenient indication of what was happening in the offshore subduction zone. Rocks like those on the bluff were deposited on the seafloor between 35 and 65 million years ago.

A big piece of the puzzle

Researchers found large amounts of calcite in these rocks. Calcite is a common carbonate mineral that, in this case, comes from ancient single-celled marine organisms, mainly foraminifera, such as plankton. The deposited calcite from long-dead tiny marine organisms could “affect how two huge tectonic plates interact mechanically.”

If the calcite can dissolve in high enough quantities, it could weaken the fault, allowing the two tectonic plates to slide easily without triggering noticeable earthquakes at the surface. If, however, it doesn’t dissolve, the fault line can lock up and store energy that can eventually be released as a larger quake.

“Calcite dissolves faster when it’s highly stressed and when temperatures are cooler,” said Dr. Carolyn Boulton, lead author of the study. “It dissolves more easily at low temperatures – say, room temperature. But it gets harder to dissolve as temperature goes up – say, deeper in the Earth. In the subduction zone, temperature increases more slowly than on land – by only around 10 ºC (18 °F) per km. So the fault is really sensitive to what calcite, those shells of old dead marine organisms, is doing. The amount and behavior of calcite from these organisms is a big piece of the puzzle of how large the next earthquake might be.”

The Hikurangi subduction zone still holds a lot of mysteries for scientists to uncover. It’s still unclear how calcite’s influence is actually playing out in the real world. And unfortunately, it’s hard to check the real subduction zone without complex drilling equipment.

“What we really want to know is: Are there slow-slip events out there we haven’t detected? Are the rocks moving without earthquakes, or are they truly locked up? That will help tell us what might happen in the next earthquake.” said Boulton.

The study has been published in the journal Lithos.