New fault system north of Gisborne sheds light on East Coast tsunami risk

A newly identified fault system north of Gisborne has implications for the region's risk from earthquakes and tsunami. Photo / Alan Gibson

Scientists have shed more light on a newly identified fault system north of Gisborne – with implications for the region's risk from earthquakes and tsunami.

A new study, led by Dr Nicola Litchfield of GNS Science, suggests that distinctive marine terraces were caused by earthquakes on multiple offshore upper-plate faults.

The team used radiocarbon and other methods to date the ages of marine terraces at two North Island sites, Puatai Beach and the Pakarae River mouth.

Previously, scientists thought the youngest terraces were created at the same time by a rupture in the Hikurangi subduction zone – a major fault area along the boundary of the Pacific and Australian plates- or a single-fault earthquake in the upper plate.

But radiocarbon dating of beach shells and layers of volcanic ash showed the uplift happened at different times.

"New research techniques allow us to build a high-resolution picture of where and how these uplifts occurred," Litchfield said.

The Hikurangi Subduction Zone. Source / GNS Science

"We're now confident that each terrace represents an individual quake – and the patterns of uplift allowed us to map new offshore faults in the region."

Litchfield said the magnitude 7.8 Kaikoura earthquake in 2016 prompted seismologists to look closely at the mechanisms behind complex multiple-fault earthquakes.

The research would add to the growing understanding of the hazards posed to the East Coast by the Hikurangi subduction zone.

"Since the newly identified faults are close to the coastline, this may mean short warning times for a tsunami," she said.

"We need to learn more about these newly mapped offshore faults, how they might rupture together, and what they mean for the overall seismic hazard."

The study was part of a wider work programme - led by GNS Science's Dr Laura Wallace and funded by a grant from the MBIE Endeavour Fund – which aimed to unlock the secrets of the subduction zone.