Journal Highlights

Imaging the Underlying Mechanics of New Zealand Earthquakes

From Research Spotlights—

Researchers create a first-of-its-kind image to map electrical properties of rocks and minerals throughout the Hikurangi subduction zone. 

New Zealand’s North and South Islands are well known for providing picturesque scenery. Beneath its bucolic veneer, however, the country is continuously being ripped to shreds by geophysical activity. Each year, New Zealand experiences around 14,000 tremors, as it falls directly on the interface between two massive slabs of Earth’s crust: the Pacific and Australian tectonic plates. 

Along the east coast of New Zealand’s North Island, a large underwater plateau on the Pacific plate is being forced (or subducted) beneath the island, which sits atop the Australian plate. This activity is gradually grinding away at the thick mountain of crust, leading to frequent earthquakes and other seismic activity. However, the physical properties of subduction interfaces like this one, and how they help control coupling (a lack of slippage caused by friction) between two plates, are not well understood. 

Because this interface lies at such shallow depths (about 10-20 miles below the coastline), scientists are able to use land-based geophysical techniques to study it. In a recent study, Heise et al. synthesize the results of magnetotelluric and geodetic studies and use earthquake data to investigate this seismically active area, known as the Hikurangi subduction margin. 

The movement of Earth’s crust is tracked by geodetic data—a set of coordinates that refers back to a known point, such as sea level. The geodetic data, collected on the Hikurangi margin, show that the coupling between the two plates is weak in the northern part of the margin, and firmly locked together in the south. In addition, in the northern part of the Hikurangi margin, large areas of slow slippage (roughly the size of a magnitude 6 or 7 earthquake) occur offshore about once every 2 years. 

To image the plate boundary at depth, the team applied a method called magnetotellurics, which uses natural variations in Earth’s magnetic and electric fields over time to measure the electrical conductivity (and its opposite, electrical resistivity) of rocks and minerals below Earth’s surface, shedding light on underlying geological structure and processes. Using magnetotellurics data from 132 measurement sites in the northern Hikurangi margin, the researchers created an image showing electrical resistivity at different depths across the region. 

To test the idea that coupling is directly related to a subduction zone’s electrical resistivity, the researchers compared the magnetotellurics data to the geodetic data, as well as to earthquake data from the region. They found that an especially electrically resistive section of the Hikurangi margin has more coupling than in the rest of the margin. 

The finding suggests that coupling behavior is controlled by the amount of fluid and sediment along the plate boundary. Clusters of small earthquakes occurring on the plate interface in this section with greater electrical resistivity suggest also a higher density of small asperities—points along the edge of a plate that are stuck in place—which increases the likelihood of subduction earthquakes forming in the area. 

This study, particularly the first-of-its-kind magnetotellurics image, is a crucial step toward understanding how fluids, sediment, and other material properties along subduction zones affect the underlying mechanics of earthquake behavior. This is particularly important in a region that is highly prone to seismic activity, from undetectable tremors to massive megathrust earthquakes. 

Mapping fluids to subduction megathrust locking and slip behavior

Commentary in Geophysical Research Letters

In subduction zones, high fluid content and pore pressure are thought to promote aseismic creep, whereas well-drained conditions are thought to promote locking and failure in earthquakes. However, observations directly linking fluid content and seismic coupling remain elusive. Heise et al. (2017) use a magnetotelluric survey to image the electrical resistivity structure of the northern Hikurangi subduction thrust to ~30 km depth, as an indicator of interconnected fluid content. The authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling. More…

By Demian M. Saffer, Department of Geosciences and Center for Geomechanics, Geofluids, and Geohazards, Pennsylvania State University, University Park, Pennsylvania, USA

-- Sarah Witman, Freelance Writer,


Recent Highlights Across AGU Publications Earth & Space Science News

View more Earth and space science news from Eos

Download the App

New Android App Available!

Google Play Store Logo

Download the Geophysical Research Letters app from the Google Play Store

iOS App for iPad or iPhone


Download the Geophysical Research Letters app from the Apple store

AGU Career Center

AGU Unlocked

Featured Special Collection

Early Results: Juno at Jupiter 

Early results from Juno's mission at Jupiter including approach to Jupiter and the first perijove pass (PJ1). Juno's scientific objectives include the study of Jupiter's interior, atmosphere and polar magnetosphere with the goal of understanding Jupiter's origin, formation and evolution. This collection of papers provides early results from Juno's measurements of the gravity and magnetic fields, deep atmospheric microwave sounding, infrared, visible and ultraviolet images/spectra and an array of fields and particles instruments as well as context for the early results with respect to current theory and models of Jupiter's formation and evolution. Topics include both Juno - Jupiter related theoretical models and data analysis as well as collaborative observations made from Earth based assets.