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Seismic stress may be changing under Wellington - 26/05/2004

Unusual surface movement measured in the hills behind Paekakariki over the past year could be linked to the recent swarm of earthquakes near Upper Hutt, scientists say.

The swarm of more than three dozen quakes over the past year was tightly bunched at depths between 24km and 29km below the surface and 4km north of Upper Hutt.

At the same time as the swarm was occurring, a highly accurate GPS instrument near Paekakariki measured a small but significant change in the steady westward movement of the Kapiti Coast.

The instrument, operated by Geological and Nuclear Sciences Ltd (GNS) through the Earthquake Commission's GeoNet Project, shows that steady westward movement of 25mm a year measured over three years from 2000, suddenly slowed to 15mm a year since May 2003.

Scientists emphasise that there is some uncertainty with this figure as spatial coverage of GPS instruments in the Wellington region is below the minimum needed for sure-footed geophysical research.

"The swarm of earthquakes near Upper Hutt and the change in motion near Paekakariki both began at the same time," GNS geophysicist John Beavan said.

"Earthquake swarms in the Wellington region are unusual and our modelling suggests that the swarm and the relatively sudden change of movement near Paekakariki are linked."

Slow earthquakes are a relatively new phenomenon that have been observed in only a handful of places worldwide, and only in the past eight years thanks to the advent of continuous recording GPS instruments.

A slow earthquake happens over weeks or months and does not cause ground-shaking
normally felt in a fast earthquake, which happens in seconds.

GNS modelling suggests the plate interface 35km under the Kapiti Coast, previously considered stuck fast, may have slipped 50cm to the east over the past year.

" A half-metre of slip at or near the plate interface would produce the observed 10mm a year change in motion at the surface near Paekakariki," Dr Beavan said.

By modelling the change, GNS seismologists have been able to show that 50cm of slip on the plate interface would have increased the tectonic stress under Upper Hutt by the right amount to trigger a swarm of small to moderate sized quakes.

GNS seismologists believe that a "slow slip" event under the Kapiti Coast would increase the stress in the Earth's crust in parts of the Wellington region. This may advance the timing of earthquakes on some faults in the Wellington region, while reducing the likelihood on others.

The good news is that the Earthquake Commission and Land Information New Zealand have been funding the installation of GPS monitoring instruments throughout New Zealand. More EQC-funded instruments are planned for the southern North Island over the next few years as part of the GeoNet Project.

Better understanding of slow slip events, and their implications for earthquakes, depends on gathering high quality data and on undertaking geological and geophysical research on it.

This will unlock secrets about the workings of the great forces at work beneath our feet, and provide insight into earthquake hazard in the Wellington region and along much of the North Island's east coast.