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Scientists here at GNS Science are still trying to unravel the details of the East Cape M7.1 earthquake. To add to this mix, both before and after this month's East Cape earthquake, there have been multiple silent earthquakes (also called slow-slip events) offshore from East Cape and the Mahia peninsula.
The recent spate of silent earthquake activity started at the end of August, around a week before the magnitude 7.1 earthquake. A silent earthquake was picked up on a few of GeoNet's GPS stations off the coast of Anaura Bay (area 1 on the map). It looked as though it was tapering off when the magnitude 7.1 quake struck, kicking the silent earthquake back into life and extended the area of movement further to the north (area 2 on the map). Judging by previous silent earthquakes in the area, this current one has moved the equivalent of a magnitude 6.5 earthquake. In the last few days, while this silent earthquake is coming to an end, another one is starting up offshore of Mahia peninsula (silent earthquakes happen so gradually it's tricky to pick exactly when they start and finish).
Auto-updating graph showing two years of movement at two of GeoNet's GPS stations at Anaura Bay and Mahia Penninsula.
The big jumps up are the silent earthquakes.
How do silent earthquakes relate to traditional earthquakes?
How traditional earthquakes and silent earthquakes interact is still a relatively new area of study - there have been many examples of small earthquake swarms associated with silent earthquakes, as well as a few examples of larger quakes and silent quakes happening around the same time. The Gisborne M6.7 earthquake in December 2007 triggered a silent earthquake. With silent quakes happening about once a year off the East Coast of the North Island, they are much more frequent than large earthquakes.
This area of New Zealand is very interesting to other scientists around the world, Japan's Kyoto University currently have sensors on the ocean bottom offshore of Gisborne. Researchers hope that these instruments will build a more complete picture of what happened in both the East Cape M7.1 quake, as well as these silent quakes. Unfortunately scientists are in for a long wait, as these instruments aren't due to be collected until mid-2017.
What are silent earthquakes?
Silent earthquakes are undetectable by both humans and GeoNet's seismographs but they can move faults the equivalent of magnitude 5+ earthquakes. A normal earthquake is over in less than a minute, but these East Coast silent earthquakes take anywhere from a week to a month to unfold. We can detect silent earthquakes only with GPS stations, which track land movement. The East Coast of the North Island is slowly pushed west. When a silent quake occurs, the land jumps back towards the east. Silent earthquakes happen at many of the subduction zones around the world - where one tectonic plate dives beneath another.
Want more info?
Have a look at the Science Learning website's animation of silent earthquakes
A small swarm of earthquakes has occurred near Kawerau in the Bay of Plenty over the last 24 hours. The busy part of the swarm lasted about 18 hours over night 19-20 September. We have recorded 20 earthquakes that are large enough to get locations in the last week. The earthquakes range from Magnitude 1.8 to Magnitude 3.1, while the depths ranged between 2 and 10 kilometres. The majority of these earthquakes are about five to six kilometres deep. Small shallow earthquakes like these will be well felt by local residents.
This swarm is typical of the earthquake activity near Kawerau. Earthquake activity in this area often occurs as a swarm which is very typical of earthquake activity in the Taupo Volcanic Zone, where swarm activity is very common.
So what is a “swarm”?
Swarms are often characterised by no one main or large earthquake, with many of the earthquakes being about the same size. Sixteen of the earthquakes in this swarm are between M 1.8 and 2.2, while the other 4 are larger. The largest is M 3.1 and occurred near the middle of the activity. Some earthquakes are so small we can’t locate them with our equipment.
Many local residents have reported feeling these earthquakes. These earthquakes are too small to be widely felt or cause any significant damage. The Kawerau area is about 25 km from Whakatane and is characterised by many active faults and a large geothermal system. GNS Science geologists have mapped numerous active faults in the area. The most famous is the Edgecumbe Fault which moved in March 1987. Many earthquakes occur in this area. GeoNet continues to monitor all activity throughout the Taupo Volcanic Zone.
A GeoNet response
At 4:40 AM last Friday morning our Geohazards Duty Officer, Bill Fry, was woken by phone alerting him to a large earthquake off the coast of the East Cape. Bill acknowledges the page and gets out of bed to review the event. At 4:45 AM, The Pacific Tsunami Warning Centre’s information statement comes out with a magnitude 7.2 earthquake located at a depth of 159 km. Something seems off, this is not what Bill is seeing on the GeoNet network. After reviewing the data he determines the earthquake is a magnitude 7.1 at 55 km depth. This shallower depth is crucial for estimating if the quake could have generated a tsunami. GeoNet’s shallower depth, coupled with a magnitude of 7.1 is right on the cusp of what could cause a tsunami. Time to contact Civil Defence and activate the Tsunami Experts Panel!
Heightened waves start coming in on the East Cape tide gauge, this confirms a tsunami was generated, but it only creates more questions... how big will it get? Which wave will be the largest? And how long will it last? This is why we have the Tsunami Experts Panel, a volunteer group of scientists from GNS Science, NIWA, Universities and industry, who assess the tsunami threat based on observations, models and their wealth of knowledge and experience. The panel then provides their scientific advice to Civil Defence.
At 8:30 AM, the main tsunami threat is over and Civil Defence could cancel the warning. This time it was a relatively minor tsunami that didn't cause any damage.
One benefit of having a national monitoring network, like GeoNet, is that it provides more accurate locations for earthquakes in and around New Zealand. We don’t have to rely on regional or global earthquake monitoring agencies like the Pacific Tsunami Warning Centre for earthquakes close to home. In fact, in this situation, inorrect information from international agencies suggested no tsunami threat was present.
Thankfully, large earthquakes like these don't happen every day, so our scientists take the opportunity to gain a better understanding of the plate boundary and how tsunami waves travel around the coastline of New Zealand.
The East Cape earthquake was a bit unusual and its offshore location poses a bit of a problem. After the Canterbury earthquakes we could send out teams of scientists to look at where the fault broke the surface and inspect damage and liquefaction. When an earthquake is under the sea we can’t do this; there are fewer clues to go on to find out what happened.
One of the biggest questions we have about this quake is simply: What happened? What motion generated the earthquake? Sounds easy and often it is but not for this earthquake. We rely on seismic waves to answer these questions; however the waves this earthquake generated are quite complicated. Also, the earthquake being located offshore means we don't have a good coverage of instrumentation surrounding the epicentre. It's like seeing a boat out at sea when you are on the shore and trying to determine whether it is travelling towards or away from you and how fast it is moving. We currently have three teams of scientists looking at different measurements to try and determine if we can learn anything more about the source of the earthquake.
- Seismologists are investigating the types of motion that could cause the seismic waves recorded on our seismometers. This is difficult due to GeoNet's network of instruments all being on land. Ocean bottom networks that rapidly supply data to scientists are only just being developed overseas and so far are extremely expensive to install and maintain.
- Geodesists are busy using GPS stations to look at how the earthquake caused the land between East Cape and Hawke Bay to move. This permanent movement is only centimetres, but it also helps to correlate with how the seafloor was moved in the earthquake.
- Tsunami scientists are looking at water height measurements from tide gauges to determine what movement of the seafloor was required to cause the tsunami and that would hopefully provide more information about the source of the earthquake.
GeoNet field technicians spent the weekend after the earthquake installing more instrumentation around the East Cape, including a new temporary site at Cape Runaway. These instruments will measure ground shaking from the aftershocks and will hopefully provide scientists with more detailed data.
GNS Science statistical seismologists are busy updating aftershock forecasts based on the additional information on aftershock frequency and size since the main earthquake last Friday.
We are also using the data recorded at tide gauges to investigate how tsunami waves behave in harbours. Like any other wave, tsunami waves interact with the coastline and seafloor geometry, which cause waves to reflect, diffract and interfere with each other. This complicated behaviour could impact how much damage a tsunami can cause in localised areas.
Needless to say, we have all been very busy here at GNS Science and GeoNet. Our work just gets started when the tsunami and initial shaking have ceased. There is still quite a lot to learn about this earthquake and the challenges we face will inform changes in the way we do research and the development of our monitoring networks.
In the coming weeks, we plan on visiting local communities around the East Cape region to give talks on what happened and provide people with the opportunity to ask questions about the earthquake.
There are very different probabilities for each scenario; some of these are more concerning than others. We recognise that while these scenarios may increase anxiety in people living in the area of the north-east corner of New Zealand, the best thing is to be prepared. Remember: If you feel a long or strong earthquake and you are on the coast, evacuate immediately.
|East Cape region long-term aftershock probabilities|
|Within 30 days||4||1 - 9||99%||0.4||0 - 2||32%||0.04||0 - 1||4%|
Issued at noon, 12 September 2016 for coming month.
Aftershock probabilities read from the table:
Within the next month:
The forecast starts from noon 12th September 2016 and is for the region of the upper East Cape and offshore (see map).
Preparing for earthquakes and tsunami
We appreciate that these scenarios may be concerning, especially scenario three. While it might seem like preparing for such large earthquakes is a waste of time, preparing for emergencies and making your home quake-safe can and does save lives.
Visit our friends at the Ministry of Civil Defence and Emergency Management and their new preparedness website www.happens.nz. Also, find out how to quake-safe your home at EQC's website. Many local and regional councils have tsunami evacuation zones maps online, so check those out if you want to know more.
About our scenarios
After reviewing and watching carefully, we’ve developed these scenarios based on our understanding of tectonics in the region, data from the current quakes, historical observations, and statistical models. The likelihood of these scenarios will change over time, based on the activity over the coming days. Be aware there is always a level of uncertainty within our models.
The magnitude 7.1 East Cape earthquake struck of the northeast coast of New Zealand at 4.37 a.m. The earthquake generated a small (30cm) tsunami, recorded on GeoNet's East Cape and Great Barrier Island tsunami gauges.
Scenarios and aftershock forecasts have been moved to a more recent story. Read them here.
'It wont happen over night, but it will happen'. It's not just a slogan for shampoo.
Another silent earthquake or slow-slip event is just getting underway off the coast of Gisborne. These silent earthquakes are undetectable by both humans and GeoNet's seismographs but they can move faults the equivalent of magnitude 5+ earthquakes. A normal earthquake is over in less than a minute, but these East Coast silent earthquakes take anywhere from a week to a month to unfold.This most recent one started around a week ago, and they happen every year or two. You can keep an eye on this one unfolding with this auto-updating graph, or by checking our 'Slow-slip watch' webpage.
Auto-updating graph of two GPS stations - Makorori and Gisborne. Note: data takes 2 days to process.
So if we can't record them on a seismograph, how do we know these quakes are happening?
We're able to record these silent quakes using GeoNet's network of GPS stations (the plot above is from two of those stations). Like lots of scientific discoveries, these silent quakes 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 (in partnership with LINZ) installed GPS stations around the North Island for mapping and surveying.
How do these silent earthquakes relate to normal earthquakes?
This is a pretty new area of study so we're always learning more. In the past we've sometimes seen swarms of small (magnitude 2 to 4) earthquakes associated with silent earthquakes that happen around the periphery. It's likely that a magnitude 4.0 quake offshore of Gisborne last week is related to the current silent earthquake.
There is worldwide interest in studying our subduction margin (where one plate dives beneath another). In terms of logistics, it's one of the easiest to study, as it's close to shore. Another factor that draws international researchers, is GeoNet's policy of making all data freely available to everyone. Japanese researchers from Kyoto and Tohoku Universities are currently out on a research vessel retrieving instruments that have been recording for a year, as well as installing more instrumentation to look at how the seafloor moves and deforms over time.
Want to know more? We've written lots of stories explaining what we know about silent earthquakes. Gisborne silent quake in 2014 has some great detail.we wrote during the last
How common are these silent earthquakes?
Silent earthquakes happen every few years in this part of the country. In New Zealand, they also happen in Hawke's Bay, Manawatu, and Kapiti. The Manawatu and Kapiti events are generally bigger and longer-lived, but happen less frequently (every 5 years). Silent quakes happens in other parts of the world with subduction margins, e.g. Japan, Costa Rica, Mexico, and Cascadia.
If more happens around this current silent quake, we'll let you know!
Facts about the earthquake
A strong earthquake occurred at 2:55 pm New Zealand time, 70 km west of Te Anau, approximately 30 km deep. So far we have received 800 felt reports from Haast to Stewart Island as at 3:20 p.m.. The highest level of impact from our felt reports so far has been MMI 6 (strong shaking) in Te Anau.
The earthquake was caused by reverse faulting that is to be related to the subduction of the Australian Plate beneath the Pacific Plate.
This earthquake was felt widely throughout the lower South Island. We have received over 800 felt it reports from Steward Island to Haast.
We'll be updating this story as more information comes to hand.
One of the questions we get asked THE MOST is: when is the next big earthquake going to happen?
Well, we don’t know. What we do know is that New Zealand has a long history of big earthquakes, and in the last 200 years, big quakes have ebbed and flowed. We explore some of that history in our new video. We also explore a bit about how researchers are tackling the big research questions. Please enjoy our lovingly handcrafted movie.
Post-script: Before anyone asks, the two-horned rainbow narwhal in this video is named Stephen.
Post-post-script: No narwhals were harmed in the making of this video.
For anyone that wants to explore the 200 years of New Zealand's big quakes further, here's an interactive graphic:
Today's quakes are a good opportunity for you to try out our beta Felt Report page.
- M4.7, moderate intensity, 28km deep, 20 km west of Masterton
- M5.2, strong intensity, 27km deep, 20 km west of Masterton
The quakes were felt throughout the southern North Island and top of the South Island. We have received over 1,500 felt reports for the first quake, and nearly 3,000 for the larger quake, so far.
Both quakes occurred at a depth of about 27km, with their epicentres virtually in the same place under the Tararua Range. This means the shaking effects from these quakes were classed 'moderate' for the first, and 'strong' for the second. Apart from minor household contents damage, we do not expect any major impact from these quakes.
Other recent quakes
A magnitude M5.2 quake struck 15km west of Masterton on 12 April at a depth of 24km.
A number of small aftershocks less than magnitude M3.0 have followed the quakes, and these will continue but lessen over the coming days.
This earthquake was reported to have been felt strongly throughout the city, and generally between Cheviot in the north and Timaru in the south. The highest level of impact from our felt reports so far has been 'damaging' at the suburbs of Somerfield and Edgeware. This could include furniture and appliances having shifted, or substantial damage to fragile or unsecured objects. We have received over 3,200 felt reports at the time of writing.
There have been no aftershocks at the time of writing.
Facts about the earthquake
A strong earthquake occurred at 7:41am New Zealand time, 15km west of Masterton, approximately 24km deep. So far we have received over 2,800 felt reports, primarily south of Taranaki and Hawke's Bay through to the top of the South Island. The highest level of impact from our felt reports so far has been MM6 ("strong"), at which level small items fall from shelves and there can be movement of small appliances. Scattered reports of this intensity were received from places in Whanganui, Manawatu, Wairarapa and Wellington.
The earthquake occurred within the descending (subducting) Pacific plate. The movement was normal faulting.
Thus far there have been over 50 aftershocks, none bigger than M3.8, and it is expected this will continue but lessen throughout the day.
The initial magnitude of M3.1 soon found its way to the more correct magnitude of M5.2. A small foreshock was mixed in with the main quake, causing this to happen. Email and GeoNet quake app notifications were affected by this, sending out their notifications based on the smaller magnitude estimate.
A further small swarm of earthquakes as occurred south of Rotorua in the Te Kopia area. This swarm lasted about 6 hours on Sunday 20 March. We recorded 23 earthquakes that are large enough to get locations. The earthquakes range from Magnitude 1.3 to Magnitude 3.0, while the depths ranged between 3 and 8 kilometres. The majority of these earthquakes are about four to six kilometres deep.
This swarm is similar but smaller than the one near Waikite Valley on from 21 to 25 February. Swarm activity like this is very typical of earthquake activity in the Taupo Volcanic Zone, where earthquake swarm activity is very common.
So what is a “swarm”?
Swarms are often characterised by no one main or large earthquake, with many of the earthquakes being about the same size. Ten of the earthquakes in this swarm are larger than M 2.0, while 13 are smaller. Some earthquakes are so small we can’t locate them with our equipment currently.
Local residents have reported feeling these earthquakes. However, these earthquakes are too small to be widely felt or cause any significant damage. The Waikite Valley-Te Kopia area is about 20-30 km from Rotorua and is characterised by many active faults. GNS Science geologists have mapped numerous active faults in the area. The largest is the Paeroa Fault. It can be traced for over 25 km and has a vertical offset of over 500 m. Many earthquakes occur in this area. GNS Science continues to monitor all activity throughout the Taupo Volcanic Zone.
We’ve continued our analysis of the earthquakes from last night and the location has moved back to the original review from the Duty Officer. This means the M 5.0 earthquake is shallower, as per the original review (10 kilometres deep), which was completed within eight minutes of the earthquake occurring. Another earthquake, shortly after the M 5.0, which was a M 4.7, shows that both were strike-slip earthquakes, meaning there was a side-to-side movement. We’ve already had a number of aftershocks and these will continue but likely won’t bring any damaging shaking.
Map of Quakes
We’ve sat down and chatted with two of our paleo-seismologists, Russ Van Dissen and Robert Langridge, who are experts in the faults of the North Canterbury and Marlborough region. Current research studies are being undertaken on these faults in Northern Canterbury and Marlborough Fault system by Russ and Rob as part of a joint New Zealand and US-funded NSF project that looks to understand the role of the Clarence among three other major faults in Marlborough.
“North Canterbury is historically and geologically very active when it comes to earthquakes. We’ve got a number of large, active faults running through that part of the South Island,” says Rob Langridge.
Bill Fry, one of our seismologists says this area is known as the transition zone.
”This area is known as the “transition” zone from the Hikurangi Subduction Zone to the Alpine Fault system. The tectonics in this area are active because of the change between these two systems. Because of this transition zone, earthquakes this size and larger are relatively common and are spread over a wide range of depths.”
A Clarence Fault landscape slightly east of the epicentre. Photo: Rob Langridge
The earthquakes have occurred near the Clarence Fault. However, due to the low number of seismic stations in this area and the uncertain location of the Clarence Fault at depths greater than a few kilometres, we cannot confirm that the earthquakes happened directly on this fault but it is the closest larger fault near the epicentre.
Increased earthquake activity is likely in or near this area until the sequence decays. We should take this opportunity to always remember that when an earthquake happens, the chance of another earthquake in the same region increases. However, earthquakes can happen anywhere in New Zealand. If you live in New Zealand, the best thing you can do is be prepared for emergencies. Here is more information about how to prepare:
Have quakes been increasing?
We're hearing this question more and more, so I thought I'd pull together our quake numbers into a few graphics and let the numbers speak for themselves. January and February this year are on par with many other previous months. There has not been a substantial increases in quakes, especially when you look at quakes about magnitude 3.
Quakes I looked at:
I looked at quakes close to New Zealand from January 2012 to March 2016, and broke them down by their month, magnitude, and depth. You can see that there are fluctuations in quake numbers over the years. Also obvious is the impact that large, shallow quakes - like Seddon and Cook Strait - have on overall quake numbers, as they produce aftershocks. The circles in the bottom left show the relative numbers of quakes by magnitude. The amount of magnitude 2 and smaller quakes make up the vast majority of the quakes we record. The real number of quakes smaller than magnitude 2 will be much higher than this - we know our network can't record them all.
The United States Geological Survey recently looked global earthquake numbers and found they were on par for 2015.
Unfortunately, this is my first experimentation with this graphic application and it turns out it doesn't work very well on mobile (which is commonly a large percentage of our readers! sorry guys, you can click on the mobile link below).
A small swarm of earthquakes, which began on 21 February, has been recorded in the Waikite Valley, near Rotorua. Most of these earthquakes are currently occurring in Waikite Valley, about 20 kilometres south of Rotorua. So far, there have been more than 50 earthquakes. The earthquakes range from Magnitude 1.0 to Magnitude 3.4, while the depths ranged between 5 and 8 kilometres. The majority of these earthquakes are about six to eight kilometres deep.
There have been two earthquakes of Magnitude 3.4, the first on 21 February at 3.26 am and the second on 25 February at 11.01 am. This is very typical of earthquake activity in the Waikite Valley area, where swarm like activity is common.
So what is a “swarm”?
Swarms are often characterised by no one main or large earthquake, with many of the quakes being about the same size. Only eight of the earthquakes in this swarm are larger than M 2.0, while 26 are between M1.5 and M2.0. Some earthquakes are so small; we can’t locate these with our equipment currently.
Several local residents have reported feeling these earthquakes. However, these earthquakes are too small to be widely felt or cause any significant damage. The Waikite Valley area is about 20 km from Rotorua and is characterised by active faults. GNS Science geologists have mapped numerous active faults in the area. The largest is the Paeroa Fault. It can be traced for over 25 km and has a vertical offset of over 500 m. Many earthquakes occur in this area. This swarm is highly unlikely to be associated with volcanism - GNS Science continues to monitor all activity throughout the Taupo Volcanic Zone.