View this blog in blog mode
Looking over the past year, there have been several bursts of activity in this earthquake swarm:
- June 10 to July 3, max M3.7
- November 22 to November 29, max M3.9
- February 23 to the present, max M4.9
The latest activity has provoked more interest, as it features the two largest magnitude quakes. They show in the accompanying map as a cluster, featuring two yellow (“strong”) quakes. The earthquakes show normal faulting, which is typical in this offshore part of the Taupo Volcanic Zone, and is due to the spreading of the crust.
We have been asked whether these quakes are related to White Island. Apart from using White Island as the reference point for the location, there is nothing to suggest that these quakes are changing activity at the volcano, especially as they are typically 100 km away from the island. The earthquake shaking at this distance would not be likely to upset the volcanic system. People in the Coromandel, at a similar distance away, have only reported light shaking for the bigger quakes. There are several undersea volcanoes north-east of White Island and these earthquakes are closer to those than they are to White Island. The earthquakes are at least 50 km away from these other volcanoes and our volcanologists don't think they are likely to be affected by the shaking either.
Swarms are a feature of the Taupo Volcanic Zone. Some may only last hours, but others like that near Matata in the mid 2000s, went on for many months, with distinct bursts of activity. This northern Bay of Plenty swarm is one of those.
At the time of writing nine quakes likely to have been felt in Gisborne have occurred. All have been between magnitude 2.5 and 2.9, but at noon today one of magnitude 4.5 interrupted this earthquake swarm. This activity is not unusual, but its proximity to a big population centre will have raised local interest. The cluster of grey epicentres are shown on the map at right, with the magnitude 4.5 shown in green (classed as "moderate").
A magnitude 4.9 earthquake occurred north of this area on December 20 2014.
Our seismologists have been busy crunching the numbers regarding the Wilberforce Earthquake. Here are the probabilities for the aftershock sequence related to the Wilberforce Earthquake:
Within 30 days
|0.06||0 - 1|
|0.005||0 - 1|
|0.0003||0 - 1||<1%|
|Within 1 year||0.46||0 - 2||37%||0.04||0 - 1||4%||0.003||0 - 1||<1%|
This table shows a forecast based on a model from international expert elicitation.
Updated 08/01/2015: Due to this sequence, the probability of a magnitude 5 or above in the next year in this region is approximately one and a half times greater
All forecasts start 6th January 2015 and are for the region from 170.5-171.9 degrees East and 42.4-43.75 degrees South.
* 95% confidence bounds.
Geographic area included in the model
Landslides around the area
Our landslide reconnaissance team has been out and about in helicopters checking out the Southern Alps, near the epicentre of the Wilberforce Earthquake. They came back with some great photographic evidence of rockfalls and landslides. The photos show small to moderate size rock
avalanches close to the Main Divide in the upper reaches of the Unknown Stream catchment (we think this is an excellent name for a stream!). Although these were not observed at the time of the earthquake - because no-one was in the area to our knowledge - their appearance on the snow-pack indicates they are only a few days old at most. As there are several of these, it is a good indication of a common trigger. As there has been no significant rain in the area this only leaves shaking from the Wilberforce earthquake as the cause.
About the name Wilberforce
Our large earthquakes get names based on the closest geographic feature or a relevant city/township. This earthquake was located in a very remote area so we named it after the Wilberforce River. That might be disappointing to some of the more literary minded people out there who thought we might be making a witty reference to the antagonists in classic kiwi novel "Under the Mountain" but...we weren't. Though we always appreciate a good book! Right, back to the science.
The Alpine Fault and the Wilberforce Earthquake
We've received a lot of questions about this! You can see that we addressed this in the scenarios below but we've got a bit more science regarding the stress levels on the Alpine Fault. Essentially, the Alpine Fault did show some change in stress levels from yesterday's earthquake: in some areas this decreased and in a small area, it increased. GNS Science has issued a media release regarding the change in stress levels.
Based on our understanding of tectonics in the area, data currently available from the sequence, historical observations, and statistical models, our seismologists considered that there are three possible scenarios that could occur over the following weeks. There are very different probabilities for each scenario. Remember, the best thing is to be prepared for whatever happens next.
Note: Scenarios are based on information that is currently available. We will closely monitor on-going earthquake activity and update our scenarios if it is required.
Scenario One - Very Likely (within the next 30 days)
The most likely scenario is for aftershocks to continue to decrease in frequency, with no future large earthquakes. This is consistent with normal aftershock behaviour. We expect aftershocks over a slightly larger region than where aftershocks have already happened. Again, it is very early in the aftershock sequence and we will know more as this sequence develops.
Scenario Two - Very Unlikely (within the next 30 days)
A very unlikely scenario is that future earthquakes similar to the M6.0 Wilberforce Earthquake (link) may occur within the general region of the main shock. Large earthquakes are not surprising in this area; it is historically a very seismically active place. In fact, it has been affected by a handful of large earthquakes during the last century. These include at least four earthquakes of M6 or greater:
This magnitude 6.2 earthquake with an epicentre near Lake Coleridge was felt over the greater part of the South Island. It was preceded by two foreshocks and followed by numerous aftershocks, the largest of which had a magnitude of 5.8. These persisted until the end of 1949.
Aftershocks for the Wilberforce Earthquake
This magnitude 6.7 earthquake occurred in the Southern Alps with its epicentre 10 km from the township of Arthur’s Pass. The earthquake was the largest in a sequence of earthquakes in the central South Island, which began with the 1992 M5.8 Wilberforce River (yes, Wilberforce is a popular area for earthquakes!) earthquake and was followed in May 1995 by a M6.1 earthquake less than 10 km to the east, and in November 1995 by the M6.3 Cass earthquake located 30 km to the east of the Arthur’s Pass main shock.
Scenario Three - Extremely Unlikely (within the next 30 days)
An extremely unlikely but possible scenario is that the Wilberforce sequence will trigger a larger magnitude quake - a magnitude 7 or greater –on another fault (e.g. the Alpine Fault). The Alpine Fault is one of the most active crustal faults on Earth. It is already known to have a high probability of rupture over the next 30 years; however it is unlikely that the occurrence of the Wilberforce earthquake has greatly increased this hazard.
About the location of the 6.0 Wilberforce Earthquake
This part of the country is no stranger to strong earthquakes. In the last 100 years, there have been several similar sized and located earthquakes (see above). The tectonics in the area are dominated by the Alpine Fault, where the Australian and Pacific tectonic plates meet, pushing together to form the mountain ranges of our picturesque Southern Alps. Surrounding the Alpine Fault there are numerous known and unidentified faults which, along with the Alpine Fault, take up stresses from the convergence of the Pacific and Australian plates. Find out more about why New Zealand is so shaky here.
Last update: 08/01/2014
Time: 1.45 p.m.
The initial location and magnitude were affected by a small (about magnitude 2.0) foreshock just five seconds prior to the main earthquake. This skewed the location further to the east, which also confused the magnitude estimation.
Our duty seismologist has confirmed that this earthquake was not associated with the Alpine Fault. The Alpine Fault is more than 20 kilometres west of the location of this earthquake. We do not currently know which fault the earthquake was on; it may be one of several already identified faults or a previously unknown fault. You can find out more information about why we may not know a fault exists until an earthquake occurs here.
The intensity of this quake is considered severe at the location. As of 12:30 p.m., there have already been more than 3,000 felt reports from as far south as Invercargill to the various places mainly in the lower North Island. There have been dozens of aftershocks located so far since the initial earthquake. The largest aftershock so far has been a magnitude 4.7. In typical aftershock sequences where the mainshock is magnitude 6.0, we can usually expect the largest aftershock to be up to magnitude 5.0.
The best advice during an earthquake is DROP, COVER AND HOLD. The Ministry of Civil Defence and Emergency Management has more information about how to respond during an earthquake.
The map to the left shows previous earthquakes of magnitude 5.8 or larger in this part of New Zealand since 1940. The epicentres from the Canterbury earthquakes are shown at right. The five nearest quakes shown are:
Updated: 12:30 p.m, Tuesday 6 January 2015
The Ministry of Civil Defence and Emergency Management have stated this earthquake has not generated a tsunami.
Bill Fry, our duty seismologist, says that some aftershocks have already occurred but future aftershocks are unlikely to make much of a felt impact on land. It is still too early to tell exactly how the sequence will progress however the most likely scenario is the aftershocks will decay in frequency and be hardly noticeable on land. So far, we've had nine earthquakes in the area that are probably associated aftershocks.
About the earthquake: Current information suggests that the earthquake was located below the mega thrust fault, which is separates the Pacific and Australian plates. This is a highly active region so earthquakes of this size and location are not surprising.
Felt reports update: Almost 3,200 felt reports have been cataloged throughout New Zealand . We now have felt reports from Dunedin, as far east as the Chatham Islands and as far north as Whangarei.
Ghost quakes have already been reported in our system, the most noteable one was a 4.8 reported in Hanmer Springs. This has been removed from our system. Any felt reports related to the Hanmer Springs ghost quake will be reassigned to the 6.5 earthquake.
(Last updated at 4.12 p.m., 17/11/2014)
(First published at 11.50 a.m., 17/11/2014)
So far, 50 people have reported feeling this earthquake.
For those of you interested if it is of volcanic origin, it is a tectonic earthquake.
The last earthquakes in Auckland that were widely felt throughout the region was 17 March 2013, when two earthquakes shook up the area. This was the most "felt" earthquake in GeoNet history with 13,917 reports.
Here are the top earthquakes in or close to the Auckland region over the last 150 years:
The first earthquake was a magnitude 3.6 on the 5 November at 3:37 p.m. (link); we received 275 felt it reports for this earthquake. The largest earthquake so far was a magnitude 3.9 that occurred at 3:43 a.m. on the 7 November (today) and 700 residents throughout the Bay of Plenty and Waikato region reported feeling the quake. These moderate earthquakes have been followed by several smaller ones. We've had only a small number of earthquakes in total; six in two days. Swarms are common in this area; we had a larger swarm in 2005, approximately 30 kilometres north east of this week’s earthquakes.
This 2005 swarm near Te Aroha was a lot more active, with more than 30 earthquakes which went on for several weeks.
Q. How frequently do we get SSEs in the Gisborne area?
SSEs large enough to detect occur about every 18 months in the Gisborne area, although these can vary in size a bit. The last SSE of this size in the region occurred back in March 2010, south-east of Gisborne and north-east of Mahia peninsula.
Q. Do you think this SSE will cause more earthquakes? Should we be preparing for more earthquakes in the region?
SSEs sometimes trigger multiple magnitude 2 to 4 earthquakes around their periphery; but in the last two weeks there hasn't been much significant seismic activity observed for this new SSE. Having said that, the science regarding SSEs is relatively new; we've only been aware of this
phenomenon for the last decade. The best advice regarding this event is to be prepared for earthquakes anyway; we can get large earthquakes anywhere in New Zealand, it’s just that some areas are more likely than others to get these. You should always be prepared for earthquakes.
Q. On the flip side of the previous question, does an SSE mean that we might have fewer earthquakes? Does it relieve the pressure on the faults?
Based on our current understanding: yes. The SSEs do relieve stress in the areas where they occur on the thrust interface, approximately 15 km below the earth’s surface, as they represent large-scale creep on the thrust fault.
Having said that, SSEs may transfer stress to surrounding areas and potentially trigger earthquakes on their periphery; what the size and strength of those potential earthquakes would be is impossible to predict at the moment.
Q. How much land movement are we talking about here? Should we be purchasing more beach front property in Gisborne with the hopes of getting extra land out of it?
Well…up to 3 cm can possibly be displaced sideways, which is about the length of a pineapple lump (it's before lunch and I'm hungry). This is mostly an eastwards direction; but less than that in the vertical direction. To compare to the previous SSE in Gisborne in March 2010, some of the GeoNet sites experienced horizontal shifts of up to 5 cm. This new SSE looks smaller than that 2010 SSE. In other words, we are very slowly moving towards Chile's coast. I'm kidding. Mostly. I mean, we might get there in about a billion years or so...never mind.
This means some beachfront property would move slightly towards the dateline but it wouldn’t be much vertically. Beachfront property in Gisborne would likely be unaffected by this SSE.
Q. How do we know when an SSE is occurring?
There is a network of continuously recording GPS stations across New Zealand, which is operated and monitored by GeoNet, which is funded by EQC, GNS Science and Land Information New Zealand (LINZ). These geodetic stations are very sensitive and can detect changes in position of the order of millimetres. When a SSE occurs, several GPS stations, (which are roughly 20 km apart) are displaced at the same time, both horizontally and vertically. From examining the GPS data we can then calculate the amount of slip involved for that SSE.
Q. So why are studying SSEs so important?
Hopefully, by observing this phenomena, we will gain important insights into why earthquakes occur, where these occur most frequently and how earthquakes are related to each other.
Q. How long ago was the last earthquake in Dunedin?
A. In 1991, a 4.1 struck off the south coast of Dunedin, near an area that had previously experienced a few Magnitude 4 earthquakes.
Q. What was the largest earthquake since 1960 in the Dunedin area?
A. The largest earthquake to occur in the area since 1960 was a 4.9 in 1974, which occurred in a similar area to the 1991 earthquake (see below).
Q. Have we had earthquakes in the same area as last night’s?
A. The earthquake that occurred last night was very similar to one that occurred in 1982; it was in almost the same location and depth.
Q. It felt more intense in some places. Why is that?
A. This earthquake occurred at a depth of 4 km. That makes it very sharply focused but only over a small area. Think of a torch shining down on a table top. If it is close to the table top, the light is strong and sharp - that's like a shallow quake, which feels like a sharp jolt. If the torch is moved further away, the light is more widespread but diffused – that’s like a deep earthquake: widely felt but more wobbly.
Here is a list of historical quakes in Dunedin from 1960 to today:
Earthquakes in the Fiordland region are due to the collision of the Australian and Pacific plates. The location and size of this one is almost identical to an earthquake on December 23 2013, nearly ten months ago. Both earthquakes were reverse faulting or thrust mechanisms resulting from the Australian plate pushing (subducting) beneath the Pacific plate. Fiordland sits upon on the Pacific plate.
The Fiordland area is a very productive region for big earthquakes, but its remoteness means that the shaking has usually lost its damaging capability by the time it has reached the nearest localities such as Tuatapere, Queenstown and Haast. Within 20 minutes over 600 people had reported the earthquake, with no more than moderate shaking. The reports were largely confined to the southern part of the South Island.
Other recent notable quakes in this region were:
- Jul 16 2009 - Fiordland quake biggest for 80 years
- Oct 16 2007 - Fiordland shaken again
- M 7.2, Fiordland, 22 August 2003
As shown on the accompanying map, it occurred a little to the east of the epicentre of the M6.0 Eketahuna quake earlier this year. It is the third earthquake of magnitude 5.0 or more to strike this region of New Zealand, with the third one occurring at the end of March in the vicinity of Waipukurau.
Over 2,000 felt reports had been submitted within the first hour of the quake, but with only one report of any damage at Terrace End, in Palmerston North, by that time.
Its epicentre close to the January shake is the main scientific point of interest. There had been very few, and only very minor, aftershocks by 4 am, 100 minutes after the earthquake.
What is a “ghost quake”?
“Ghost” quakes appear on our network typically after a large regional source earthquake. We have very sensitive seismic equipment that picks up the various waves that earthquakes create and we can pick up these waves even if it is very far away. For example, we also picked up seismic waves from the Alaska quake this morning. Our equipment gets confused by these waves and interprets these as being a smaller, locally-sourced earthquakes close by.
Why do we get “Ghost Quakes”?
These quakes are an unfortunate side effect of getting information out to the public as quickly as possible, instead of waiting up to a quarter of an hour for a person to locate and ensure these are authentic earthquakes. This started when we introduced “GeoNet Rapid”; the up side is that the system is highly efficient and quick with earthquake reporting times of just minutes after the quakes.
It is sharply arriving S-waves - that our automated system confuses for P-waves - that are causing the incorrectly reported earthquakes. Our seismologists are easily able to see the confusion and gradually mop up the false earthquakes on the website.
How can we stop “Ghost Quakes”?
We are working on it but it's tricky. If we make the system too picky on the quakes it reports, we might not get rapid information about real earthquakes that occur. If we let it report on whatever information it picks up, we get “ghost quakes”. At GeoNet, we have erred on the side of speedy reporting of everything because we know how important it is to get information as quickly as possible to everyone.
Having said that, we are working on finding out what we can do to better identify them and so exorcise “ghost quakes” from our system. Until then, please be patient with us, and keep up the sense of humour, New Zealand!
(Image: provided by Stuff.co.nz)
It's a question we get a lot here at GeoNet given that all of New Zealand's significant earthquakes in the past few years - Canterbury, Cook Strait and Eketahuna - have struck on previously unknown faults. And the answer is... complicated.
There are currently four simple ways to tell if a fault exists: we can see it (land deformation), we've heard about it (it's in our written or oral histories), we've dug into the ground and can see it in the soils and rocks, or we've recorded an earthquake on it (read below why this isn't necessarily useful for finding large faults). If it's not one of those four ways, it becomes difficult to know if a fault exists in an area and if it can cause a significant earthquake. There is a fifth way but I'll get to that in a moment...
How often do earthquakes happen on previously unknown faults?
Unfortunately the answer is a lot; a magnitude 6-7 earthquake occurring on an already identified fault is the exception rather than the rule. Luckily, the odds of larger earthquakes occurring on identified faults are much better; once we look into magnitude 7.8+ earthquakes, researchers are confident that all faults in New Zealand capable of these sized quakes have already been identified and mapped. Why? Because we can see them clearly in our landscape. A big earthquake requires a fault of significant length; large faults with the potential to generate magnitude 7.8+ earthquakes - like the Alpine Fault - will form a significant feature of New Zealand's landscape and are therefore much easier to find.
Earthquakes are happening all the time - don't they show us where faults are?
As we all well know after the Canterbury quakes, parts of the country can be relatively quiet for decades but still capable of large quakes; we'd need 20,000 years' worth of earthquake records to have the complete picture!
Every earthquake happens on a fault, and with a searchable database of over 480,000 earthquakes we know where a lot of faults are. The trouble is that all these earthquakes don't nicely line up and illuminate individual sub-surface faults. The accompanying map showing just three years of shallow earthquakes in the middle of the country illustrates this. Also, as large quakes are infrequent, only 30,000 of the 480,000 are magnitude 4+ quakes so it's impossible to know whether the small quakes are occurring on a small fault, or they're small quakes occurring on a small portion of a larger fault.
Our current New Zealand active faults database contains 532 mapped faults that are capable of magnitude 6+ earthquakes. Although the amount of known active faults has increased significantly in the last few decades, there are still a large number of unknown faults capable of producing large earthquakes. Some research conducted by Dr. Andy Nicol at GNS Science suggests that there could be as many as 3500-4000 faults around the country capable of producing magnitude 6+ earthquakes, though the vast majority of these faults will be in remote areas of the country.
How come we, the people paid to know where faults are and what will happen, don't know?
The reason that so many faults are still undetected is that faults capable of magnitude 6-7 earthquakes aren't present at the Earth's surface and are therefore difficult to map. Earthquakes generally originate at depths greater than 5km, so only the very large, long-lived faults fracture rock all the way up to the surface. Faults that do not reach the surface are called 'blind' faults, and we aren't the only country with this problem. Other seismically active places around the world also have to contend with significant earthquakes occurring on these unknown blind faults. One famous example was the magnitude 6.7 Northridge Earthquake that struck suburban northern Los Angeles in the early morning of 17th January 1994. The quake resulted in 60 deaths, and more than 40,000 damaged buildings. Although it was widely known that California has many faults, this particular fault and its proximity to the densely populated city was unknown.
Finding all of the potentially thousands of blind faults in New Zealand is currently impossible. Geophysical surveys - similar to rudimentary MRI scans - are the way we traditionally find blind faults. These surveys look to find layers of rock offset below the surface. Although the technology does exists for us to go out and find most blind faults capable of large earthquakes, currently it would be prohibitively expensive to do this over the whole country. Even using this technology, faults which move very infrequently, like the Greendale fault - one of the faults responsible for kick-starting the Canterbury earthquake sequence - are particularly hard to identify as the subsurface rock layers are minimally offset.
Scientists do sometimes carry out such geophysical surveys to understand earthquake risk and to gain a better understanding of New Zealand's subsurface, and generally focus on finding faults that have the highest potential for devastation, especially in areas near high population and/or high likelihood of earthquakes. Geophysical surveys are also carried out by industries (such as oil and gas) and are utilised by scientists looking for earthquake generating faults. Surveys from the oil industry were used to locate faults around the Canterbury Plains (the Greendale Fault was not found, but other faults were known pre-2010). So this approach certainly isn't perfect.
So what do we do all day if we can't tell you where all faults are?
Researchers tackle the problem of unknown faults in a few ways. Firstly, they understand that knowing where these individual faults are only goes so far; we'd also need to know how often large quakes reccur on that particular fault, and when the last big one was on that fault. Research focusing on tectonic regions as a whole can offer more relevant insights into different areas' earthquake potential. We know that areas like Wellington and the Alpine Fault accumulate more strain at a faster rate from the collision of tectonic plates than other areas of the country (and therefore must release this strain more often via earthquakes). Secondly, as large earthquakes are inevitable (but infrequent), researchers focus on ways to mitigate the effects, such as building design and developing resilient communities.
Right now, it is like playing a constant game of catch-up with the earth, knowing that it has billions of years ahead of you and you only have 50 years of research behind you to unravel its secrets. We want to be able to tell everyone one day where all the faults are in New Zealand, when these will rupture and how big they will be, but until we can do this, it is important for all of us to be prepared for earthquakes.
For those of us in the middle of the country, it was hard to miss that mid-2013 to early 2014 was an especially active period when it came to earthquakes. It kicked off in July 2013 with the Cook Strait sequence – which included magnitude 6.0, 6.5, and 6.6 earthquakes – followed by the magnitude 6.2 Eketahuna earthquake in January 2014, and the magnitude 5.2 Waipukurau quake in March 2014.
2013 was also an especially active time for ‘silent earthquakes’, also called slow-slip events, which are similar to earthquakes as they involve fault movement. However, unlike earthquakes which occur in a matter of seconds, slow-slip events happen over weeks to months. In the last decade, slow-slip events have been discovered at plate boundaries around the world and GeoNet’s continuously operating GPS network in New Zealand has enabled the detection of slow-slip events at the North Island's Hikurangi subduction zone.
Last year four slow-slip events occurred around the North Island, including the largest slow-slip event ever recorded in New Zealand - it was offshore from the Kapiti Coast and equivalent to a magnitude 7.1 earthquake. A slow-slip event in Hawke’s Bay, equivalent to a magnitude 7.0 earthquake, was the largest on record for that region. Just like earthquakes, slow-slip events release strain, but without our GPS instruments, slow-slip events would be unreported as they can’t be felt and do not noticeably deform the ground. Nevertheless, the altered stresses around a slow-slip event can impact shallower faults that break via traditional earthquakes.
The extent to which recent slow-slip activity is related to the recent large earthquakes (and vice versa) is an area of active research, as we learn more about these relatively new phenomena (see below).
The slow-slip events of 2013
The Kapiti event started in early 2013 and is still going on. It is New Zealand’s largest slow-slip event ever recorded, equivalent to a magnitude 7.1 earthquake. We have 12 years of records in this region, which show three Kapiti events, each occurring roughly every five years. So far, this current event has as much as 25cm of movement over an area approximately 100km by 200km. The previous event in 2008 was equivalent to a magnitude 7.0 earthquake, but the movement was more concentrated: 30cm of movement over a smaller area near Kapiti Island. The 2013 slow-slip event evolved over the course of the year, moving north-eastward towards the North Island, with movement diminishing in late 2013-early 2014.
In February 2013, Hawke’s Bay experienced the largest slow-slip event for the region in a decade of recording. This slow-slip event occurred over several days and was the equivalent to a magnitude 7.0 earthquake. It was also associated with a swarm of over 100 earthquakes larger than magnitude 3 recorded during the period of slow slip, with the largest being of magnitude 4.8. These earthquakes occurred over a period of hours, and migrated deeper over time.
Two slow-slip events were recorded in the Gisborne region in 2013: one in July, the other in October. Both events were smaller than the Kapiti and Hawke’s Bay slow-slip events. The July slow-slip event occurred to the northeast of Gisborne, whereas the October event occurred to the south. The amount of movement and exact location of these events are not precisely known yet, but a temporary deployment of sensors on the ocean bottom placed offshore from Gisborne are being retrieved this week, and these should have captured how these events deformed the seafloor above. While these sensors are being retrieved, a larger set of ocean bottom pressure sensors and seismometers are being deployed as part of a joint NZ/USA/Japan project. These will offer our best insight yet into Gisborne slow-slip events - read a blog of the deployment here.
The relationship between slow-slip events and earthquakes
There are several examples from both New Zealand and overseas of earthquake swarms accompanying slow-slip events. Because slow-slip events occur over a large area, the amount of stress they transfer to other faults is diffuse. This is unlike a large traditional earthquake that has a large stress transfer concentrated in a relatively small region. For this reason, a magnitude 7.1 slow-slip event is probably not going to have anywhere near the associated triggered earthquake activity that we saw after the magnitude 7.1 Darfield earthquake, but it will increase stress in surrounding areas, and could push an already stressed fault closer to rupture. In essence, it can be the straw that breaks the camel’s back. This is possibly what happened with the January 2014 Eketahuna quake, where the Kapiti slow-slip event loaded stress on the fault that broke. The fault in the Eketahuna earthquake would most likely have ruptured in the near future, but the added stress may have caused it to rupture earlier.
It is important to note a few caveats when looking at the transferred stress events from slow-slip events (or regular earthquakes):
- Slow-slip events do not universally increase stress on surrounding faults, they also relieve stress in some areas, and therefore may postpone an earthquake in an area of decreased stress.
- As we can’t directly measure the plate movement (because it is well below the surface and most often offshore) we have to use models to determine what is going on and how the stress is being distributed. As such, the model will hopefully give a good indication, but will not be perfect, as there are many details that we cannot resolve using solely land-based instruments.
Why we care when slow slip causes no shaking
Before discovering slow-slip events, earthquakes were thought to be the only way the Earth’s crust could relieve the pent-up stresses caused by the moving tectonic plates. With the discovery of slow-slip events, this thinking has been drastically altered, as slow-slip events accommodate a large proportion of the effects of the converging plates without knocking a single ornament off a shelf. Slow-slip events in themselves don't pose any risk to people, but they are a major part of how the tectonic plates move in a subduction zone. The other major part is earthquakes. So if we better understand the slow-slip events, we should better understand the earthquake potential of subduction zones.
The subduction zones where slow-slip events occur (in our case where the Pacific and Australian plates meet) are responsible for generating the world’s largest earthquakes – ‘great earthquakes’ or ‘mega-thrust earthquakes’ – which have a magnitude greater than 8. These types of earthquakes can also produce tsunami with deadly consequences as we’ve seen in recent times in Japan and Sumatra. Scientists think that a future mega-thrust earthquake with a magnitude of 8 or larger is possible on New Zealand’s northern subduction zone – the Hikurangi subduction zone. An earthquake this large would produce damaging shaking for much of the North Island, and could produce a significant tsunami affecting much of the country (and also some coastal regions around the Pacific). However, scientists still don’t know how much, and how often, the Hikurangi subduction zone ruptures in megathrust quakes. The best evidence at the moment suggests they are relatively rare, happening every 1500 years. This is an area of active scientific research in New Zealand, and the more we know about the slow-slip events, the more we understand the subduction zone as a whole, and ultimately the better prepared we, and other nations, can be.
I have recently read an excellent article by John Stenmark that answers this question. It has the added bonus of using examples from our part of the world: