Big Alpine Fault rupture due in the next century or two

The Alpine Fault is a big feature of South Island geography and seismic activity. It last ruptured in 1717 and there is a big earthquake, on average, about every 300 years, but the times vary so there is thought to be about a 30% risk of the next ‘Big One’ in the next fifty years.

When I was woken up by an earthquake in the middle of the night on 4 September 2010 I wondered if it was ‘the big one’ – it was big enough but that was centred close to Christchurch.

This image clearly shows the fault running up the West Coast, where the Australian plate dives under and drives up the Southern Alps which line along the rising edge of the Pacific Plate.

Project AF8 have been preparing for a severe earthquake on the Alpine Fault for two years.

Stuff has the latest information on this: Videos show devastating impact across South Island if Alpine Fault ruptures

Video simulations demonstrate widespread destruction across the South Island if New Zealand’s most dangerous fault line ruptures, and there’s evidence the country is due for another big one.

Project Alpine Fault Magnitude 8 (AF8) released a series of videos on Wednesday, warning there’s compelling geological evidence to show it produces a significant earthquake of magnitude eight or greater every 300 years on average.

The last rupture was believed to have happened in 1717.

That sounds dramatically imminent, but the intervals have actually varied between 140 and 510 years, so it may or may not happen in our lifetimes.

The Stuff has the latest videos.

Extensive details are at Project Alpine Fault 8

The Alpine Fault is the biggest but just one of many fault lines affecting New Zealand. This is a few years old but is a good look at plate tectonics in New Zealand:

Dunedin is about as far as you can get in the South Island from the fault but when the big one strikes it is certain to be felt here (we feel larger Fiordland earthquakes as well as the bigger Christchurch quakes) and it is likely to have  major effects across the South Island, and also in the lower North Island.

North Island slow-slip follows South Island quake

There has been a lot evidence of land movement during and after what is now referred to as the M7.8 Kaikoura earthquake – it was initially said to be centred close to Culverden and Geonet still shows it as ’15 km north-east of Culverden’ (Kaikoura is about four times further away).

It is thought that the initial quake caused a chain reaction along other fault lines in the South Island, and Wellington was also affected.

Geonet has now revealed that since this movement happened there has been a ‘slow slip’ occurring further north, along the Hawkes Bay and Gisborne coast. Slow slips have been detected there before but only after North Island earthquakes.

Today Geonet posted on Coastal Uplift: How has the Kaikoura Coastline Changed which shows the extent of land movement over a large area of the South island.

Much of the northeastern coast of the South Island was uplifted during the 14th of November 2016 earthquake. We know this from photos of rock platforms covered in seaweed and marine animals such as crayfish and paua stranded above tide levels.

Our records measured the tide gauge at Kaikoura was lifted up by 1 m, and continuous GPS monitoring sites at Kaikoura and Cape Campbell were also raised by 0.7-0.9 m. At this stage we can estimate that the coast was raised between 0.5 m and 2 m from about 20 km south of Kaikoura all the way north to Cape Campbell.

The startling uplift of ~5.5 m at Waipapa Bay is a localised block pushed up between two traces of the Papatea Fault and is thankfully not representative of the whole coastline.

There was greater horizontal movement, reported to up to 10 metres in places. All those ground movement happened along a long stretch of coastline up the north east of the South Island.

kaikouraearthquake_uplift_21nov2016

Uplift and horizontal movement happened on a long stretch of coastline up the north east of the South Island.

Also today Geonet posted Gisborne and Hawke’s Bay slow-slip event follows M7.8 Kaikoura Quake

GPS stations have detected a slow-slip event under the Hawke’s Bay and Gisborne regions in the days following the Kaikoura M7.8 Earthquake.

These silent earthquakes or slow-slip events are undetectable by both humans and GeoNet’s seismographs. They can move faults the equivalent of magnitude 6+ earthquakes over a period of weeks to months, without any detectable shaking.

The ongoing slow-slip event off the North Island’s east coast has moved some GPS stations up to 2-3 centimetres.

So far. It’s only a week since the M7.8

This movement is similar to what has been observed in previous East Coast slow-slip events over the last 15 years, so is not necessarily abnormal. We see events in this area usually every 1-2 years.

We have also observed other slow-slip events happening in response to large earthquakes.

The last slow-slip event offshore of Gisborne followed the Te Araroa earthquake in September 2016 (related GeoNet story http://info.geonet.org.nz/x/ZIAvAQ).

A slow-slip event also occurred following the 2007 M6.7 Gisborne earthquake.

But this time the slow-slip began after a more distant quake.

It is possible that passing seismic waves from the M7.8 earthquake caused stress changes that triggered the slow slip event. GNS Science and GeoNet and scientists are keeping a close eye on the event as it evolves.

So the Culverden quake may have triggered the Kaikoura and Seddon quakes (and three other fault line breaks), nudged across Cook Strait to Wellington and may rearranged stresses enough up the east coast of the North Island to start the slow-slip.

slowslip_hb_and_gisb

This instability covers a large area in which many of New Zealand’s significant earthquakes have occurred.

nz_faults

The initial Culverden quake was on the Hope fault line which reaches back to the top of the Alpine Fault where it breaks apart into Marlborough’s mess of mountains.

Also today Geonet updated it’s statistics based scenarios and forecasts which includes the probability of aftershocks:

  • 99% M6.0-6.9  in the next year (89% within 30 days)
  • 38% >=M7 in the next year (20% within 30 days)

There is no way of knowing, if another large quake occurs, where it would be. There is a lot of uncharted territory here.

What to do about it?

If you feel an earthquake:

Don’t run outside, many injuries are caused by things falling from buildings. Beware of breaking glass.

And if you are close to sea level near the coast don’t wait for a knock on your door or a warning siren, move inland or to higher ground.

And hope that the slow-slip eases the pressure gradually so nothing major gives suddenly.

Earthquake update – Thursday

Geonet 5 am Thursday update:

  • 19 earthquakes in last hour
  • 290 earthquakes in last 12 hrs (11 over M4)
  • 2070 earthquakes since the M7.8 (570 since 4 am Wednesday)

The rate and size of quakes is slowly reducing. The larger quakes overnight:

  • 4.3 – 7:45:51 pm 20 km south of Seddon
  • 4.3 – 7:46:33 pm 20 km north-east of Kaikoura
  • 3.8 – 9:27:10 pm 5 km north of Culverden
  • 4.8 – 10:15:14 pm 20 km east of Martinborough
  • 4.3 – 1:19:45 am 30 km north-east of Arthur’s Pass
  • 3.8 – 3:20:09 am 25 km north-east of Hanmer Springs
  • 4.0 – 3:41:33 am 30 km south of Seddon
  • 4.9 – 4:03:49 am 20 km south-west of Kaikoura
  • 4.0 – 5:02:32 am 10 km north of Culverden

Notable points: while reducing in size they are still spread over the three main areas, Culverden/Hamner, Kaikoura and Seddon, but with two other locations, east of Martinborough on the Wairarapa faultline (the one that went M8+ in 1855) and one north-east of Arthur’s Pass (Alpine Fault territory).

We can hope that these are reducing stresses rather than increasing them on other faultlines.

Yesterday Geonet upgraded the size of the initial quake (which may have been two consecutive quakes in different locations) from M7.5 to M7.8.

Kaikoura earthquake update: Magnitude revised

What has changed since our initial review

Based on our findings and in discussion with international researchers, early indications are that this is one of the most complex earthquakes ever recorded on land. This complexity means we have had to take extraordinary efforts to determine the magnitude, depth, and locations.

The very long time it took for the faults to rupture (over one minute) meant that the standard methods of calculating magnitude were insufficient to capture the full energy released.  

Due to the size of the quakes, we’ve gathered data from our entire network of seismic stations. All of these stations would not normally need to be included in magnitude estimates.

Further, our techs at GeoNet went out to several sites which we lost communication with and we have now been able to upload this information, so we have a more complete understanding of the ground deformation and strong-motion data.

Finally, our science teams have been working tirelessly, going up and down the affected areas and measuring the length of faults and how much they moved.  Their efforts have provided us with a clearer picture as to the size and length of the ruptures.

Based on all these ongoing efforts, we can say with some confidence that the earthquake was an M7.8.  This is consistent with estimates from several  other international agencies, specifically the USGS. Their early model provided us important information and we used all our additional data sets to confirm the magnitude. 

What this means

The new magnitude just tells us what we think most people who felt the earthquake already know: it was powerful, and went on for a long time over a large distance. It doesn’t change what happened but it does provide us with more knowledge about how significant the event was.

Our recent analysis confirms the complexity of this event. It does not change any of the observations of strong ground motion, fault breaks or GPS recorded movement of the earth’s surface – these are physical observations independent of the magnitude of the earthquake.

We are in the process of revising our probabilities and scenarios based on this new information and should have this released within the next 24 hours.

RNZ continues to have good coverage, including:

Rescue efforts in Kaikoura continue as helicopters and NZ and US defence force ships arrive in the quake-hit town with supplies.

Wellington is also dealing with the aftermath of Monday’s 7.8 magnitude quake, with several central city buildings deemed unsafe.

The aftermath of the 7.8 magnitude quake so far

Govt to consider funding for 24/7 quake monitoring

‘Absolutely gutted’: Tiny community of Mt Lyford devastated by quake