Scenarios for the M6.0 Wilberforce Earthquake: What is going to happen next?

UPDATE: We've added a map so you can see what geographic area we are including in the probability model. Also, we added some pictures of landslides that our team of landslide experts sent to us.

Probabilities 

Our seismologists have been busy crunching the numbers regarding the Wilberforce Earthquake. Here are the probabilities for the aftershock sequence related to the Wilberforce Earthquake:


 

M5.0-5.9M6.0-6.9M ≥7.0
Average
number
Range*

Probability of
one or more

Average
number
Range*

Probability of
one or more

Average
number

Range*

Probability of
one or more

Within 30 days

0.060 - 1

6%

0.0050 - 1

<1%

0.00030 - 1<1%
Within 1 year0.460 - 237%0.040 - 14%0.0030 - 1<1%

This table shows a forecast based on a model from international expert elicitation.

Updated 08/01/2015: Due to this sequence, the probability of a magnitude 5 or above in the next year in this region is approximately one and a half times greater
than what we would normally expect from our National Seismic Hazard model. As the aftershock rates decrease, this difference will decrease also.

All forecasts start 6th January 2015 and are for the region from 170.5-171.9 degrees East and 42.4-43.75 degrees South. 

* 95% confidence bounds.

Geographic area included in the model

Landslides around the area

Our landslide reconnaissance team has been out and about in helicopters checking out the Southern Alps, near the epicentre of the Wilberforce Earthquake. They came back with some great photographic evidence of rockfalls and landslides.  The photos show small to moderate size rock

avalanches close to the Main Divide in the upper reaches of the Unknown Stream catchment (we think this is an excellent name for a stream!). Although these were not observed at the time of the earthquake - because no-one was in the area to our knowledge - their appearance on the snow-pack indicates they are only a few days old at most. As there are several of these, it is a good indication of a common trigger. As there has been no significant rain in the area this only leaves shaking from the Wilberforce earthquake as the cause.

 About the name Wilberforce 

Our large earthquakes get names based on the closest geographic feature or a relevant city/township. This earthquake was located in a very remote area so we named it after the Wilberforce River.  That might be disappointing to some of the more literary minded people out there who thought we might be making a witty reference to the antagonists in classic kiwi novel "Under the Mountain" but...we weren't. Though we always appreciate a good book! Right, back to the science.

The Alpine Fault and the Wilberforce Earthquake

We've received a lot of questions about this! You can see that we addressed this in the scenarios below but we've got a bit more science regarding the stress levels on the Alpine Fault. Essentially, the Alpine Fault did show some change in stress levels from yesterday's earthquake: in some areas this decreased and in a small area, it increased. GNS Science has issued a media release regarding the change in stress levels.

Scenarios 

Based on our understanding of tectonics in the area, data currently available from the sequence, historical observations, and statistical models, our seismologists considered that there are three possible scenarios that could occur over the following weeks. There are very different probabilities for each scenario. Remember, the best thing is to be prepared for whatever happens next.

Note: Scenarios are based on information that is currently available. We will closely monitor on-going earthquake activity and update our scenarios if it is required.

Scenario One - Very Likely (within the next 30 days)

The most likely scenario is for aftershocks to continue to decrease in frequency, with no future large earthquakes. This is consistent with normal aftershock behaviour. We expect aftershocks over a slightly larger region than where aftershocks have already happened. Again, it is very early in the aftershock sequence and we will know more as this sequence develops.   

Scenario Two - Very Unlikely (within the next 30 days)

A very unlikely scenario is that future earthquakes similar to the M6.0 Wilberforce Earthquake (link) may occur within the general region of the main shock. Large earthquakes are not surprising in this area; it is historically a very seismically active place. In fact, it has been affected by a handful of large earthquakes during the last century. These include at least four earthquakes of M6 or greater:

Lake Coleridge earthquake, 27 June 1946 

This magnitude 6.2 earthquake with an epicentre near Lake Coleridge was felt over the greater part of the South Island. It was preceded by two foreshocks and followed by numerous aftershocks, the largest of which had a magnitude of 5.8. These persisted until the end of 1949.

Aftershocks for the Wilberforce Earthquake

Arthur’s Pass earthquake, 18 June 1994 

This magnitude 6.7 earthquake occurred in the Southern Alps with its epicentre 10 km from the township of Arthur’s Pass. The earthquake was the largest in a sequence of earthquakes in the central South Island, which began with the 1992 M5.8 Wilberforce River (yes, Wilberforce is a popular area for earthquakes!) earthquake and was followed in May 1995 by a M6.1 earthquake less than 10 km to the east, and in November 1995 by the M6.3 Cass earthquake located 30 km to the east of the Arthur’s Pass main shock.

Scenario Three - Extremely Unlikely (within the next 30 days)

An extremely unlikely but possible scenario is that the Wilberforce sequence will trigger a larger magnitude quake - a magnitude 7 or greater –on another fault (e.g.  the Alpine Fault). The Alpine Fault is one of the most active crustal faults on Earth. It is already known to have a high probability of rupture over the next 30 years; however it is unlikely that the occurrence of the Wilberforce earthquake has greatly increased this hazard.

About the location of the 6.0 Wilberforce Earthquake

This part of the country is no stranger to strong earthquakes. In the last 100 years, there have been several similar sized and located earthquakes (see above). The tectonics in the area are dominated by the Alpine Fault, where the Australian and Pacific tectonic plates meet, pushing together to form the mountain ranges of our picturesque Southern Alps. Surrounding the Alpine Fault there are numerous known and unidentified faults which, along with the Alpine Fault, take up stresses from the convergence of the Pacific and Australian plates. Find out more about why New Zealand is so shaky here

  • The Ministry of Civil Defence and Emergency Management advises people to drop, cover and hold during an earthquake. More information about what to do before, during and after an earthquake is at www.getthru.govt.nz
  • The Earthquake Commission advised that as aftershocks continue, it’s important to fix and fasten any large furniture or valuables in homes to keep families safe. One step is to visit the EQC website and look through information in the Fix. Fasten. Don’t Forget section.
  • Your city or district council is responsible for Civil Defence in your area. You can get more information about hazards in your area from your council and on the GNS Science and GeoNet websites.

Last update: 08/01/2014

Time: 1.45 p.m. 

 

 

 

 

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