Slow Slip Events

More than a dozen slow slip events (also known as "silent" earthquakes) have been recorded in New Zealand between 2002 and 2012. Scientists have only been able to detect them recently due to the advent of global positioning system (GPS) equipment which can detect sub-centimetre changes in land movements. As part of the GeoNet project in New Zealand, continuously operating GPS have been installed throughout the country. The GeoNet cGPS data show that these silent earthquakes occurring deep under New Zealand are changing the shape of parts of the North Island over time periods of weeks to years.

Plate interface cross section

The Pacific plate descends westward beneath the eastern North Island, but for most of the time the descending Pacific plate and the over-riding North Island are stuck together at their interface, which causes large parts of the eastern North Island to be pushed to the west.

The silent earthquakes are a sign that some of this tectonic stress is being relieved by ‘slow slip’ occurring on parts of the plate interface at 15 – 50 km depth beneath the North Island. These slow slip events tend to occur just below the area of the plate interface that is “stuck” and building up stress to be relieved in future earthquakes.

Slow slips and the locked plate interface in the North Isalnd

Offshore the Gisborne and Hawke’s Bay regions, slow slip events are very shallow (<15 km depth), and occur over a period of around 2 weeks approximately every two years. Beneath the Manawatu and Kapiti regions, much deeper slow slip events (60-25 km depth) are observed over periods of 1-1.5 years. The most recent events beneath the Manawatu, Gisborne, and Hawke’s Bay regions occurred in 2010/2011, while the last slow slip event beneath the Kapiti area and the Marlborough sounds was in 2008. If the plate boundary slip that occurred during the Manawatu 2010/2011 event occurred suddenly (rather than over a year), it would have been equivalent to a magnitude 7.0 earthquake.

In the New Zealand slow slip events, large areas of land have been observed to move eastward by up to 30mm over days, weeks, or months. Some scientists believe that these movements can shift stress within the Earth's crust and trigger earthquakes, so they are not necessarily benign events.

This 'time lapse' graphic animation shows the slow slip events that occurred around the North Island between 2009 and 2012. Small red circles represent earthquake epicentres.

Slow slip animation

Slow Slip events under the North Island from late 2009 to early 2012. Courtesy of Noel Bartlow, Stanford University and GNS Science