Cook Strait 2013 earthquake

According to GNS Science, GPS observations have shown that parts of Marlborough moved up to 5 cm to the east during the 21 July 2013 magnitude 6.5 earthquake, which shook Wellington and the top of the South Island, causing minor damage.

Unknown offshore fault

GNS Science has reported that the Cook Strait earthquakes have struck on a previously unknown offshore fault that is approximately 40 km long. At first it was thought the fault was possibly an extension of the London Hills Fault – a fault that runs from near Ward and enters Cook Strait between Lake Grassmere and Cape Campbell. However, data from 14 permanent GeoNet-operated GPS stations between the outer Marlborough Sounds and Cape Campbell has shown the part of the fault that ruptured was a 19 km long north-east south-west trending slash in the Earth’s crust.

Its southern end came within a short distance of the coastline at Clifford Bay. The rupture plane was nearly vertical and came to within about 6 km of the seabed. The bottom of the rupture plane was about 18 km below the seabed, indicating that the earthquake started at the bottom of the rupture and moved upwards and outwards, with its tearing action travelling at several kilometres a second. This puts the rupture in between the London Hills Fault and the Hog Swamp Fault. Both are little-known on-land faults with no known offshore extensions. Little is known about these neighbouring faults other than the London Hills Fault is believed to rupture once every 3500–5000 years.

Cook Strait and its coastlines have many known active faults. These include the Awatere Fault, which had a major earthquake in 1848, and the Wellington Fault, which runs through Wellington and Hutt Valley. However, at this stage, there is no indication that the Cook Strait earthquakes have caused any activity on these faults.

Weaker ground accelerations than Christchurch earthquake

A recent update from GeoNet explains that, although the Cook Strait earthquake was larger in magnitude than the M6.3 quake in Christchurch in February 2011, the ground accelerations felt in Christchurch were about 10 times as strong as those felt in Marlborough and Wellington due to how close the city was to the epicentre and the geometry of the fault that failed. In addition, the way the earthquake is felt depends on local conditions such as the type of soil and building.

“The Christchurch February 2011 earthquake recorded peak ground accelerations of more than 2g, while the strongest readings for the Cook Strait earthquakes were approximately 0.2g at Ward (about 16 km south-east of the epicentre) and even less (up to 0.16g) in Wellington and Porirua. [g is shorthand for the Earth’s gravitational acceleration constant.]”

In an interview with the Science Media Centre, Euan Smith, Professor of Geophysics at Victoria University of Wellington, said that earthquakes are likely to continue near Wellington for some time – no-one can say how long. “Some of them could be as large, or larger, than the ones we have just had. Therefore, anyone with property or possessions that are vulnerable to being knocked over or down and damaged should take steps to secure them. In our land of recurrent earthquakes, this is always good advice.”

Cook Strait earthquakes not a surprise

At a press conference in Wellington on 22 July 2013, GNS Science seismologist Dr Ken Gledhill said, “Unlike the Canterbury region, an event like this is not a surprise. It is something that happens every few decades if you actually look at the historical record. These things happen periodically. What we know about this series of earthquakes, and particularly the one last night, was that they were in the overlying Earth’s crust.”

“The first two big events on Friday and on Sunday morning were thrusting events, a pushing up motion. The one last night was a strike slip, the faults sliding past each other. What happens is, when you get something that slides half a metre or more over a large area more than 10 km and with some depth, you are going to strain the area around it, and that is what we are seeing at the moment."

Dr Gledhill says that the maximum shaking in Wellington was felt in Karori and was 14% the force of gravity. Christchurch at its maximum was two times the force of gravity, and in the CBD of Christchurch, a little under one times the force of gravity. The maximum shaking recorded was in Ward, a town near Seddon on the other side of Cook Strait, at 21% the force of gravity.

Aftershocks

“The first earthquake on Friday set off its own aftershock sequence. The Sunday morning one was particularly energetic and went on all day Sunday, up to the main one on Sunday evening. It was a different kind of earthquake because it was a sliding past earthquake. But all of these earthquakes are past 10–20 km depth under the seafloor. We’ve basically got three aftershock sequences building up on each other. It’s not main shock, aftershock, we’ve ended up with, if you like, aftershock sequence stacked on aftershock sequence.”

“Every time you had a 5.5M earthquake, you had an aftershock sequence for it. This is not unusual behaviour for the Cook Strait region.”

Dr Gledhill said there was a 39% chance the region would have a magnitude 6.0 or higher aftershock in the next year and a 99% chance of a magnitude 5.0 to 5.9 aftershock. This figure was downgraded on 25 July 2013 by GNS Science to a 97% chance of a magnitude 5.0 to 5.9 aftershock and a 30% chance of a magnitude 6.0 or higher aftershock in the next year. As time progresses with few large aftershocks, these percentages decrease.

In a press release on 24 July 2013, GNS Science reported that the maximum amount of surface displacement occurred in a large area between Blenheim and the Awatere Valley, with smaller horizontal movements in other parts of Marlborough. Horizontal movement on the seabed in southern Cook Strait, above the epicentre, was up to 8 cm.

It’s like Grand Central Station down there. It’s a challenge to understand the interactions among these faults.

Seismologist Dr Stephen Bannister