Predicting the likelihood and severity of earthquakes is important, but it’s hard to account for all factors at play. Researchers in New Zealand have now uncovered a previously overlooked factor that could affect the impact of the next big quake – fossils of tiny sea creatures.
The Hikurangi Subduction Zone is New Zealand’s largest fault line, running off the east coast of the north island. It marks the boundary of the Pacific and Australian plates, with the former diving underneath the latter as they collide. This makes the region capable of generating some powerful earthquakes, with events stronger than magnitude 8 on record and as high as magnitude 9 thought possible.
Closer examination of the subduction zone is necessary for more accurate predictions of earthquakes, but its offshore location and depth makes that tricky. So for the new study, researchers at Te Herenga Waka – Victoria University of Wellington investigated similar rocks of limestone, mudstone and siltstone on a nearby bluff on land.
In these deposits, the team found large amounts of calcite, a common carbonate mineral that in this case comes from the shells of tiny, ancient marine organisms. And this could play a bigger role than it’s given credit for. If the calcite can dissolve in high enough quantities, it could act like lubricant for the two tectonic plates, allowing them 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.”
While the team has identified this potential new factor, 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. Ideally, the hypothesis would be tested by checking whether the plates are sliding past each other in gentle slow-slip movements that are hard to detect, or if they’re stuck and potentially building up to a powerful future quake.
“What we really want to know is: Are there slow-slip events out there we haven’t detected?” said Boulton. “Are the rocks moving without earthquakes, or are they truly locked up? That will help tell us what might happen in the next earthquake.”
The research was published in the journal Lithos.
