Using shells to date earthquakes

A team of scientists from California State Polytechnic University Pomona has been mapping and sampling marine terraces at Waimarama and Ocean Beach that were uplifted in ancient earthquakes.

They are searching for marine shell samples to use for radiocarbon dating back in the lab, so they can figure out the timing of the uplift and past earthquakes.

Like archaeologists, these scientists dig into the earth to look for information from the past. 

“During large earthquakes, the coast is uplifted, shifting the rocky intertidal platform up above high tide. Organisms living on the platform, beach, or in estuaries die and are buried and preserved in the shore deposits at that site,” says Professor Jeff Marshall of Cal Poly Pomona.

These long-dead organisms are the evidence of earthquakes that occurred hundreds or even thousands of years ago, and are preserved in the buried landscape beneath the surface. 

“We are interested in learning about the timing of past earthquakes to help us understand long-term behaviour of the Hikurangi subduction zone,” says Prof. Marshall.

The Hikurangi subduction zone lies off the East Coast of the North Island and is where the Pacific tectonic plate subducts (or dives) under the Australian tectonic plate. Subduction zones are where the largest and most powerful earthquakes occur.

Scientists want to find out more about how these two tectonic plates behave as they move past each other. This research is part of a larger science project which involves looking for evidence of past earthquakes, geological and geophysical mapping, ocean bottom seismology, and computer-modelling.

From Waimarama and Ocean Beach, the team will be heading to the Pourerere - Aramoana beaches of Central Hawke’s Bay to research uplift there.  

The wider project called Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) is funded by the National Science Foundation (United States) and involves scientists from University of Texas, GNS Science, NIWA, Cal Poly Pomona University, University of Southern Mississippi, Pennsylvania State University, University of Southern California, Victoria University of Wellington, Imperial College of London, JAMSTEC, and University of Tokyo.

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