Slow earthquake near Hawke’s Bay, New Zealand

A slow earthquake occurs in the Hikurangi subduction zone, a tectonic boundary between the Australian and Pacific plates. This zone runs along the east coast of New Zealand’s North Island and plunges beneath the landmass, forming a complex and active geological system.

Global Navigation Satellite System (GNSS) stations near the Mahia Peninsula north of Hawke’s Bay recorded land shifts of about 4 cm (1.6 in) east and 1 cm (0.4 in) south in the last three weeks of December .

Stations between Wairoa and Tolaga Bay showed similar movements, with some locations moving as much as 5 to 8 cm (2 to 3.1 in). It is a tectonic plate movement lasting up to two years that takes place in just three weeks.

The last recorded slow earthquake in the region occurred in June 2023, demonstrating the recurring nature of these phenomena in the Northern Hawke’s Bay and Mahia regions.

The ongoing SSE is a focus of international research. Since 2014, over 50 offshore instruments, including seafloor seismometers and seafloor pressure sensors, have been deployed to study the subduction zone. The tools enable precise monitoring of tectonic movements and related phenomena.

These sensors, part of an international collaboration between researchers from New Zealand, Germany, Japan and the USA, aim to map the spatial extent of SSEs, detect associated seismicity and refine knowledge of subduction processes.

The drillship JOIDES Resolution installed two observatories in 2023 up to 500 m (1,640 ft) below the seafloor, near the current south-southeast location. The observatories are designed to capture multi-year data cycles of slow-slip events.

A recent U.S.-funded project also installed seafloor flowmeters to measure subseafloor water movement associated with SSEs. The instruments will be collected and analyzed to provide detailed insights into subduction zone processes.

In early 2024, a remotely operated underwater vehicle collected data from the observatories. This dataset will help scientists understand the mechanisms of SSEs and their relationship to regular earthquakes.

The Hikurangi subduction zone extends along the east coast of New Zealand. The fault is known for frequent slow-slip events, which are different from normal earthquakes.

The subduction zone is the largest fault in New Zealand and experiences movement of the Pacific Plate at rates of 2 to 6 cm (0.8 to 2.4 in) per year. SSEs were first discovered here in 2002 after GeoNet established permanent GNSS stations along the coast.

SSEs involve a gradual release of energy over weeks or months, without the ground shaking associated with traditional seismic activity.

SSEs relieve stress in some areas of a subduction zone, but can increase stress in adjacent regions. The interaction can trigger smaller, shallow earthquakes. Several magnitude 2 to 4 earthquakes have already been recorded near the Mahia Peninsula, correlating with the current south-southeast orientation.

SSEs are not unique to New Zealand but are prominent in the tectonic landscape.

They play the main role in adjusting tectonic plate movement and redistributing stress, although they occur too slowly to be noticeable.

References:

¹ Hawke’s Bay’s slow slip awakens – GNS – December 20, 2024

² A slow earthquake is currently occurring off the East Coast – East Coast Lab – Accessed December 20, 2024

Rishika holds a Masters in International Studies from Stella Maris College in Chennai, India, where she received a gold medal, and an MCA from the University of Mysore, Karnataka, India. Previously, she worked as a research assistant at the National Institute of Advanced Studies, Indian Institute of Science in Bengaluru, India. During her tenure, she served as a junior writer Europe Monitor on the Global Politics website and as Assistant Editor for The world this week. Her work has also been published in The Hindu Newspaper that demonstrates its expertise in global affairs. Rishika is also the recipient of the District Level Women Empowerment Award in Haryana, India in 2022.