Shooting the SAHKE Seismic Survey across the North Island

The blasts from the seismic survey were detectable by GeoNet as very small local ground tremors along the seismic line last week:


                   PRELIMINARY EARTHQUAKE REPORT

                            GNS SCIENCE
                         GeoNet Data Centre
                      Lower Hutt, New Zealand
                      http://www.geonet.org.nz


       The following earthquake has been recorded by GNS Science:

       Reference number:        3511346/G
       Universal Time:          12 May 2011 at 10:24
       NZ Standard Time:        Thursday, 12 May 2011 at 10:24 pm
       Latitude, Longitude:     41.15°S, 175.38°E
       Location:                10 km south-east of Featherston
       Focal depth:             0 km
       Richter magnitude:       2.3

       Web page: http://www.geonet.org.nz/earthquake/quakes/3511346g.html

Man-made explosion as part of a science experiment in lower North Island                                    
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 Here is a video in which Stuart Henrys explains the seismic survey :

SAHKE - Seismic Array Hikurangi Experiment

About a dozen field teams have been out over the weekend  deploying geophones along the 90 kilometre transect of the SAHKE seismic survey.

The first photo shows some of the Orica contractors  loading and priming one of the transect shot holes. 500 kilogrammes of explosive emulsion is being pumped down a 50m bore hole. The pile of gravel in the foreground is used to back fill the hole on top of the explosives. The idea is that the shock wave is directed downwards into the earth rather than up into the air.  (Photo by Stuart Henrys)

It is very important that every geophone is in perfect working order and set up in exactly the right way, as  there will be no possibility of repeating the survey if anything goes wrong.

The second photo shows Stuart Henrys, project co-ordinator, with some of the equipment being prepared at GNS Science, Lower Hutt. Getting all the equipment set up and deployed is a huge organisational feat. Stuart is holding one of the many hundreds of geophones that will be embedded in the ground along the survey line.

Apart from the New Zealand participants (Victoia University and GNS Science), a large amount of equipment and expertise is being contributed by the Earthquake Research Institute at Tokyo University, Japan, and the University of Southern California


The map shows the actual location of the seismic survey line, with the positions of the shot holes indicated as stars. Depending on the time required for putting all the geophones in place, the detonations will be set off overnight during this week. The explosions are detontated at night to avoid too much interference from vibrations caused by traffic on the roads.

When the geophones are deployed they have to be pushed into the ground so that they are well embedded. This ensures a solid contact. In this photo by Margaret Low (Photo Librarian at GNS Science) Vaughan Stagpoole is burying one of the 900 geophones alongside a road in the Wairarapapa.

Check out our time-lapse of the busy science teams preparing the equipment for the SAHKE survey. Two days compressed into just over a minute to the music of Lykke Li!

Wellington's Stuck Plate Boundary

Ever since 1855, when New Zealand's largest ever recorded earthquake (magnitude 8.1) shook the Wellington Region, a lot of effort has gone into understanding the earthquake risk in and around New Zealand's capital city. There are several large fault lines in the area, including the Wellington Fault. This is the most active fault of the system, and stands out clearly, passing along the Hutt Valley and right through Wellington City itself. For more background information check here or watch this video.

However, the largest fault of all, the interface between the Pacific and Australian Plates, underlies the whole region.


The two dimensional map shows  the line of the boundary between the plates east of the North Island. In three dimensions, it is a sloping boundary (known as a subduction zone), with the Pacific Plate dipping under the Australian Plate. Plate collision is occurring at an oblique angle rather than head on, which is why there is such a large component of strike slip (sideways) motion in the North Island Fault System.


The hidden, subsurface plate boundary has been mapped over the years using evidence from thousands of small or medium sized earthquakes generated on or nearby to  it. Seismometers are used to locate these earthquakes, and the seismic waves give information about the geological structures and rock types that make up the two interacting plates. Under Wellington the boundary dips gently down to the North-West at an angle of about 9 degrees, and is about 25 kilometers deep under the city.

Over New Zealand there is a widespread array of GPS stations continually monitoring their location with great precision. This station is set up in the Tararuas, not far north of Wellington and the Hutt Valley.

Scientists also carry out GPS campaigns to make repeated measurements at a large number of locations when they want more detailed coverage.

Over time these recordings show that the surface of the landscape is being deformed by tectonic movements. These measurements indicate that a large segment of the crust of the Australian Plate in the Lower North Island is stuck to the underlying slab of Pacific Plate, and is being dragged along to the west faster than the Hawkes Bay or East Cape areas. There have been different reasons for this proposed by scientists, but it is believed to be caused mainly by friction on the interface between the two plates.

It is very important for us to develop our understanding of the nature of this plate interface and the earthquakes that it produces, as subduction zone ruptures potentially create the most destructive earthquakes and tsunamis worldwide. The recent earthquake in Japan is one such example.

In this coloured image, the red colour indicates a high "slip rate deficit" or high degree of coupling between the subducting and overriding plates in the Lower North Island. This segment of stuck plate boundary is about 70 km wide and 140 km long.


 If it ruptured it would produce an earthquake of magnitude 8 or above. It is even possible for larger sections (eg the length of the North Island) to rupture occasionally in a single massive earthquake. For more information about the locked plates under the North Island, check out our website here.

In order to improve our knowledge of the plate boundary, a major GNS Science co-ordinated project is being carried out next week. This involves a 90 km seismic survey crossing the lower North Island from one side to the other. Instead of listening out for natural earthquakes, the survey will use explosives, detonated down boreholes, to produce the seismic waves. Hundreds of geophones, spaced 100 metres apart, will pick up reflected sound waves to map the plate interface, faults and other features in the crust. Scientists from GNS Science, Victoria University, Tokyo and California are collaborating in this project.

For some more background to this project, have a look at our media release, or listen to Tim Stern of Victoria University in this radio interview.