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article: Mar 29 2010 - More slow-slip events in the Gisborne region

Following a dearth of slow-slip events in 2009, the GeoNet continuous GPS network has recorded new slow-slip events off the east coast of the North Island between January and March of this year.

Normally the interface between the subducting Pacific Plate and the overlying New Zealand crust is locked, which causes the Gisborne region to be pushed slowly westwards (at a few millimetres per year) due to the westward motion of the Pacific Plate. But during slow-slip events the locking is temporarily released, allowing the overlying crust to move back eastwards at rates of a few millimetres per day. GNS Science geodesists have modelled the surface movements at a number of GPS stations in the region and have determined how much movement must have taken place at the interface in order to get the surface observations. The modelling has revealed three events over the past three months. The first two events occurred during a similar time period:

  • The first event occurred north of the 2002 and 2004 Gisborne slow-slip patch and lasted from mid-January to mid-February.
  • The second event occurred under northern Hawke Bay to the south of the 2002-2004 Gisborne patch, and lasted from late January to mid-February.

The third event is currently under way:

  • It lies between the first two events, and started in mid-March.

Maps of the inferred subduction interface slips are shown at right:

  • In the left-hand map, maximum slip on the interface was about 100 mm in the northern event and about 60 mm in the southern event.
  • In the right-hand map showing the March slow-slip event, maximum slip to date is about 70 mm. The corresponding surface movement at Paritu Road (PARI) continuous GPS station is about 35 mm to date. The station, south of Gisborne, is built very close to the zone of maximum slip shown on the right-hand map.
The two areas of slow-slip occurring during mid-January to mid-February.

The two areas of slow-slip occurring during mid-January to mid-February (dashed contours show the depth to the interface of the two plates).		The area of slow-slip occurring during March.

The area of slow-slip occurring during March.




Locations of the four GPS stations shown on the plot.

Locations of the four GPS stations shown on the plot.Plots showing daily positions of four of the continuous GPS sites in the Hawke's Bay to East Cape region.

 

A plot showing the daily positions of four of the continuous GPS sites in the Hawke's Bay to East Cape region. The usual motion of these sites is slowly towards the west. But when a slow-slip event occurs on the plate interface it causes nearby GPS stations to shift as much as 30 millimetres to the east over a period of days or weeks. The first (northernmost) event starts in mid-January as shown by the change in motion of site CNST. This event takes place over a time period of about 3 weeks. While this event is still happening the second (southernmost) event starts at the end of January and lasts for a few days. This event can be seen on station MAHI. Then, in mid March, a bigger event takes place in between the first two. The third event shows especially strongly at sites PARI and GISB, but is also detected at MAHI and CNST.