Earthquakes of magnitude 8 or higher are called ‘megathrust’, and new research has found that tiny, ancient marine organisms can have a big impact on these earthquakes.
Researchers studying New Zealand Hikurangi subduction zone have found that calcite deposits left behind by single-celled marine organisms tens of millions of years ago may control movement and friction levels between the Pacific and Australian plates.
It depends on whether the calcite is soluble or not. If the calcite is soluble like sugar in tea, the tectonic plates can slide over each other more easily. If they don’t, they stop plate movement and shut off the energy that would later be released in a sudden rupture.
What is calcite?
Calcite is often the main component of the shells of marine organisms, such as plankton (e.g. Coccolith and planktonic foraminifera), hard parts of red algae, some sponges, brachiopods, echinoderms, some serpulidae, most bryozoa, parts of shells, and some by-valves (like sip & rodist) etc.
How does calcite affect earthquakes?
“Calcite dissolves faster when it’s under a lot of pressure and when the temperature is cold,” says Caroline Bolton, a structural geologist in Te Herenga Waka at Victoria University of Wellington, New Zealand.
“It melts more easily at low temperatures, but becomes more difficult to dissolve as temperatures rise, such as deep in the Earth.”
Deep in the subduction zone, the temperature increases gradually with depth, warming by about 10°C for every kilometer. Calcite shells that fail to dissolve below the surface can have a significant effect on the movement of a fault line (the meeting point of two Earth’s plates).
The fault itself is difficult to reach and requires expensive drilling equipment to access, so the researchers used exposed layers of limestone, mudstone, and siltstone on a local shoreline southeast of Martinborough, on the North Island – as a proxy.
Rocks southeast of Martinborough on the North Island of New Zealand contain calcite from marine organisms, mainly a type of oyster called foraminifera.
“How much of that calcite is in the subduction zone, and the amount and behavior of calcite from these organisms is a big part of the puzzle of how big the next earthquake might be,” Bolton says.
Geologists know less about the Hikurangi subduction zone than other New Zealand faults because it cannot be closely monitored. The record of past earthquakes is not as comprehensive, and knowledge of its location is not as complete, making it difficult to predict the next major earthquake.
Researchers say there is a 26 percent chance of a large earthquake along the fault in the next 50 years, which could generate a large tsunami.
All kinds of factors are at play, but the study shows how plate motion can be slow and gentle, or fast and large, and the more we know about the structure of underwater calcites, the more we can understand. They will be better able to figure out what is going to happen next.
“Just think, these tiny, long dead organisms could affect how two large tectonic plates interact mechanically,” says Dr Caroline Bolton.
