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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. 


20/01/2017 1.30 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 a lot since the last forecast.  There is now a 25 percent chance of one or more M6.0-6.9 earthquakes occurring within the next month; this has decreased from 54 percent from our last forecast (19 December 2016).  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 25% 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.

What’s happened so far

We’ve had a total of 11956 earthquakes since the M7.8 Kaikoura earthquake stopped shaking our islands (we ran the numbers at 10 a.m. on the 19 January, so these will change). 452 of those earthquakes were M4-4.9, 53 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 January 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. Our previous forecast, from 19 December, is here

 

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

3.2

0-9

89%

0.3

0-2

25%

0.03

0-1

3%

 

within one year

16.6

7-29

>99%

1.5

0-4

77%

0.1

0-1

12%

 

Forecast for rectangular box with the coordinates -40.7, 171.7, -43.5, 171.7, -43.5, 175.5, -40.7, 175.5 at 12 noon, Thursday, 19 January. * 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 25% chance of one or more M6.0-6.9 earthquakes occurring within the next month. 

The current rate of magnitude 6 and above earthquakes for the next month is about 10 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.

Why has the forecast dropped so much?

There are two reasons to explain the drop in expected numbers: A month has passed since our last update and we have included another model into our forecast mix. The longer a sequence goes on, the more information we have (based on location, depth and size of earthquakes) to improve our forecasts based on what we are seeing from the Kaikoura earthquakes. We know more now and, as time goes on, we’ll continue to refine the models further to give you the best information we can. The more we observe, test, develop, and refine, the better we can forecast what happens next.

For example, for the 19th December forecast we estimated 8.4 earthquakes of M5.0-M5.9 within the next 30 days. With the updated model, that number for December would have been 5.2 earthquakes in the M5.0-M5.9 range; this was closer to the actual observed earthquakes of that size, which was three.  

Scenarios

The scenarios specifically address the probabilities of what we might see happen within the next year and were estimated in mid-December 2016 (this differs from the forecast table above which is more recently updated). The scenarios cover a wider geographic area than the aftershock probability forecast area. The probability numbers in the table above are slightly different to the scenarios.  This is because 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 January 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. 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

Monday morning’s M7.8 earthquake near Kaikoura has caused massive permanent displacement of the land in the northern half of the South Island (GeoNet Story on Coastal Uplift). Detailed knowledge of these land displacements provide us with critical clues to help determine which faults ruptured during the earthquake, and how much movement occurred on them. Although many GNS and GeoNet scientists are currently “on the ground” trying to obtain information about the earthquakes and its impacts, sometimes we can get an even better picture of what happened in the earthquake by stepping back and viewing it from space. 

To do this, we must turn to specialized satellites that collect radar data that can be used to track these land movements in great detail. A technique called InSAR, which stands for ‘Interferometric Synthetic Aperture Radar’, utilises radar satellites orbiting ~700 km above the earth to precisely measure the distance between the ground and the satellite. If the ground moves between two subsequent satellite passes, due to an earthquake or volcanic eruption, then the distance between the ground and the satellite changes. Observing these changes in the positon of the land with InSAR enables us to generate detailed maps of ground movement, often with centimeter-level accuracy.

The InSAR images that have been coming out from the European and Japanese satellites are astonishing, and give us the most detailed view yet of what happened in the M 7.8 quake.  The satellite images reveal huge changes in land movement across the Hope and Kekerengu faults, as well as several other faults in the region.  To the east of these faults, the land went mostly southwest (see blue area in the figure on the left). In contrast, to the west of these faults the land moved mostly northeastwards (see red area in figure on left). Sharp changes in land movement are visible on the InSAR images, and show us where the faults ruptured to the Earth’s surface.

The satellite images clearly show that Monday’s M7.8 earthquake is one of the most complex earthquakes that has ever been observed. Consistent with observations from geologists of fault displacements on the ground, the InSAR results suggest that the earthquake ruptured at least four different faults, and probably more. The biggest displacements are seen on the Kekerengu, Hope, Hundalee, and Papatea faults.

Scientists at GNS have done computer simulations to replicate the ground displacements observed by the satellites and GPS measurements (GeoNet Story on GPS). To fit the InSAR and GPS displacement data, the computer simulations require up to 10 metres of slip across the Kekerengu fault north of Kaikoura. Our scientists are working hard to pull all of the details that they can out of these exciting and important satellite images to better inform our understanding of what happened in the massive quake.

 

 

A perspective 3-D view towards the east coast of the South Island of the results from the computer simulation to determine the amount of movement on faults in the earthquake.  The model shown fits the InSAR and GPS displacements well. The faults are shown as a mesh of rectangles. The colors show the amount of slip on the faults in the model (in meters).  Some portions of the faults (representing the Kekerengu fault) accommodated up to 10 metres of slip

 

Contact Scientist: Ian Hamling, i.hamling@gns.cri.nz


After the Kaikoura Earthquake, the GNS landslide and paleoseismology teams were quick to take to the air on the 14th November to find evidence of the earthquake effects in the landscape. The team were fascinated by the sights they saw from the helicopter. The effects were widespread over an area of 7,000 km2 with estimates that there has been between 80,000 to 100,000 landslides triggered by the earthquake and subsequent aftershocks.

Along the coastline damage became apparent south from Cape Campbell, becoming more intense south of the Clarence River, with several large (100,000 – 500,000 m3) landslides disrupting both SH1 and SIMT railway (e.g. Ohau Point and Waipapa Bay areas). South of Kaikoura, on the steep slopes near Twin Tunnels, damage was similar to areas north of Kaikoura, again covering and or disrupting both SH1 and SIMT. 

Landslide Dams

Since Tuesday, GNS has been working to compile a complete inventory of landslide dams in the area affected by landslides after the earthquake. This work has two parts. The first involves using remotely sensed imagery to locate landslide dammed lakes in inland areas. The second involves a systematic search of vulnerable areas. 

A satellite image from WorldView-2 image was received and processed. This image covers part of the area north of Kaikoura and has identified 12 landslide dams. Only one was known prior to processing this image.

GNS has also started systematically flying landslide affected areas to visually identify landslide dammed lakes. The southeast flank of the Seaward Kaikouras, was flown on the 16th November and seven landslide dams were found. These flights will continue over the next three or four days. The purpose of these flights is to systematically cover affected areas where no data is currently available. The second purpose is to photograph the landslide dams to allow assessment of the hazard at each site.

As the data comes in it is being forwarded to NIWA to undertake dam-break modelling, which will help determine what will happen if the dam-breaks. From this people and assets at risk can be identified.  

 

Safety around Landslides

Please take precautions and stay away from regions of landslides and landslide dams. Avoid rivers or streams that have been blocked by landslides. Stay away from steep cliffs and slopes in the Kaikoura and Marlborough region. 

Director of the Ministry of Civil Defence and Emergency Management, Sarah Stuart-Black, advises that people in those areas to be especially vigilant and to keep clear of river valleys and outlets. Landslide dams can break quickly, and release large volumes of water and sediment as a flood wave. Homes at potential risk have been advised to evacuate.

For more information from the Ministry of Civil Defence and Emergency Management, go to http://www.civildefence.govt.nz/.

 

GeoNet, with funding from LINZ (Land Information New Zealand), operates a large network of continuously recording GPS sites in New Zealand to track land movements on a daily basis.  Within a couple of hours of the M7.8 earthquake, GeoNet was able to use the GPS data to estimate the initial displacements of the Earth’s surface that occurred during the earthquake.  Tracking these types of land movements as the result earthquakes is a critical piece of the puzzle needed to determine which faults ruptured, and by how much.

Shifting of the land near the earthquake

 

 

What the GPS revealed was astonishing. It turns out that the earthquake shifted the land at Cape Campbell (the northeast tip of the South Island) to the north-northeast by more than 2 m, and up vertically by almost 1 m. This means that Cape Campbell is now more than 2 m closer to the North Island than it was before the earthquake. Similarly, Kaikoura has moved to the northeast by nearly a metre, and has been lifted upwards by 70 cm. Hanmer Springs, which was our closest GPS site to the quake epicentre, jumped eastward by approximately 50 cm. All of this movement happened during the earthquake in a matter of seconds.
Movements of the Earth’s surface near the earthquake recorded from GPS have been incredibly important to help diagnose what motions were involved in the earthquake. A key observation is that although the earthquake fault rupture began near Culverden, by far the largest motions of GPS sites occurred at Cape Campbell. This supports the idea that the ruptured north over a very long distance from where it started.

A lot of New Zealand has moved

 

Not only did the earthquake shift landmasses in the northern South Island, but it also caused movements across most of the country.  In the lower North Island, the east coast has shifted west by 1-5 cm, while the Wellington and Kapiti regions were shunted 2-6 cm to the north. Christchurch and Banks Peninsula didn’t miss out on the action, either—they are now approximately 2 cm further south than they were the day before the quake. Some parts of the west coast of the South Island have been shifted eastward by as much as 10 cm. The northern North Island and southern South Island only moved a few millimeters.

 

Updated 1pm, 21/11/2016

UPDATE Monday 21st November

Over the weekend we conducted further reconnaissance trips via helicopter to refine how the Kaikoura Earthquake uplifted areas of the East Coast of the upper South Island. This is a map of preliminary coastal uplift measurements along the Kaikoura coastline from the Conway River to Cape Campbell. We have compiled three sets of uplift observations for this map. The most accurate information is from the tide gauge (green dot), but we only have one tide gauge in the area, located at Kaikoura. The measured points (red dots) show locations where GNS Science and University of Canterbury teams surveyed the coastal uplift. Biological markers of pre-earthquake tide levels were surveyed relative to the post-earthquake tide levels. The range in values reflects the elevation range of the biological markers and the uncertainty in surveying methods and the tidal correction. The helicopter points (blue dots) represent areas where we estimated the amount of uplift from our helicopter observations during high tide and low tide survey flights, these have the largest amount of uncertainty. Survey teams will revisit the area again this week to increase the density of our observations.

Our preliminary insights from this work is that coastal uplift is highly variable along the Kaikoura coastline. 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. The surveys teams would like to thank the residents of the Kaikoura coastline who have been so welcoming to scientists, hearing the resident’s observations of the coastline and the impact of the earthquake has been hugely valuable and we hope that our post-earthquake science survey can contribute to our knowledge base of these very rare, but hugely impactful events.

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. Scientists plan to find out more about this coastal uplift.

What is coastal uplift?

Coastal uplift is when the land is raised above the sea by tectonic forces. It can happen gradually over geological timescales or suddenly by an earthquake.

How does it happen?

Sudden coastal uplift happens as the result of large earthquakes. Vertical movement on a fault can cause land to be pushed up. This is the type of movement that has built many of the mountain ranges in New Zealand. When such vertical movement on a fault happens near the coast, land is raised above sea level. Sea level is a powerful horizontal marker for measuring tectonic movement because it is very obvious which land used to be under water before the earthquake!

 

 

Is coast uplift normal?

Coastal uplift is normal for large earthquakes near the coast that include some vertical movement on a fault. Most movement on the faults that ruptured in the Kaikoura M7.8 earthquake was horizontal, but there was some vertical movement too, so it is not surprising that there has been some coastal uplift from this event.

In New Zealand there is evidence preserved in the landscape that shows many parts of the coast have been repeatedly uplifted through time. For example, raised marine beaches and terraces along the Kaikoura Peninsula, Wairarapa coast, Cape Kidnappers, Mahia Peninsula, north of Gisborne and East Cape, are evidence of former beaches that were uplifted from the sea by earthquakes in prehistorical times. Many of these have been or are currently being studied to find out the size and age of past earthquakes.

 

 

Historical examples

In historical times we have had several examples of earthquakes causing coastal uplift that, although devastating in the short term, have led to some benefits. For example, in 1931 the M7.8 Hawkes Bay earthquake raised land around Napier by 1-2 m above sea level. Land that used to be estuary now provides space for Napier airport. In 1855 the M8.2 Wairarapa earthquake raised much of Wellington by 1-2 m. The road from Wellington city to the Hutt Valley became viable as a result of the new land around the harbour’s edge.

Will the land go back down?

Probably not. The newly raised coastline of the Kaikoura coast is most likely a permanent feature. Historical and pre-historical examples show that in many parts of New Zealand these raised beaches remain high above sea level. However, in some parts of the world, raised beaches have been known to gradually drop down again (over centuries) or be dropped down suddenly in a large earthquake with a different sense of movement.

What will happen to all the sea creatures?

There will be a major shift in the community of marine species. Many seaweeds and animals that would normally be permanently covered by water will now be struggling to exist in a zone of transition between air and sea. Some of the animals that have been raised will be accustomed to air exposure for short periods of time, but not the full tidal cycle that they will now experience. As the animals not suited to this environment die from the reef, they will be replaced by seaweed and animals appropriate for the new tidal level. 

What are scientists doing to study the coastal uplift and its impact? 

There are several studies being planned by geologists and biologists. Over the next weeks and months geologists will be undertaking surveys along the coastline to measure the amount and extent of coastal uplift that has occurred. Recording this information will help us to understand which faults ruptured in the earthquake, and how much slip occurred on those faults. Over the coming weeks marine ecologists and fisheries scientists will be surveying areas on the rocky shore to understand the sequence of change. The surveying will continue for in the long-term for changes to be captured.

The information collected by scientists will be publicly available.

Can the public contribute their photos and observations?

YES! The coastal uplift covers a long stretch of coastline and the scientists will not visit every location. We are interested in photographs of: 

  • displaced coastal features such as the uplifted reefs and rock platforms;
  • displaced marine life especially before and after photos if animals are being rescued and removed from the reefs;
  • uplifted man-made structures such as jettys and boat ramps particularly if they have tide level markers on them.

We are also interested in people’s observations, particularly residents who have familiarity with the coastline pre- and post-earthquake. To be of most use, we would like to know the location, date, time of the photograph and if possible, context and scale.

As always your safety comes first, please do not put yourself at risk to collect this information.

Photographs and information can be sent to: k.clark@gns.cri.nz

Citation for map: Clark KJ, Goldstein S, Villamor P, Berryman KR, Gerrity S. 2016. Preliminary coseismic coastal uplift measurements from the 2016 M7.8 Kaikoura earthquake. GNS Science. http://dx.doi.org/10.21420/G2H59W

Maps updated with additional station data 11am,  

I think we can all agree there has been a lot of shaking over the past few days. Scientists from GNS Science and University of Auckland, GeoNet field technicians and our platform team, have been busy at work collating the data to figure out exactly how much shaking occurred. While we still haven't retrieved all of data, here is what we have so far.

Ground shaking caused by the Kaikoura Earthquake reached over MMI VIII near the fault rupture, on the Modified Mercalli Intensity Scale and is considered severe. In Wellington, some areas experienced shaking up to MMI VI-VII, which is considered strong. This is based on the current Shakemap calculation, which includes both measured data and estimated values. This may be updated as more information is acquired.

PGA Information

The strongest ground shaking measured by GeoNet instrumentation so far was a peak ground acceleration of (PGA) 1.3g in Ward. Since publishing this story, data has come in from the WTMC station in Waiau with a vertical PGA over 3g. As yet, we're unsure if this is a reliable value, it may have been contaminated by other effects.

 

 

The highest ground accelerations extend in a band trending northeast from the epicentre. This aligns with where the largest fault movements have occurred, based on field surveys.

In the Wellington region, PGAs exceeded 0.2g in parts of the CBD and Lower Hutt. This is similar to that experienced during the Cook Strait sequence (earthquakes of M6.6), however the duration of shaking was significantly longer (around four times) and there was more long period energy.

In Christchurch, the recorded PGAs were lower (less than 0.1g) even though the city is closer to the epicentre. This is related to the fault rupturing from the epicentre northwards away from Christchurch. This is our understanding based on what we know so far about the faults.

We are still acquiring strong motion data from the closest stations to the rupture.

 

 

Shakemap

Shakemap is a combined model of the estimated ground shaking based on instrumental observations from GeoNet sites and ground motion predictions based on the current fault models. As we get more information from InSAR (Interferometric Synthetic Aperture Radar) the fault models are likely to be updated.

Shakemap also takes into account soil conditions that can amplify shaking in certain areas. This can be seen on the map as areas experiencing higher shaking despite being further from fault rupture.

 

Before the foul weather set in, a team of GNS Science geologists, Nicola Litchfield and Pilar Villamor, went out on helicopter flights trying to pin down exactly which fault lines ruptured in the Kaikoura Earthquake.  Their observations confirmed what our seismologists have been saying – that this was a very complex earthquake. On their trip they observed several different faults rupturing the land. They mostly followed State Highway 1 as we flew down the east coast of the upper South Island. Roads are very useful markers for telling how much a fault has moved.
 

The trip in their own words:

 

Kekerengu and Waipapa Bay

“The first fault rupture we saw was the Kekerengu Fault. The fault shifted State Highway 1 and the railway line by at least 2.3 m horizontally and 1 m vertically. We followed the fault rupture from the beach inland for 2km to Bluff Station. Here, the earthquake had dislocated hills, fences, roads, buildings and the river bed, some by as much as 10m horizontally. A house was also spectacularly moved off its foundations - the occupants were shaken up, but otherwise okay. This was as far as we flew along this fault so the surface rupture could well have continued past this point.

We continued flying south. At Waipapa Bay the road and railway line were shifted by 1m. Further south, we came to the Hope Fault. From the air it looked like this has offset the road by about 1m. We didn’t follow the Hope Fault inland on this flight, but we’ve heard from other scientists who did fly along here earlier, that they didn’t see any disturbance of the land

 

 

Hundalee, Emu Plains, and The Humps fault zone (yes, someone long ago named a fault 'The Humps')

Then it was on to the Hundalee Fault. Again, we saw the fault had disturbed State Highway 1 and the railway line as well as the beach south of Kaikoura.

We then flew to the epicentre area (The Humps fault zone and Emu Plains) and found some fault ruptures on the north side of the Waiau River, some on previously unknown faults. It was less clear exactly how these faults had moved compared to the others further north, but it looked like some horizontal movement had occurred. At the western end these ruptures had affected some small streams.”

Field trips like this one can only find where fault lines have broken through to the surface. It’s likely that other faults below the surface (or longer lengths of the faults mentioned above) have also moved. Other investigations including satellite images will help to show any subsurface fault movements.

What Next?

Now that the wind and rain have subsided, additional teams are out in the field - they’re hoping to cover areas that Nicola and Pilar didn’t get to.


Fault Map by: Litchfield NJ, Barrell DJA, Begg JG, Benson A, Berryman KR, Clark KJ, Cochran UA, Cox SC, Gasston C, Glassey, PJ, Heron DW, Howarth JD, Langridge RM, Little T, Lukovic B, Nicol A, Pettinga J, Ries WF, Rowland J, Stirling MW, Townsend DB, Upton P, Van Dissen RJ, Villamor, P. 2016. 14th November 2016 Kaikoura Earthquake. Preliminary earthquake geology observations. GNS Science. http:dx.doi.org/10.21420/G2MW2D


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