Slow Earthquake Near Gisborne

A slow earthquake is currently occurring near Gisborne, the third such event in the area this year.

Tuesday 30th March 2010

Earthquakes are normally thought to be sudden events resulting from changes in stress along earthquake faults, tectonic plate interfaces and tension in rocks. The sudden change in stress results in energy being released and shaking is felt nearby or, if it is a large release, over a wide area.

The advent of global positioning satellite technology has enabled scientists to study changes in the surface of the Earth with much more immediacy and in greater detail than the traditional manual surveying techniques which involved teams of surveyors with theodolites systematically measuring distances between known landmarks.

GeoNet operates a continuous GPS (Global Positioning System) network that uses information from artificial satellites to systematically record the position of designated landmarks across the surface of New Zealand. The data gained is useful in studying land deformation – land being compressed or stretched by tectonic forces.

Normally, this shows the Gisborne region being pushed westwards at the very slow rate of a few millimetres per year due to the Pacific Plate moving westwards. As the Pacific Plate slides beneath the Australian Plate offshore to the east of Gisborne, the process occurs in fits and starts. Most of the time, the interface between the two plates is stuck causing the Australian Plate, which carries the North Island, to slowly buckle and move slowly westward.

The compression continues for lengthy periods, and is normally released in abrupt events that we recognise as earthquakes. Depending on the amount of accumulated strain, the quakes can be small or dramatically large such as the Hawke’s Bay earthquake of 1931.

Monitoring of the GPS data during 2009 showed the Gisborne area being slowly compressed with the plate interface locked while the Pacific Plate continued its westward movement. However, this changed abruptly in January this year, with an area off the coast near Tolaga Bay beginning a slow movement eastward. The movement began mid-January and ceased in the middle of February, by which time a maximum eastward drift of 100 mm had been recorded.

The movement occurs so slowly, that people in the area usually do not feel any movement or earthquakes. Small quakes of magnitude 2.9 and 3.5 struck the area 10 km north-west of Tolaga Bay on the 20th and 22nd of January, and a magnitude 3.3 event was recorded on January 25th but otherwise the activity went unnoticed by locals.

A second slow slip event began in Hawke’s Bay, near Mahia Peninsula in late January. A maximum slip on the plate interface of 60 mm was recorded before activity ceased mid-February.

A third slow slip event is currently in progress at a location between the two earlier events. This is centred on the coast near Bartletts, south of Poverty Bay. Activity began mid-March, with a maximum slip of 70 mm being recorded to date. The Paritu Road GPS Station has shown 35 mm of surface movement so far.

The first recorded slow slip event in New Zealand occurred near Gisborne in 2002, when the area moved 20 mm eastward over eight days during October. The detection was made possible following the installation of a second GPS station in the area in July of that year.

In 2004 the same area began moving eastward at about 2 mm per day. The activity began on October 31st and, when it ceased mid-November, the Gisborne GPS station had moved eastward and slightly south by 20 mm.

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

Slow slip is thought to alleviate the stresses acting on faults in a much less conspicuous manner than typical earthquakes, but whether it lessens the likelihood of future damaging earthquakes or acts to trigger them is uncertain. A number of scientists have suggested that slow slip events on deeper subduction interfaces may trigger large earthquakes in the seismic zone immediately above. However, the evidence for this is scarce, and the examination of slow slip events using GPS technology has yet to build up sufficient historical data to allow a detailed study to be made.

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.

GeoNet’s initial report on the current Gisborne event can be found here.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST.]

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