Slow slip event occurring along the East Coast of the North Island

Published: Fri Sep 29 2017 3:00 PM

There’s been a recent swarm of small earthquakes in the East Coast area and, yes, we think these are related to another slow slip event.

Earthquake swarm norh of Gisborne recorded over the last month

Earthquake swarm norh of Gisborne recorded over the last month

Over the last month, the GeoNet seismograph network has recorded a number of small shallow earthquakes (a swarm of earthquakes is a sequence of nearby earthquakes striking in a short period of time) north of Gisborne.

At the same time - our GPS (Global Positioning System) instruments have recorded really slow land movement of up to 1 centimetre over the last 2 weeks. This is in the same area as the earthquake swarm (Tokomaru Bay and Anaura Bay areas). This slow movement indicates to us that there is a slow slip event occurring on the Hikurangi subduction zone beneath that region - (see the blue circle on the image below).

Slow slip event recorded by the Anaura Bay GNSS instrument. The triangles represent the instrument location (the yellow is the selected ANAU instrument at Anuara Bay)

Slow slip event recorded by the Anaura Bay GNSS instrument. The triangles represent the instrument location (the yellow is the selected ANAU instrument at Anuara Bay)

The movement was also recorded on other GPS instruments. See below

Time series of GPS instruments from Ahititi and Anaura Bay

Time series of GPS instruments from Ahititi and Anaura Bay

Our scientists have modeled the GPS displacements to determine the slip on the subduction zone during the slow slip event (see image below).

This map shows the amount of movement between the plates on the Hikurangi subduction zone over the last 1-2 weeks. The hot colors show movement in centimetres (see scale).  The earthquakes over the last 3 weeks (green dots) form a cluster in the area of the ongoing slow slip event.

This map shows the amount of movement between the plates on the Hikurangi subduction zone over the last 1-2 weeks. The hot colors show movement in centimetres (see scale). The earthquakes over the last 3 weeks (green dots) form a cluster in the area of the ongoing slow slip event.

If you look back in time, it is quite typical for this area of New Zealand to have a slow slip events occuring at least once or twice a year (see here, here and here). And often, these past slow slip events in the Tokomaru Bay area have been accompanied by swarms of small earthquakes, similar to what we have seen over the past few weeks.

Slow slip events are happening off the North Island’s east coast along the Hikurangi Subduction Zone , which is where the Pacific Plate dives or "subducts" below the Australian Plate. At the interface between the two plates, they can become locked together in some places. Over time it slowly unlocks and the plates can slip past each other by centimetres to tens of centimetres over a period of weeks, which is a slow slip event.

Keep an eye out for slow-slip events showing up on our GPS instruments using our slow slip watch page. The different graphs you see plotted on this page are the different instruments we have located across the country that are monitoring slow movement.

If you would like more information about how New Zealand's ground movement is collected continuously there is a great story here about how the measurements are plotted as graphs.