A massive, magnitude-7.5 quake centred near Hanmer has led to at least two deaths, caused widespread damage and triggered tsunami alerts. Sara McBride of GNS explains what we know about the science behind the disaster, and what to expect next.

A more recent update is here.

What do we know so far?

This earthquake was the largest recorded in New Zealand since the M7.8 Dusky Sound earthquake in 2009. But, given its location, it was more widely felt and more damaging. This earthquake unsettled many, many people and that is perfectly normal; earthquakes can be upsetting events. The best advice we have is to be prepared for earthquakes.

We can say one thing with certainty: there will be more earthquakes to come in this area.

Which faults ruptured?

Rapid field reconnaissance indicates that multiple faults have ruptured:

• Kekerengu Fault at the coast – appears to have had up to 10m of slip
• Newly identified fault at Waipapa Bay
• Hope Fault – seaward segment – minor movement
• Hundalee Fault

In the simplest case an earthquake is a rupture on a single fault plane.

What we are finding in New Zealand is that quite a few of our larger earthquakes involve jumping from rupture on one plane to another in a complex sequence. We first saw that with the Darfield Sept 2010 EQ where multiple segments together ruptured together as a single earthquake. We appear to have seen this again overnight.

It looks like there were two main quakes: one was a strike-slip and the other was a thrust fault.

What are the scenarios and probabilities?

Most earthquake aftershock sequences decay over time, with spikes of activity and occasional larger earthquakes. We have updated our probabilities of larger or similar sized earthquakes; we use probabilities as we cannot predict earthquakes. These probabilities describe the likely progression of the sequence within the next week, month and year.

Scenarios

There are very different probabilities for each scenario; some of these are more concerning than others. We recognise that while these scenarios may increase anxiety the best thing is to be prepared. Remember: If you feel a long or strong earthquake and you are on the coast, evacuate immediately.

We’ve developed three scenarios based on what we know so far but be aware that our understanding is evolving as we do more analysis and receive more data.

Scenario One: Very likely (80% and greater)

A normal aftershock sequence that is spread over the next few months. Felt aftershocks (e.g. M>5) would occur from the M7.5 epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few weeks and months. This is the most likely scenario.

Scenario Two: Likely (60% and greater)

In the next month, it would be likely that rupture of earthquakes of about an M6 in the North Canterbury and Marlborough regions will occur, as well as potentially offshore in Southern Cook Strait and offshore Kaikoura.

Scenario Three: Unlikely (less than 40%)

The least likely scenario is that in the next month, (it is unlikely but still possible) there would be rupture of longer known faults (with earthquakes of about M7), in the Marlborough and Cook Strait regions.

Within this sequence, aftershocks will most likely occur anywhere in the affected areas (see coordinates for a box covering this area, below the probability table). It is this geographical region for which the modelling is done. It is important to understand that earthquakes can and do happen outside this box but the box represents the most likely area related to this sequence.

Peak Ground Acceleration from Ward, the closest equipment near the epicentre

Ward recorded 1.2g Peak Ground Acceleration. Ward is the closest equipment location (25 kilometres from the epicentre), that we have. For reference, the 22 February 2011 Christchurch earthquake was a PGA 2.2g.

The Tsunami

Our earliest reports are that there is only a little bit of tsunami damage on the coast. Happily, it is not as much as we were expecting.  However, further impact assessments need to be performed before we can conclusively say this around for the whole East Coast.

Landslides

Early reports indicate that these earthquakes have triggered very large landslides to the north and south of Kaikoura. One was at least 1 million cubic metres. GNS Science has three landslide specialists in helicopters today gathering information about the landslides throughout the affected areas.

What about the supermoon?

Some people have raised concerns about a link with the supermoon. In large groups earthquakes exhibit slight associations with lunar cycles, but this is not reliable for forecasting. We have two tides a day throughout New Zealand and at any one place there is no clear association in location. The occurrence of the full moon around the globe does not allow us to say how big, when and where any earthquake might be.

Why were NZ’s magnitudes different from the USGS?

We have currently established that the New Zealand local magnitude for this earthquake is 7.5, but this may be reviewed over time when more detailed research is undertaken. This may bring it closer to the internationally-derived value (eg Mw7.8 by the US Geological Survey).

Be prepared – both physically and mentally

Beyond physical preparedness is the emotional and psychological support for these earthquakes. The All Right? Hotline (0800-777-846) is a great resource where you can talk about any anxieties or concerns that you have regarding the earthquakes.

Check out the Ministry of Civil Defence and Emergency Management’s website for more information.

The above is drawn from Geonet updates here and here.