Moderate Aftershock Rocks Seddon

Moderate Aftershock Rocks Marlborough Region

Mb 3.7, 5km north-east of Seddon, South Island, New Zealand

The Mb 3.7 Seddon Aftershock, visible just right of the 60 minute timestamp.

A magnitude 3.7 earthquake has struck a short distance north-east of the township of Seddon in the Marlborough Province of the South Island, New Zealand. The earthquake had a focal depth of about 9 kilometres and struck at 11:05am NZST. Felt reports have been received from the local settlements, including Blind River, Seddon, Blenheim, Spring Creek & Renwick. The maximum intensity reported was MM6 in Blenheim; however epicentral shaking intensities of MM5 in both Seddon & Blind River are more in keeping with the expected shaking intensity and are therefore regarded by myself as the maximum felt intensity.

It has been over 15 months now since the Cook Strait sequence began with a moment magnitude 5.5 earthquake in the morning of the 19th July 2013. Since then there have been numerous major earthquakes in the Cook Strait & Seddon regions, including a Mw 6.6 earthquake offshore on the 21st July 2013 & a Mw 6.6 earthquake underneath Lake Grassmere on the 16th August 2013. There were over 4500 recorded quakes in this sequence until the end of September 2013, over 850 of them over a magnitude of 3.0 and thus likely felt. Today's earthquake is one of the smaller aftershocks, but owing to its timing within the aftershock sequence is of considerable merit and interest.

Since the beginning of 2014 there have been 9 earthquakes in the general Cook Strait aftershock zone over a magnitude of 3.5. Of these, four were over a magnitude of 4, the latest a Mb 4.3 quake a considerable distance offshore of Cape Campbell on the 19th June. This earthquake was located within the subducting oceanic Pacific Plate at a depth of 38 kilometres and was widely felt Paraparaumu down to Kaikoura.

The June M4.3 quake was the last earthquake in the general aftershock zone, but within the main quake zone the last over M3.5 was a magnitude 4.1 earthquake about 30 kilometres north-east of Seddon, not too far from the location of the first Mw 5.5 quake in mid-July 2013. This means that today's M3.7 earthquake is the first major Seddon aftershock in 8 months.

Seddon's aftershock sequence has been very consistent with that expected of a highly energetic earthquake swarm. There is a lot of evidence indicating that the Cook Strait earthquakes increased stresses on many Marlborough faults, including the Clarence Fault, Awatere Fault and Wairau Fault. The former may indeed be the culprit which caused the Seddon Sequence.

Whilst this earthquake appears to be just a normal aftershock occurring a relatively long time after it's mainshock, there is the potential for further large earthquakes in the north-east part of the South Island.

This article was written on the 29th October 2014 by J H Gurney.