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09/03/2017

Taupo, like Rotorua, Reporoa, Ngakuru and Turangi, lies within the area called the Taupo Volcanic Zone. This is an area where many large caldera volcanoes and geothermal systems are present and is also an area where small earthquakes are frequent. The earthquakes often occur as swarms. Earthquake swarms are defined as a sequence of many earthquakes striking in a relatively short period of time in a localised area. They are differentiated from ‘normal’ earthquakes followed by aftershocks by the fact that no single earthquake in the sequence is obviously the main shock. The larger or largest event can come early in the sequence towards the middle or at the end. The rate that earthquakes occur can also vary through the sequence.

The residents of Taupo often feel small earthquakes and Tuesday afternoon was no different. Two small events, 2 minutes apart were widely felt locally. The earthquakes were small (under M2.3), shallow and located to the north of Centennial Park. Around 200 people reported each event. Small earthquakes are very common in this area and tend to cluster about the Wairakei-Tauhara and Rotokawa geothermal areas. Some also occur out in the lake area. In the last year GeoNet has recorded and located 152 events. Only four of them are larger than M3; the largest event is M4 and occurred at 10.12 pm on 2 January 2017 near Rotokawa. 

The residents of the Turangi-Waihi-Pukawa-Omori area have also been experiencing earthquakes over the last few weeks. A swarm has been on going to the west of there since Monday 13 February. To date GeoNet has recorded and located 587 earthquakes in this swarm, the largest event is a M3.8 on 21 February at 9.35 pm. There have only been four events larger than M3 in the swarm. The number of events per day is slowly declining and only 52 were located in the last week. The events are occurring between about 4 and 10 km depth and most are smaller than M2 (550 of the 587 to date). The Taupo Volcanic Zone is a rifting area, growing wider each year by 6-9 mm. These earthquakes are located on the western boundary and are likely to be related to the long-term ‘tectonic’ stretching of the Zone. Currently, there are no indications that the earthquakes are related to volcanic activity, being located well away from the active volcanoes. As usual, we continue to closely monitor the activity.

06/03/2017

The results of NIWA’s sea floor mapping off the Kaikoura and Marlborough coast are now in, adding even more fault ruptures to the fault map of November’s magnitude 7.8 Kaikoura earthquake. Scientists on board NIWA’s research vessel Ikatere took to the water in January with state-of-the-art equipment to map the sea floor between Cape Campbell and Spyglass Point to the south of Kaikoura.   Ikatere is a smaller boat than Tangaroa so it can get closer in to the shore to map how the onshore and offshore faults link up.  You can see the team at work in this NIWA video.

What did the Ikatere voyage uncover?

The scientists discovered a previously unknown fault in the sea bed about 10km north east of Kaikoura Peninsula.  This new fault has been named the Point Kean Fault after Point Kean at the tip of the peninsula.  While it is not clear how much this fault moved during the earthquake, it is likely to be the fault responsible for the uplift around Kaikoura Peninsula.

To the north, the Papatea Fault rupture leaves land and breaks into a complex network of seafloor scarps up to 6 metres high continuing for about 5km offshore.  The image to the right shows the onshore lidar (high resolution topography) data stitched together with the offshore lidar and new sea bed mapping to reveal two traces of the Papatea Fault cutting the beach and the sea bed. 

While movement on the Needles Fault had been detected by scientists on board Tangaroa shortly after the earthquake, Ikatere was able to map it in more detail.  Rupture along the fault was traced along the sea bed for a total of 34 km, from Cape Campbell in the north to where it connects up with the onshore Kekerengu Fault in the south.

The southern-most offshore fault, the Hundalee Fault, was mapped from where it enters the sea at Oaro to near the head of Kaikoura Canyon.  Comparing the bathymetry data here to pre-earthquake data from 2013 shows a clear 2 metre high scarp formed in the sea bed, but it doesn’t appear to go all the way into the canyon.

As well as mapping fault scarps, the NIWA scientists also mapped the sea bed in the Kaikoura Canyon, just to the south of Kaikoura Peninsula, to determine any changes to its shape.  Comparing this new data to the 2013 data reveals that huge amounts of mud have been shaken from the top of the canyon – more than 1000 landslide scars have been mapped along 30km of canyon rim.  This mud tumbled down into the canyon floor and flowed over 350km along the deep sea Hikurangi Channel, wiping out everything in its path.  You can read more about the impact of these mudslides on the Canyon ecosystem on the NIWA website.

Meanwhile, back on land...

Geologists from GNS Science, University of Canterbury and Victoria University have continued their field mapping of fault ruptures from North Canterbury through to Marlborough.  A new record displacement for the North Canterbury faults of 4 metres was measured on the North Leader Fault.

And back in the office, two new surface ruptures have also been discovered after analysing the lidar data along the coast to the north of Kaikoura – one north of the Kekerengu Fault at Tinline Downs and one crossing Papatea Point.  There’s lots more lidar to come, so geologists could yet discover more small surface ruptures like these that can be hard to see in the field.

Science contacts: Dr Joshu Mountjoy (NIWA) and Dr Nicola Litchfield (GNS Science)

Deployment of Ikatere was made possible by MBIE funding to the Natural Hazards Research Platform for immediate response to the Kaikoura earthquake.

02/03/2017

At 1:42pm on 2 March 1987 a large earthquake occurred in the northern Bay of Plenty, near the town of Edgecumbe. The M6.5 event was soon named after the town. It was, with the exception of the Inangahua earthquake in 1968, the most severe and damaging earthquake to hit New Zealand in 45 years.

We sat down with our Brad Scott, who was there at the time. He was one of the first science responders on the scene.

Q. How widely was the earthquake felt and what did you feel?

Brad: The quake was felt over most of the North Island, including Rotorua, Hamilton, Taupo, Napier and Gisborne. An interesting feature of the Edgecumbe event was the foreshock sequence.

At the time of the earthquake Steve Sherburn (one of our other volcanologists) and I were installing a portable seismograph in a quarry in the Papamoa Hills to record the earthquakes that were happening near Te Puke. We were bouncing down the 4x4 trail at the time and we didn’t realise it had happened. The VHF radio from the DSIR office soon woke up to inform us, then asked us to head towards Edgecumbe.

Q. What do you think were some interesting facts about Edgecumbe?

A. It was preceded by a number of foreshocks in the Matata and Thornton areas. Of the 126 located foreshocks, 93 occurred of these occurred off the Te Puke coast, and the DSIR was focusing its attention in that area. One of the largest foreshocks was of M4.9, and occurred only 7 minutes before the main shock.


Q. How much damage was caused by the earthquake and how were people affected?

A. Luckily, a number of buildings were evacuated in response to this foreshock and were consequently empty when they were damaged in the main earthquake. There were in fact no fatalities, but 25 injuries requiring medical attention were recorded. One vision I’ll always remember was the large number of holes in roofs where chimneys had fallen through.

Q. What was it like when you went out in the field? What kind of earth movement did you see?

A. In the Rangitaiki Plains the quake ruptured the ground to create a spectacular 15 km long surface fault rupture. The fault displaced the ground vertically and up to 3m of height change was measured in places. The Rangitaiki Plains area is part of the Taupo Volcanic Zone (TVZ); a rift ranging from Ruapehu in the south to White Island of the Bay of Plenty.  The TVZ rift opened 1.2 m during the Edgecumbe earthquake. That is, the distance between Matata and Whakatane increased by 1.2 m. Today the GeoNet GPS’s record about 9 mm of extension each year across the TVZ in this area.

Steve and I spent about 2 hours trying to navigate ourselves into the area, however every road we tried was block by landslides. Eventually we by-passed some down on the coast by driving along the railway line. At Te Teko we could see the fault rupture across the other side of the river but the bridge was moving 6-10 inches from its abutment and we didn’t want to take it on. The bridge is famous as it was one of the first to use the DSIR lead-rubber bearings and was shown to be undamaged.

 

 

Q. Were there aftershocks after the initial earthquake?

A. Yes. Like all large earthquakes the Edgecumbe event was followed by an aftershock sequence. In the 3 years after the main shock over 470 events were located.

 

Want to know more about Edgecumbe? Find out more here:

Links:

https://goo.gl/PoYVXd

https://goo.gl/uJ1utV

 


 


22/02/2017

A very common characteristic of the greater Rotorua-Taupo area, known as the Taupo Volcanic Zone is earthquake swarms. Several minor ones occur every year. Since around 6 pm (Monday 13 February) the GeoNet seismometer network between Taupo and the Tongariro National Park has been recording a swarm of small earthquakes. They locate about 10 kilometres north west of Tokaanu. Since the swarm started we have located over 290 events. The largest been a M3.8 at 9.35 pm Tuesday Feb 21.

Earthquake swarms are defined as a sequence of many earthquakes striking in a relatively short period of time in a localised area. They are differentiated from ‘normal’ earthquakes followed by aftershocks by the fact that no single earthquake in the sequence is obviously the main shock. The larger or largest event can come early in the sequence towards the middle or at the end. The rate that earthquakes occur can also vary through the sequence. 

The current swarm west of Tokaanu (Turangi) has comprised three phases. The third phase has included the four largest events (M3.0, 3.4, 3.6 and 3.8) and started Tuesday evening around 5 pm, continuing overnight. We have located about 200 events so far in this phase of the swarm. In total we have located 291 events so far (22 Feb 9.30 am). Their magnitudes have ranged from about M 0.6 to M 3.8, while the depths ranged between 1 and 11 kilometres, with most being 5-7 kilometres deep. As the earthquakes are quite shallow they will feel stronger than the magnitudes indicate.

The Taupo Volcanic Zone is a rifting area, growing wider each year by 6-9 mm. These earthquakes are likely to be related to the long-term ‘tectonic’ stretching of the Zone. Currently, there are no indications that the earthquakes are related to volcanic activity, being located well away from the active volcanoes. As usual, we continue to closely monitor the activity.


21/02/2017

We get asked two questions a lot: what is an earthquake forecast and why do I need to know about it? Answering that second part quickly: some people need to know this for their work, some people are interested as to what we think could happen next, while others just want to move on and not hear about earthquakes anymore. But, for those of you who want more detail, we thought we’d take a moment and answer some questions about the forecasts. 

What’s the difference between a forecast and prediction? 

GNS Science has been producing earthquake forecasts since the late 1990s, but it wasn’t until the 2010/11 Canterbury earthquakes that people got really interested in them.

The earthquake forecasts we produce are not earthquake predictions. A forecast is a probability of something happening over a certain period of time.  A prediction gives a specific timing and location for something to happen.

Some people say they can predict earthquakes. However, at GeoNet we stick to our knitting: we are a science organisation and base our work on things we can observe and measure.  At present there is no scientific way to accurately and reliably predict when and where a big earthquake is going to happen next.

Another way to think about the forecasts: Introducing our Grandma’s China-ometer!

"Grandma China-ometer" (patent pending) Levels

LevelAction
4

The china's probably best securely packed away in a box somewhere safe just at the moment

3Keep the china in the china cupboard
2The china is ok to put out again, but we'd suggest using Blu Tack
1The china's probably fine to put out without Blu Tack (but nowhere in NZ is 100% safe from earthquakes, so for peace of mind perhaps use a non-slip mat!)

What many people want to know is “what should I do with this forecast”?  Perhaps you could think of the forecasts as a sort of Grandma’s China-ometer – should I put Grandma’s heirloom tea set back on the mantelpiece or bookshelf yet?  I'm not an engineer or a seismologist, so I use the forecasts like this: I have a special china tea set that my grandmother left me when she passed away. In the Kaikoura Earthquake, some of the tea set moved (but didn’t fall…thank you Blu Tack!). My grandmother’s tea set means a lot to me, so, as an extra precaution, I wrapped the tea set in tissue paper and put it in a cardboard box. I look at the numbers and think to myself “maybe just a few more months until I’ll risk putting my tea set back up".

At the moment (three months since the M7.8 Kaikoura Earthquake), I think we are still at Level 4 on the Grandma’s China-ometer in the North Canterbury/Kaikoura area, but this will gradually lower back to a Level 2, which is the background, normal level for this area.  In Wellington, we are probably at Level 3; but I’ve still got my tea set in a box packed away. I’m naturally a bit of a pessimist, so I act like Wellington is at Level 4.

How do you produce the forecasts?

The past gives us clues for the future.  Much like detectives putting together evidence to solve a crime, scientists use evidence from observations and models to understand the processes happening in the earth.

The models that GNS Science use to generate the earthquake forecasts are based on observations of how earthquake sequences work, from all around the world over more than 100 years.  In general, most aftershock sequences decay, which means the number of earthquakes decreases over time. This is called Omori’s law. Although, a large aftershock can cause a spike of activity any time.

These models tell us about the average behaviour of aftershock sequences, but we learn more as a particular sequence unfolds.  Think of it like family behaviour: we might expect that your family might behave in a certain way at a family get together. But if we randomly grab one member, we might get your weird Uncle Kevin or more stable Aunt Caroline. At this point, we think the Kaikoura sequence is more like stable Aunt Caroline, but crazy Uncle Kevin can still show up and ruin the family gathering.

The initial aftershock model we use was developed by GNS Science. It was based on one used by the USGS that one of our scientists developed (we loaned him out for a few years and then brought him back). The model has been improved over the last decade to suit New Zealand’s unique conditions.  If you want the technical details of the models, they are explained on GNS Science’s Earthquake Hazard Modelling page.

We don’t know exactly what is coming when, however, knowing what is most likely can help us make decisions as individuals and communities. Want to know more technical details about earthquake forecasting for the Kaikoura earthquake sequence? Go here.

These numbers don’t really help me, who uses them?

The earthquake forecast probabilities are really useful for engineers, infrastructure managers, private companies, Civil Defence, government planning, and insurance organisations, including EQC.  Infrastructure managers and Civil Defence can use the probabilities to plan for the next few months – they only have so much time and resources, so knowing what is likely (or not) helps them decide where to focus their efforts and what to plan for.  The probabilities are fed into new building standards (as they were after the Canterbury earthquakes), so that our buildings will be more resilient to earthquakes in the future.  And when probabilities are quantified like this they can be used by risk assessors at insurance companies to compare risks from different hazards (e.g. flooding, snowstorm and earthquakes). Some members of the public also want the numbers to know what to expect about how many earthquakes they might feel and how many might be large enough to cause more damage.

Other people admit that they don’t really understand the numbers, but they say that the numbers provide reassurance; they are comforted by the thought that some people understand what is going on and what is happening is generally within the range of the forecast.  Others would rather get a poke in the eye than see another forecast.

What is important is having a general indication of what we can expect and figuring out how to live around the possibility of another large earthquake – either as part of the current earthquake sequence, or a separate one (we live smack bang on the top of a tectonic plate boundary, so getting big earthquakes every now and then is not surprising).  The best thing we can do is take a few steps to help ourselves.  As the probability of a moderate-sized earthquake in the aftershock area is still significant three months on from the Kaikoura earthquake, you might want to be extra careful and prepared.

What do I do about these earthquakes? These earthquakes are really getting to me.

As fascinating as they are, earthquakes can be really scary for some people.  Even if you are not that disturbed by the earthquakes themselves, just constantly getting a fright every time one arrives can be enough to rattle your nerves.  Or you may just be plain scared of them, and it is normal to be scared of something that is scary.  If you are anxious about the earthquakes and this is affecting your ability to go about your daily life 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. Remember to also seek support with friends and family, and to take time out to do things you enjoy. 

If you want more advice on how to prepare your household, you can follow our friends at the Ministry of Civil Defence & Emergency Management on Twitter and Facebook for the latest earthquake and tsunami preparedness information. EQC also have a great guide to Quake Safe your home. You can also follow your regional Civil Defence Emergency Management Groups.

Story written by: Helen Jack, Sara McBride, Annamarie Christophersen, Matt Gerstenberger 

China stunt coordinator: Helen Jack

 

A small earthquake swarm started around 6pm (Monday 13 February) about 10 kilometres north west of Tokaanu (Turangi). Since around 7am today (Monday February 20) earthquake activity has restarted. We have been able to locate 52 of the events so far. Their magnitudes ranged from about M 0.8 to M 2.5, while the depths ranged between 1 and 10 kilometres, with most being 5-8 kilometres deep.

The largest event to date occurred at 05.16 am on 20 February, being M2.5. There have been two felt reports, reporting weak shaking. Swarms are often characterised by no one main or large event, with many of the events being about the same size.

Earthquake swarms are very common in the greater Rotorua-Taupo area, known as the Taupo Volcanic Zone. In 2008 there was a larger swarm in the same area, while in March 2015 there was one about 9 km to the north and another in October 2015 to the south. 

Earthquake swarms are very common in the greater Rotorua-Taupo area, known as the Taupo Volcanic Zone. Since around 6pm (Monday 13 February) the GeoNet seismometer network between Taupo and the Tongariro National Park has been recording a swarm of small earthquakes. They locate about 10 kilometres north west of Tokaanu. We have been able to locate 19 of the events so far. Their magnitudes ranged from about M 0.8 to M 1.7, while the depths ranged between 1 and 9 kilometres, with most being 3-5 kilometres deep. The quakes are likely to be related to the long-term tectonic stretching of the Taupo Volcanic Zone. Currently, there are no indications that the earthquakes are related to volcanic activity. As usual, we continue to closely monitor the activity.

The largest event to date occurred at 10.04 pm on 16 February, being M1.7. There have been no felt reports. Swarms are often characterised by no one main or large event, with many of the events being about the same size.

In 2008 there was a larger swarm in the same area, while in March 2015 there was one about 9 km to the north and another in October 2015 to the south. 


20/03/2017 12.00 p.m.

We’ve been busy crunching the numbers based on all the seismic activity in the aftershock area of November’s Kaikoura Earthquake. So what’s the big news? The expected numbers of earthquakes have dropped since the last forecast.  There is now a 15% chance of one or more M6.0-6.9 earthquakes occurring within the next month; this has decreased from 18% from our last forecast (19 February 2017).  We like this downward movement in our forecast; it is good step in the right direction. 

But does this mean we are all in the clear and don’t need to worry about more big earthquakes? No, absolutely not. Another big earthquake is still well within the probabilities in our models. A 15% chance in a month is still a concerning probability. We need to continue to be prepared for earthquakes as these will go on for years to come. The ongoing Canterbury Earthquake Sequence is an example of aftershocks that can last for years after the initial mainshock (which was the M7.1 Darfield quake in 2010). 

Remember: Drop, cover and hold in an earthquake. If the earthquake is long or strong and you are near the coast, evacuate as soon as the shaking stops. Our friends at the Ministry of Civil Defence and Emergency Management advise to not wait for an official warning or sirens.

Want to know why we produce aftershock forecasts?  As well as helping people to understand what they might expect next, many people use the information in their work.  This includes engineers, infrastructure planners, Civil Defence planners and insurance risk assessors who use our forecasts in their planning and decision making.  See more in our Why do we do earthquake forecasting story.

What’s happened so far

We’ve had a total of 14796 earthquakes since the M7.8 Kaikoura earthquake stopped shaking our islands (we ran the numbers at 11 a.m. on the 20 March). 473 of those earthquakes were M4-4.9, 56 were M5-5.9 and 4 have been M6.0 or greater. Yes, that is a lot of earthquakes but line up with what we’ve been forecasting.

Aftershock Forecast starting on 19 March 2017 

While no one can yet scientifically predict earthquakes, we can provide forecasts of future aftershocks (based on probabilities), as well as some scenarios from what is most likely to happen to what is very unlikely, but still possible. Most earthquake aftershock sequences decay (i.e. the number of earthquakes generally decreases) over time, with spikes of activity that can include larger earthquakes. For a comparison, here's our older forecast from 19 December. 

 

Average number of M5.0-5.9

Range* of M5.0-5.9

Probability of 1 or more M5.0-5.9

Average number of M6.0-6.9

Range* of M6.0-6.9

Probability of 1 or more M6.0-6.9

Average number of M≥7

Range* of M≥7

Probability of 1 or more M≥7

within 30 days

1.9

0-7

73%

0.2

0-1

15%

0.02

0-1

1%

within one year

13.1

5-26

>99%

1.1

0-4

68%

0.1

0-1

10%

Forecast for rectangular box (see map on the right) with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5 at 12 noon, Sunday, 19 March; 95% confidence bounds.

The aftershocks of the magnitude 7.8 Kaikoura earthquake are mostly occurring throughout a broad area from North Canterbury through to Cook Strait that surrounds the faults that ruptured in that earthquake, although a few have occurred in the lower North Island. We forecast aftershock probabilities for the area in the red box on the map to the right. The area near the centre of the box (around Kaikoura) is more likely to experience felt aftershocks than areas towards the edge of the box. See the MMI map below for more information on the forecast shaking for the Wellington area. Earthquakes can and do happen outside this box but the box represents the most likely area for aftershocks in this sequence. 

For example, there is a 15% chance of one or more M6.0-6.9 earthquakes occurring within the next month. We estimate there will be between 0 and 1 earthquakes in this magnitude range within the next month.

The current rate of magnitude 6 and above earthquakes for the next month is about 5 times larger than what we would normally expect for long term seismicity represented in our National Seismic Hazard model. As the aftershock rates decrease, this difference will decrease as well.

 

Scenarios

Note that the scenarios are not updated as frequently as the forecasts above. The scenarios below were estimated in December 2016 so the likelihoods are higher than the forecast table above.

The scenarios specifically address the probabilities of what we might see happen within the next year and were estimated in mid-December 2016. The scenarios cover a wider geographic area than the aftershock probability forecast area. The probability numbers in the table above differ to the scenarios. This is because they were estimated at a different time in the aftershock sequence, and we have used new information we have gathered from the slow-slip events, and their potential impact on the plate interface and other faults, to help define our probabilities in scenario three. 

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

Scenario One: Likely (approximately 70% within the next year)

The most likely scenario is that aftershocks will continue to decrease in frequency (and in line with forecasts) over the next year and no aftershocks of magnitude 7 or larger will occur. Felt aftershocks (e.g. over magnitude 5) can occur in the area from North Canterbury to Cape Palliser/Wellington.

Scenario Two:  Unlikely (approximately 25% within the next year)

An earthquake smaller than the mainshock and between magnitude 7.0 to magnitude 7.8 will occur. There are numerous mapped faults in the North Canterbury, Marlborough, Cook Strait and Southern North Island areas capable of such an earthquake. It may also occur on an unmapped fault. This earthquake may be onshore or offshore but close enough to cause severe shaking on land. This scenario includes the possibility of an earthquake in the Hikurangi Subduction Zone. Earthquakes originating from here or in the Cook Strait have the potential to generate localised tsunami. The Hawke’s Bay earthquake sequence in 1931 provides an analogy to scenario two, as a magnitude 7.3 aftershock occurred approximately 2 weeks after the initial magnitude 7.8 earthquake.

Scenario Three:  Very unlikely (5% within the next year)

A much less likely scenario than the previous two scenarios is that recent earthquake activity will trigger an earthquake larger than the magnitude 7.8 mainshock. This includes the possibility for an earthquake of greater than magnitude 8.0, which could be on the plate interface (where the Pacific Plate meets the Australian Plate). Although it is still very unlikely, the chances of this occurring have increased since before the magnitude 7.8 earthquake, and have also been also been slightly increased by the slow-slip events.

Initially our scenarios covered what might happen over the next 30 days, but we are now shifting to covering what might happen over the next year.  This is because the aftershocks are generally becoming smaller and less frequent (decaying) over time, and this lower aftershock rate increases the uncertainty of what might happen over shorter time periods. The change in forecast does not hugely affect the scenarios at the moment; we will review these again later in the year. While we will continue to update the aftershock probabilities regularly, we will not update the scenarios as often.

Can't get enough technical information? Here's the fine print on how we model aftershock probabilities.

Aftershock shaking forecasts

We have also calculated the probability of damaging earthquake shaking from aftershocks over the next year (starting 19 March 2017).  Damaging earthquake shaking is defined as MM7 on the Modified Mercalli Intensity (MMI) scale.  The MMI scale is different to earthquake magnitude – it describes the intensity and impacts of the shaking, which depend on the magnitude of the earthquake, how far away the earthquake was and the type of ground you are on.  At MM7 intensity shaking levels it is difficult to stand, furniture and appliances move, contents are damaged, there is minor building damage and liquefaction can occur in susceptible sediments.

The maps show the probability of MM7 shaking within the aftershock region, which includes Wellington. Over the next year the probability of MM7 shaking around the wider Kaikoura/northern area. In comparison, the probability of MM7 shaking in the Wellington area is around 3% (dark blue) in the next year. While this probability is considerably lower in Wellington than in the areas around Kaikoura, it is possible for shaking similar to what occurred during the mainshock to happen again in Wellington. 

Christchurch's aftershock probabilities are not greatly affected by the magnitude 7.8 Kaikoura earthquake.  The most recent update of the Christchurch aftershock probabilities are here. We update the Christchurch aftershock probabilities annually, as now they do not change much from month to month. 

Tsunami and landslide hazards

A tsunami was created by the magnitude 7.8 Kaikoura earthquake, and our scientists are still analysing the tsunami data and collecting information on its impacts. Remember, if an earthquake is too strong to stand up in, or lasts longer than a minute, move inland or to higher ground immediately. Do not wait for a siren or an official warning.

The earthquake also caused tens of thousands of landslides in North Canterbury, Kaikoura and Marlborough. These landslides remain dangerous and can move at any time. Please be careful around landslides and cracks in slopes. Heavy rain can pick up and carry landslide material and cause debris flows and debris floods (flash floods). Landslides have also dammed several rivers. These dams could breach, particularly in heavy rain. Please be careful and avoid riverbeds downstream of dams.

Take care of yourselves and others – physically and mentally

Earthquakes can be scary.  It is normal and okay to be a bit scared about things that are scary.  But the best thing you can do is take action and be prepared. 

You can follow our friends at the Ministry of Civil Defence & Emergency Management on Twitter and Facebook for the latest earthquake and tsunami preparedness information. EQC also have a great guide to Quake Safe your home. You can also follow your regional Civil Defence Emergency Management GroupsIf you are anxious about the earthquakes and this is affecting your ability to go about your daily life 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. Remember to also seek support with friends and family, and to take time out to do things you enjoy. 

Written by Kate Clark
Earthquake Geologist
k.clark@gns.cri.nz

 

Extensive coastal uplift occurred during the Nov 14 2016 Kaikoura earthquake. The uplift occurred almost instantaneously – all during the two minutes of shaking caused by the earthquake. 

Following the earthquake, a team of scientists from GNS Science and the University of Canterbury headed out into the impacted area to survey and record the amount of coastal uplift that occurred. Knowing the amount and pattern of uplift helps us understand aspects of the earthquake such as how much movement occurred across faults that cut across the coastline, and the presence or absence of offshore faults that also moved in the earthquake. Studying the uplift in the recent earthquake can also help us understand more about earthquakes that occurred in the past. For example, we can compare the amount of uplift that occurred in November 2016 earthquake with tectonic uplift recorded by uplifted marine platforms that were thrust out of the sea by earthquakes many thousands of years ago.

Our survey of coastal uplift showed that most of the coastline from Oaro to Lake Grassmere uplifted, that’s about 110 km of coastal uplift. The uplift varies along the coastline is broadly controlled by where faults ruptured the ground surface during the earthquake. The following paragraphs describe the pattern of coastal uplift from south to north along the coast.

Coastal uplift begins immediately north of where the Hundalee Fault crosses the coastline at Oaro. We observed the uplift from our helicopter surveys and also measured a point near Goose Bay where we recorded about 1.6 m of uplift. This amount of uplift is fairly steady along the coast toward Kaikoura. We measured a lot of points around the Kaikoura Peninsula and recorded about 0.8 – 1 m of uplift, which is consistent with the tide gauge there, which uplifted 0.95 m during the earthquake.

It was difficult to tell if uplift had occurred along the steep gravel beaches between Kaikoura Peninsula and the Hapuku River, but north of the Hapuku River uplift increased. Around Halfmoon Bay and Ohau Stream the uplift was measured at 2-3 m. The Hope Fault comes across the coastline near Halfmoon Bay and there was a small amount of surface rupture on the fault but the amount of uplift either side of the fault did not vary substantially. Just north of Waipapa Bay, two strands of the Papatea Fault cut across the coastline and the block in between these two fault strands was dramatically pushed up about 4.8 – 5 m high. This is the greatest amount of uplift recorded in this earthquake.

North of the Papatea Fault there appeared to be very little coastal uplift. Uplift increased markedly just north of where the Kekerengu Fault crosses the coastline. Uplift of around 2.5 – 3 m was measured at Needles Point, Ward Beach and Chancet Rocks. Between Chancet Rocks and Cape Campbell the amount of coastal uplift gradually decreased, there were a couple of sharp ~0.5 m steps where minor fault ruptures cut across the coastline. Around the corner from Cape Campbell, uplift was still moderate (~1 m) at Marfells Beach.

We noticed big changes in the coastal landscape each time we revisited locations, the uplifted seaweeds are quickly bleaching and falling off the rocks, and new hightide and storm beach lines are being established on the uplifted beach faces. This has been a phenomenal tectonic event and is causing numerous problems for local residents, fishermen, boat operators and other coastal users whose coastline has shifted so suddenly.  We are working closely with other scientists to build up a complete understanding of the earthquake, and also keeping in touch with our marine biology colleagues at the University of Canterbury who are working hard to understand the consequences and biological shifts that will be seen in the marine environment.

Since the morning of 14 November earthquake geologists have been flying, walking, driving, and sailing all over North Canterbury, Kaikoura and Marlborough, mapping and measuring the faults that moved during the magnitude 7.8 earthquake.  It’s been a collaborative effort, involving scientists from GNS Science, NIWA, the Universities of Auckland, Canterbury and Otago, Victoria University as well as overseas researchers – 54 people in all!

This is one of the most complex on-land earthquakes ever recorded – to date, at least 12 faults have been mapped where they have broken through to the ground or sea floor. 

Unlike the cracks associated with landslides, these breaks start kilometres down in the crust, and come all the way up to the ground surface, shunting land sideways and upwards.  Some fault displacements are just wee steps or small cracks in the ground, while others have produced metres-high cliffs, or land has been pushed sideways by many metres relative to land on the other side of the fault.  New Zealand Geographic magazine has a great story about the fault ruptures and the geologists investigating them, with some amazing photos illustrating how the faults have moved.

So far, most of the fault mapping and has been done on foot, or by helicopter or drone.  Offshore, NIWA have used  a multibeam echosounder and high frequency sub-bottom profiler (fancy machines with interesting names that map the sea floor) on the Tangaroa to detect the movement on the undersea Needles Fault. 

The initial field reconnaissance of the faults has now finished, but LiDAR – a type of high-resolution ground mapping – is now being flown over much of the area affected by the earthquake.  And early in the new year NIWA scientists will be back out on a smaller boat the Ikatere, which can get closer to shore than the Tangaroa, to map how the onshore and offshore fault ruptures join up.

After taking a well-deserved break over the holidays, the geologists will begin analysing the mountain of data collected, to work out their piece of this earthquake story.  It’s likely that once the LiDAR and Ikatere data are analysed even more fault ruptures will be discovered, and they’ll then have a better idea of the total movement that has taken place.

Further down the track they’ll also be looking at the connections and gaps between the faults and what this means for seismic hazard modelling.  Is there evidence for these faults having ruptured together before? And is this normal?

We'll update you next year on this research, which gives us a window into understanding not only this earthquake but how New Zealand reshapes itself through earthquakes over millions of years.

Updated 3pm, 9/12/2016

This is an important, but difficult question that our experts here at GNS Science, along with Victoria University have been working on answering. The slow-slip events (or silent earthquakes) cover a large area of the plate boundary underneath the North Island and have made calculating the likelihood of future large aftershocks trickier.

Although still very unlikely, we now estimate that the probability of a magnitude 7.8 or larger earthquake in the coming year has increased to approximately 5% – due to ongoing slow-slip events. This is a small increase in the likelihood which is generated by the Kaikoura Earthquake alone. This is approximately 6 times greater than it was prior to the Kaikoura Earthquake.

Due to the large extent of slow slip, the adjusted forecast covers a larger region than the standard aftershock area to now include the lower half of the North Island and the upper South Island. There are several faults in these areas capable of large quakes, including the subduction zone and crustal faults like those that ruptured during the Kaikoura earthquake.

 
Cross section of the North Island of New Zealand showing how the
Australian and
Pacific Plates meet. The slow-slip events (orange-yellow
patches) are superimposed
onto the cross-section. Bottom right Insert
shows a map view of the slow-slip events.

What does this mean?

Our forecasts tell us what is likely (or unlikely) to happen in the future, but they can never definitively say if a large earthquake will occur or not. We’re aware that these messages could be unsettling, and that’s a very normal reaction. What we do want you to take away from this (and this applies to all New Zealanders, at all times—not only now) is to follow Civil Defence’s advice and make sure that you’re prepared for earthquakes and tsunamis. We know that being prepared makes a real difference in helping you get through an event and recovering afterward. Many of you have already got you and your family prepared, so well done you guys!

Tell me about slow-slip events again

The current slow-slip events that have followed the M7.8 Kaikoura earthquake are occurring along the fault between the Australian and Pacific Plates, known as the Hikurangi subduction zone.  The movements along the fault are equivalent to a magnitude 7.0 earthquake in the Hawke’s Bay-Gisborne region, and magnitude 6.9 earthquake in the Manawatu-Kapiti region.

We have observed many similar slow-slip events in these areas of this size, but this is the first time we’ve observed slow-slip occurring simultaneously in multiple areas around the North Island in the 15 years we've been detecting them. This is also the first time we’ve been able to observe slow slip in New Zealand after a magnitude 7.8 earthquake, so it’s possible this is a normal pattern after such a large quake.

The Hawke’s Bay-Gisborne region slow-slip event only lasted about a week, and it has mostly finished. Slow-slip events in these regions happen less than 15 kilometres deep. There were a number of earthquakes offshore Porangahau in the southern Hawke's Bay in the 2-3 weeks following the Kaikoura earthquake that were likely triggered by the east coast slow slip event.

The slow slip event beneath the Kapiti and Manawatu regions appears to be ongoing at a relatively steady rate since the Kaikoura M7.8 earthquake. Slow-slip events in these regions tend to occur between 25-45 kilometres deep.

If we can’t feel slow-slip events, why are you focusing on them?

Slow-slip events occur beneath the North Island, where the Australian and Pacific Plates meet. In the lower North Island, the slow-slip events happen slightly deeper than the part of the subduction zone where the plates are currently stuck together.  These “stuck zones” are thought to periodically rupture in large earthquakes. When slow-slip events occur, stresses are applied to this stuck-plate zone. This increased stress happens during all slow-slip events. The number of earthquakes during some slow-slip events can increase, but this doesn’t always happen.

There have been hundreds of slow slip events observed at subduction zones around the world that have not triggered larger, damaging quakes. So, if slow-slip events do trigger large damaging quakes, it is very rare indeed. In New Zealand we typically have at least 2-3 slow-slip events each year. Scientists have only discovered in the last 15 years that slow slip events occur, so trying to understand their relationship to larger, damaging quakes is still in its very early stages.

How did we go about incorporating slow-slip events into aftershock forecasts?

In New Zealand, scientists at GNS Science have been developing aftershock forecasts for the several large earthquakes since the Canterbury earthquake sequence. We’ve refined these over the last 6 years, but have never had to incorporate slow slip events into the mix.  

Experts from GNS Science and Victoria University evaluated many strands of evidence to determine the likelihood of an earthquake equal to or larger than the Kaikoura Earthquake. We have also consulted with several international experts who study slow-slip phenomena in their respective countries to provide additional perspective. 

This is our first run at including slow slip into the forecasts, and it is probably the first attempt worldwide to implement this, so it is definitely a work in progress and our estimates have large uncertainties. As scientists’ understanding of this phenomena improves we hope to develop better ways to incorporate the mechanics of slow-slip events and their relationship with earthquakes into our forecasts.

Being prepared

Civil Defence have great resources on how to prepare your home, how to make an emergency plan for your family, and what to do during and after an earthquake. Check with your local and regional council for your region's tsunami evacuation zones - remember, these zones apply regardless of where the tsunami is coming from. Knowing where these zones are can help you plan your evacuation route before a tsunami occurs.  You will not have time to do this if an earthquake occurs.  Remember if you are near the coast and you feel a long OR strong earthquake, then you should evacuate inland or to higher ground immediately.

As always, we'll keep you updated with any new information as it comes to hand. We'll also be updating our aftershock forecasts periodically. 

Written by Ursula Cochran

Last year Tim Little of Victoria University of Wellington and Russ Van Dissen of GNS Science thought it would be a good idea to find out more about the Kekerengu Fault in North Canterbury. Through previous work they had identified the Kekerengu Fault as likely to be the fastest slipping fault within 100 km of Wellington city apart from the Hikurangi subduction zone.  

They knew this meant it posed a significant seismic hazard to the northeastern South Island and also to Wellington if linking faults in Cook Strait ruptured at the same time as the Kekerengu Fault.

In February of this year, with funding from the Natural Hazards Research Platform, Tim and Russ excavated three trenches across the Kekerengu Fault to look for evidence of past large earthquakes. The main aim of their project was to better constrain the seismic hazard posed by this major active fault.

In these trenches Tim and Russ found evidence that at least three past large earthquakes had occurred in the last 1250 years. These initial results confirmed that the Kekerengu Fault was capable of producing large earthquakes frequently (on average, about every 300 or 400 hundred years), and was likely to do so again in future.

Then, two weeks ago, as if to say, “Don’t underestimate me!” the fault ruptured right through those same trenches. Tim was awe-struck. As a geologist working on active faults he said, “I had often wondered what it would look like if a fault moved while we were working on a trench cut across it, but I had never expected this to happen to me.”

When the Kekerengu Fault moved as part of the M7.8 Kaikoura earthquake the impacts on the landscape were dramatic. One side of the fault has moved as much as eleven metres with respect to the other side. Tim did not expect quite this amount of slip on this fault during a single earthquake. Russ, though, was less surprised – he says it fits with the long-term slip rate calculated for the fault – but he is still amazed to see such fault movement in action.

So, we knew about this fault, we knew it posed a seismic hazard, we even thought it was possible that it would rupture jointly with other faults – New Zealand’s National Seismic Hazard Model specifically includes scenarios that involve joint rupture of the Jordon, Kekerengu, and Needles Faults. And this, now confirmed by NIWA’s offshore survey of the Needles Fault, is exactly what happened on Monday 14th November. What we had not foreseen is that even more faults would be involved in a single earthquake sequence.

Currently we have evidence for seven faults rupturing in the M7.8 Kaikoura earthquake so work on the Kekerengu Fault is just a small part of the earthquake geology response. There are teams from University of Otago, University of Canterbury, University of Auckland, Victoria University, GNS Science, and NIWA, not to mention volunteers from overseas, currently mapping and measuring the faults that moved last week. We want to understand what happened in this event but, most importantly, what it means for future events.

The Kekerengu Fault has been speaking to geologists-in-the-making for generations because Victoria University’s third year structural field geology course is held near its northern end. I still clearly remember the Kekerengu Fault back in the early 1990s as a subtle, curious line in the landscape that our professor – Tim Little – stood astride inciting us to notice. Today, the fault has spoken and it is impossible not to notice.

13/12/2016 10:00 a.m.

Probabilities and scenarios…we’ve been getting a lot of questions about how these work and what it all means. Specifically, people want to know if the sequence is behaving as we have forecast.

So are we getting the number of aftershocks expected in the forecasts? The short answer is: we are on the low side of what we’ve forecast, but the numbers are mostly still within the forecast range.  There’s a bit more to the story, so let’s back up for a second and take a look at the big picture. 

How many aftershocks have there been?

By noon on Monday 12 December we had detected 8,735 aftershocks from the M7.8 Kaikoura earthquake (with the area of detection being the forecast area represented by the box). Most of these aftershocks have been small (8,669 earthquakes <M4.9) and would have only been felt close to the epicentre. As of Monday 12th December, there were also 49 aftershocks in the M5.0-5.9 range, and 3 aftershocks in the magnitude M6.0-6.9 range.

When a large earthquake like M7.8 Kaikoura occurs, aftershocks happen thick and fast. When that happens, our seismic detection network can miss smaller aftershocks as their energy is overshadowed by the larger aftershocks - so not all aftershocks are detected. Our seismic network is very sensitive and typically picks up even the smallest of shakes. But now, due to the big earthquakes coming through, it is more difficult to detect all of the quakes. Imagine a clear lake. Most of the time, when you skip small pebbles on the clear lake, you can spot the associated ripples easily. But then a giant rock is thrown into the clear lake. The splash and ripples it creates can cause the smaller ripples from the pebbles to go unnoticed. Once the giant rock’s waves subside, the smaller pebbles and their ripples become noticeable again. That is similar to what is happening here.

At this stage, we haven’t been able to detect all of the smaller aftershocks in amongst the waves of the larger earthquakes. Therefore, the total number of aftershocks in the earthquake catalogue below magnitude 5 is currently lower than what has actually happened. The total of 8,735 aftershocks is the bare minimum of what we’ve detected. When we have more time for data processing we will likely find further small aftershocks in the seismic waves of the mainshock. Our earthquake catalogue will be changed to reflect this in future.

 

 Observation and forecast time interval

M5.0-5.9

M6.0-6.9

M≥7

Date

Time

Duration

Forecast

Detected

Forecast

Detected

Forecast

Detected

14 Nov

00:15*

11.75h

14-32

24

0-5

2

0-1

0

14 Nov

12 noon

24h

5-17

12

0-3

1

0-1

0

15 Nov

12 noon

24h

1-10

4

0-2

0

0-1

0

16 Nov

12 noon

24h

1-10

1

0-2

0

0-1

0

17 Nov

12 noon

24h

0-8

0

0-2

0

0-1

0

18 Nov

12 noon

24h

0-7

1

0-2

0

0-1

0

19 Nov

12 noon

24h

0-6

0

0-1

0

0-1

0

20 Nov

12 noon

24h

0-5

1

0-1

0

0-1

0

21 Nov

12 noon

24h

0-5

0

0-1

0

0-1

0

22 Nov

12 noon

24h

0-5

1

0-1

0

0-1

0

         
21 Nov12 noon7 days3-1910-300-10
28 Nov12 noon7 days1-1310-200-10
5 Dec12 noon7 days0-1100-100-10

Total number of aftershocks by noon, 12 Nov

 

46

 

3

 

0

For example, at 12 noon on 14 November, we forecast that there would be between 5 and 17 aftershocks in the M5.0-5.9 range, for the following 24-hour period. Once this time period had finished, 12 aftershocks in this magnitude range were actually detected, which is around the average of what we forecast

Forecast and detected earthquakes for a rectangular box with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5. Note: Our aftershock forecast models are based on previous New Zealand aftershock sequences.   

* The first forecast is calculated is about a quarter of an hour after the mainshock to avoid the period of most undetected aftershocks.

How do the detected aftershocks compare to our forecasts?

At  the moment, the aftershock sequence is falling within or just below the lower end of our forecast range.  The table above shows the range in the number of aftershocks that we have forecast for 24-hour time intervals, compared to the number of earthquakes that we have actually detected so far, for three magnitude ranges.   

The  graph shows the number of aftershocks over M5.0 that we can expect per day, on average (with the uncertainty range in grey). The stars show the actual number of detected aftershocks that have occurred on each day. The graph shows that the forecast number of aftershocks will continue to decrease on average. There may be the occasional spike of activity as larger aftershocks occur with their own aftershock sequences. This follows the normal pattern of what we can expect following an earthquake.

We are working to understand why the Kaikoura sequence has been less productive than the average New Zealand earthquake sequence. However, the variability in aftershock productivity between sequences can be up to a factor of around ten. So far the aftershocks have decreased with the initial productivity of the sequence and as such the sequence is behaving normally.

In general, more small aftershocks will continue to occur than big ones. As a rule of thumb, there is a tenfold increase in the number of earthquakes for every one-magnitude decrease. For example, for one M6.0 earthquake, we expect around 10 earthquakes of M5.0-5.9 and around 100 earthquakes of M4.0-4.9 on average. This applies to all seismicity experienced, as well as that occurring as part of the aftershock sequence.

What does this mean?

In summary, the aftershocks are at the lower end of the forecast range. Just because we are in lower end of the forecast, it doesn’t mean that this will stay that way.

What you can do about forecasts: be prepared!

We know that these events can make people anxious or worried. That is perfectly normal; earthquakes can be scary! If you are feeling overwhelmed by the earthquakes or the forecasts, there are people who are there to listen and support you on 0800-777-846..

Our best advice is to be prepared for future aftershocks.  You can find out more about getting prepared by visiting our friends at the Ministry of Civil Defence and Emergency Management.

Can't get enough technical information? Here's the fine print. 

(This story will be next updated on the Monday 19th December, when the current weekly forecast period is over) 

 

The GeoNet network of seismometers and continuously operating GPS instruments have allowed an unprecedented view of the propagation of the M7.8 Kaikoura earthquake in real time.  Computer simulations using GeoNet seismological data allow us to watch the progress of the rupture as it punched its way up the east coast of the South Island over an approximately two minute period early in the morning of November 14th

Because Geonet’s seismic network had numerous seismic stations situated very close to the rupture (just like its Canterbury Seismic Network during the 2010-2011 earthquake sequence), the data from these sensors allow us to essentially travel back in time and watch the event unfold.  Computer simulations of the seismological data (using two very different methods—see figures below) suggest that the rupture started in the south, on a fault at approximately 15 km depth near Culverden. The initial rupture near Culverden subsequently triggered a domino effect, as the earthquake rupture jumped from fault to fault, essentially “unzipping” along a 150 km length of the northeast coast of the South Island.  The data suggests that many faults were involved in the rupture. 

Chronological evolution of the fault rupture as the earthquake progresses. Colour scale in metres of slip.

Data (seismograms) show the vertical acceleration at GeoNet strong motion stations. 

Movie showing migration of maximum energy released during the earthquake.

Coloured circles represent progression with time (in seconds following the main shock) of the energy release.

 

The earthquake rupture picked up a head of steam as it passed Kaikoura, with the largest fault movements occurring between Kekerengu and Cape Campbell.  This pattern is confirmed by GPS and satellite radar observations. Near the end of the earthquake’s runaway journey, it jumped onto a series of faults offshore. NIWA has found clear evidence of large earthquake displacement on one of these--the Needles fault (NIWA story).  The earthquake rupture terminated abruptly just before reaching Cook Strait.  There is no evidence at this time that the Hikurangi subduction zone was significantly involved in the rupture.

The Kaikoura M7.8 earthquake is arguably the most complex earthquake rupture ever to be observed in this level of detail with modern instrumentation (seismic sensors, GPS instruments, and satellite radar data).  Multiple datasets (seismology, geodesy, and surface observations of faulting, uplift, and tsunami) are rapidly converging to give us a single, consistent view of exactly what happened in the earthquake. Preliminary models, will of course be refined, as new information continues to come in.

Science contacts: Caroline Holden and Bill Fry, GNS Science

Update 6pm, Saturday 26th November:

The slow-slip event detected earlier this week of the east coast now has company. More slow-slip movement on the Hikurangi subduction zone (where the Pacific and Australian plates meet) has now been detected by GeoNet and PositioNZ cGPS sites in the Kapiti and Manawatu regions. We have recorded multiple slow-slip events in all of these regions previously (since they were first discovered in early 2002), and sometimes one region slips after another, however we have never detected slow-slip simultaneously in multiple regions. We have also never monitored slow-slip following a central New Zealand earthquake as big as the M7.8, so this could be typical bahaviour in the aftermath of such a large earthquake. We are continuing to monitor the event closely as it unfolds, and we will provide regular updates. The Kapiti-Manawatu slow-slip event has involved movement across the Hikurangi subduction zone plate boundary of between 5-7cm,  equivalent to a magnitude 6.8 earthquake in the last two weeks.  The Gisborne-Hawke’s Bay event has involved slip across the plate boundary up to about 15cm, equivalent to a magnitude 7.2 earthquake.  The cGPS data suggest that the Gisborne-Hawkes Bay slow slip event appears to be tapering off in the last day or two. 

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.

What is a slow slip event and how are they detected?

GeoNet, in partnership with LINZ run a network of GPS stations around the country that are able to detect land movement as little as a few millimeters resulting from silent earthquakes. 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.

These slow-slip events occur below the earth’s surface where the Pacific Plate meets the Australian Plate, along the Hikurangi Subduction Zone.
More info: on the GNS webpage and GeoNet Q&A about slow-slip events.

The slow-slip event following the Kaikoura Earthquake

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

This slow-slip event is particularly interesting as it appears to involve slip along the plate boundary from Hawke's Bay up to East Cape at the same time. Normally we see slow-slip events in these regions but they are separated in time or happen one after the other, as was the case after the Te Araroa Earthquake. 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.

 

Porangahau Earthquakes

 

Update: Saturday 26 November

As of Saturday 26th November, 225 earthquakes have been recorded in the Porangahau region since the Kaikoura Earthquake. Most of the earthquakes have been smaller than magnitude 3.

A magnitude 5.5 earthquake struck on 22nd November , 65 km south-east of Porangahau.

This is in the same area as the slow-slip event detected offshore of Hawke's Bay and Gisborne (see below).

Clusters of earthquakes, generally less than magnitude 5, have occurred multiple times in this area offshore of Porangahau, including 2001, 2006, and 2011. During the 2006 and 2011 cluster we also observed a slow-slip event in the same area (GeoNet started installing GPS instruments in the Hawke's Bay in 2002 so we don't know if the 2001 cluster was also associated with slow-slip event, though we suspect it might be).

Previous earthquake clusters in this area have gone on for weeks.

What does this mean?

Unfortunately, no one can give a definitive answer to this question as the precise linkage between slow-slip events and standard earthquakes is not well understood - this is still an area of active research. Like lots of scientific discoveries, these slow-slip events were stumbled upon while investigating something else entirely. They were first discovered in North America a few decades ago, and only discovered in New Zealand in the early 2000s when GeoNet and LINZ began installing GPS stations around the North Island.

Large earthquakes can happen any time in New Zealand, so it's essential to be prepared for them. Check out Civil Defence for advice on how to be prepared.

As always, we will update this story as the event evolves.

Contact scientist: Laura Wallace l.wallace@gns.cri.nz


GeoNet is a collaboration between the Earthquake Commission and GNS Science.

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