Studying Slow Slip Earthquake Events

The EQC Fellow in Seismic Studies, Dr John Townend, is leading a project to determine whether a low-pitched rumble accompanies slow slip earthquakes in New Zealand.

Slow slip earthquakes are a newly recognised phenomenon during which a fault slips by as much as several tens of centimetres during the course of a day, weeks or months. Also known as “silent earthquakes,” they have only been accurately identified since the advent of Global Positioning System (GPS) instruments.

The GPS instruments allow automated measurements of land movement, and have been deployed to monitor earthquake faults, landslides and other geographical features. During October 2002, newly installed GPS instruments detected land near Gisborne moving eastward by about 20 mm during an eight day period without the shaking and damage caused by a normal earthquake. Prior to the slow-slip event, the land had been moving in the opposite direction, westward, at about 5 mm per year as it came under pressure from the Pacific tectonic Plate offshore.

The Gisborne event of 2002 was followed by a similar-sized slow event 2 years later, indicating that these slow slip quakes may occur quite regularly.

Between January 2004 and June 2005, a slow slip event beneath the Manawatu region was recorded by GPS sites near Ashurst, Wanganui and Dannevirke.

Now that scientists have measured the events in New Zealand, their research has progressed to determining the impact of slow slip earthquakes and whether they can be detected by other means. Slow slip is thought to alleviate the stresses acting on faults in a much less conspicuous manner, but whether it lessens the likelihood of future damaging earthquakes or acts to trigger them is uncertain.

Current research indicates that the slow slip near Gisborne in 2004 triggered very small earthquakes beneath the Mahia Peninsula. It is also thought that the Manawatu event may have triggered some of the many small to medium-sized earthquakes which struck in the lower North Island early in 2005.

The EQC project involves Dr Martin Reyners of GNS Science, Dr Garry Rogers of the Canadian Geological Survey and MSc student Emily Delahaye who is conducting work to refine the locations of the earthquakes.

Slow slip events have been observed at several locations overseas, often associated with a low pitched rumble known as seismic tremor. The EQC project will investigate whether a similar rumble occurs here. Results obtained to date indicate that the Gisborne event of 2004 triggered small earthquakes rather than causing seismic tremor.

[sources ”“ Institute of Geological & Nuclear Sciences media release, 19 December 2002. GNS Science, Geonet News issue 5, December 2005. Earthquake Commission, Ru Whenua issue 15, March 2007.]

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