A hazard mortgage advisers can't ignore
New research is helping reveal how earthquakes can change the course of rivers, damage stop banks, and increase the risk of flooding – and where in New Zealand this is most likely to happen.
University of Canterbury (UC) PhD student Erin McEwan has created the world’s first database showing how earthquakes have affected rivers and flood protection in the past. She hopes the work will help emergency planners and councils better prepare for future disasters.
“This is a hazard that hasn’t had much attention before,” McEwan said. “But in New Zealand, where we have thousands of rivers and lots of active fault lines, it’s inevitable that they will intersect and interact.”
Over 3,700 rivers in New Zealand could be at risk of earthquake-related flooding if the fault lines beneath them rupture. More than 450 of these rivers are the size of, or larger than, Canterbury’s 35-kilometre-long Hororata River.
How earthquakes reshape rivers
McEwan’s research shows that earthquakes can cause rivers to shift, flood, or get blocked entirely – sometimes creating permanent dams and lakes. This can lead to serious flooding, not just during the earthquake but for months or years afterwards.
The 1987 Edgecumbe earthquake is one example. It caused major damage to stop banks and dropped parts of the land into the floodplain of the Rangitaiki River. The area has experienced several damaging floods since, including a major flood in 2017 that forced 3,000 people to evacuate.
Other major quakes, like those in Christchurch and KaikÅura, also changed how water moves across the land, redrawing flood maps and putting new areas at risk.
“It’s a very real hazard that we see again and again after large earthquakes,” McEwan said. “This research gives us tools to start predicting where those risks might be highest.”
Building the database
To build her model, McEwan and her team studied 52 earthquakes around the world where rivers were affected by fault ruptures. They combined this global data with New Zealand’s fault maps and existing flood modelling techniques to test future scenarios.
“If this method was used across New Zealand, it could help communities prepare more earthquake-resilient flood risk mitigation,” McEwan said.
She acknowledges it can be unsettling to think about earthquakes undoing years of flood protection work, but says planning ahead is key.
“We can’t stop earthquakes or floods — but we can help people be ready for them,” she said. “My goal is to get this into the hands of councils and planners so they can run their own ‘what if’ scenarios and make smart decisions for their communities.”
Why this matters for property and lending
For mortgage advisers, the research highlights how natural hazards can change property risk profiles overnight. Homes and developments near rivers or flood-prone land could face increased long-term risk after a major earthquake event, with implications for insurance, property values, and loan security.
Placing homes and important infrastructure on or near active fault lines and flood zones is risky, and McEwan’s research could help show how those risks might increase after an earthquake and where it might be safer to build in future.
Access the report on the Natural Hazards Commission website.
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