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The GeoNet seismographs in the Auckland region recorded two small earthquakes on Saturday at Waiheke Island. Both were located towards the western end of the island. The larger occurred at 11.56 am, being a magnitude 2.5 earthquake at 8 km depth. So far 48 people have reported feeling the event. Reports are mainly from on the island or the city area (including Maraetai).
The second was a magnitude 2.2 event at 7 km depth. Twenty-three people have reported feeling this one, again mainly from the island, Takapuna and the city area. Most felt earthquakes in the Auckland area come from events in the Hauraki Gulf area. The previous event in this area was on 13 November 2014, a magnitude 2.7 earthquake at 4 km depth near Motutapu Island. It was reported felt by 60 people.
The earthquake's location was very close to a magnitude 4.0 quake last October. Last night's quake is highlighted on the map shown on the right, with last October's epicentre the other circle close by. As quakes in this part of New Zealand are uncommon, we wrote up a history of earthquakes that have affected Dunedin in the past.
The ShakeMap for the earthquake shows the modeled expected strong shaking, mainly expected in spots along State Highway 87. There were a few felt questionnaires reporting items coming off shelves in various parts of Dunedin city. The majority of reports were from Timaru to Invercargill, and west to Queenstown.
- At the shallow depth of 5km, this classes the earthquake as severe. Due to the distance from populated centres, it is not expected to be greatly damaging.
- The magnitude was reviewed from 6.0 to 5.8 upon reception and analysis of data from more remote stations throughout New Zealand.
It has been felt all over the bottom half of the South Island. Reports of strong shaking have come in from Queenstown, Dunedin, Cromwell, Invercargill, and of course Wanaka.
There are no known active faults in this part of New Zealand. This earthquake is unlikely to have ruptured the surface.
- The most likely scenario is that there will be no earthquakes larger than the mainshock of magnitude 5.8.
- There is a 4% probability that there will be an earthquake of magnitude 6.0 or larger within the next 30 days†.
Probabilities of further quakes within about 50km of today's earthquake epicentre
Within 1 week
|3||0 - 6|
|0.2||0 - 1|
|0.02||0 - 1||2%|
Within 1 month
|4||0 - 9|
|0.4||0 - 2|
|0.04||0 - 1||4%|
† Updated on 6 May 2015 for the coming week and month
* 95% confidence bounds
GeoNet landslide response
A GeoNet landslide response was undertaken on Tuesday 5 May. Aerial reconnaissance reported very few signs of landslides or ground damage in the east branch of the Matukituki River, or throughout the immediate epicentral area. About twenty landslides were seen, all of them small.
Last update 3:00pm on 6 May 2015
It struck at 3:36 pm, and has a revised depth of 52 km and a magnitude 6.2. This classes the earthquake as severe but due to the depth of the quake and distance from populated centres, it is not expected to be greatly damaging.
It's been felt moderately from Wellington to Christchurch, but with some reports indicating strong shaking, which is indicative of items falling from shelves. It will have been felt over a wide area. Reports of lighter shaking are in from Auckland through to Dunedin.
The epicentre is in a very remote part of the north-east South Island, which has a very complex geology featuring a number of significant faults. The faulting type is oblique - this is common for these earthquakes in the upper South Island.
Deeper earthquakes such as this usually have only a few aftershocks. We would not expect any of these to be bigger than about magnitude 5.0.
Relationship to July/August 2013 Cook Strait earthquakes
Yesterday's earthquakes were certainly in the vicinity of the sequence that occurred back in 2013. Today's earthquake is outside that general zone, but we cannot be certain that it hasn't been triggered by stresses from all those quakes over time.
Last update 4:55pm on 24 April 2015
- 10:36 am, M4.4, moderate intensity at the epicentre
- 10:42 am, M5.1, strong intensity at the epicentre
Where were they felt?
Both have been reported felt from across the top of the South Island, throughout the lower North Island, and up the west coast of the North Island to New Plymouth. A few reports have also been received from Canterbury. No structural damage would be expected, but a few reports indicating that items may have fallen from shelves have been received.
What sort of earthquake faulting were they?
Analysis of the M5.1 quake shows the fault moved as a 'strike-slip fault', meaning that one side of the fault moved alongside the other.
Could a tsunami have been generated?
These earthquakes are much too small to cause a tsunami in their own right. An offshore earthquake needs to be at least magnitude 7.5 for a tsunami to be considered possible.
Will there be aftershocks?
Only a few small aftershocks have occurred since these two quakes, with the next largest being of magnitude 3.2.
Are these earthquakes unusual?
Last updated 11:45 am, 23 April 2015
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