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The western Bay of Plenty coast area near Pikowai and Pukehina has been rattled by an earthquake sequence to day. The sequence started this morning at 5.02 am when a magnitude 3.1 event was recorded. We have received 34 felt reports from the local community associated with that event. Since the first event we have recorded numerous small aftershocks (see drum plot below) and have been able to locate 15 of them, though many are too small to locate. They have ranged in size from M1.7 to M2.6 at depths of 5-7 km. This type of earthquake activity is common in the Bay of Plenty area.
In the last 12 months we have located 178 earthquakes in this area and almost 1400 in the last 5 years. Offshore of Pikowai has been busy, with about half of those been located in that area. These earthquakes are occurring in an area about 12-14 km west of Matata, which was busy in the 2005-2008 period. We located over 2500 events in that earthquake sequence.
Today’s earthquakes appear more as a main shock-aftershock sequence, that is the largest event is at the start of the sequence and smaller events follow. In a swarm the largest event can be at the start, in the middle or near the end of it. Often we do not know until it is over if we are dealing with a main shock-aftershock sequence or a swarm. The area where these earthquakes are occurring lies just outside of the Taupo Volcanic Zone, the zone of active volcanic activity that extends from White Island (Whakaari) to Mt Ruapehu. Earthquake swarms are common in volcanic areas.
Earthquake swarms are very common in the greater Rotorua-Taupo area, known as the Taupo Volcanic Zone. Since around 3am today (Thursday 29 October) the GeoNet seismometer network between Taupo and the Tongariro National Park started recording a swarm of small earthquakes. They locate about 8 kilometres south west of Turangi. We have been able to locate over 60 of the events so far. Their magnitudes ranged from about M1.0 to M 3.4, while the depths ranged between 5 and 16 kilometres, with most being 7-8 kilometres deep. The quakes are likely to be related to the long-term tectonic stretching of the Taupo Volcanic Zone. Currently, there are no clear indications that the earthquakes are related to volcanic activity. As usual, we continue to closely monitor the nearby volcanoes to assess any changes.
The largest event occurred at 3.14 am and has been reported as felt by 12 people. They have reported ‘moderate’ shaking (MM5), which is typical of events in this area. The events were too small to be widely felt or cause any significant damage. The largest event so far is near the start of the swarm. Volcanologist Brad Scott commented that this is very typical of earthquake swarm activity, when many earthquakes are recorded over a period of time. Swarms are often characterised by no one main or large event, with many of the events being about the same size. Four of the events in this swarm are larger than M3, another seven are larger than M2. The rest are smaller than M2.
In 2008 there was a similar swarm about 3-5 kilometres north of where this swarm is occurring, while in March this year there was one about 9 km to the north.
The M5.7 quake struck at 9:05 pm on a Monday evening, and was centered 15 km east of Pongaroa, under the south-east of the North Island. The quake was felt strongly in the North Island and we have received over 5000 felt reports from the public. The focal mechanism shows it to be a reverse fault earthquake.
An aftershock sequence is ongoing following this earthquake and includes numerous magnitude 4 events. As with any aftershock sequence, we cannot rule out the possibility of occurrence of future larger earthquakes.
Like many parts of New Zealand the eastern Wairarapa is a seismically active region and has experienced many earthquakes in the past. In 1990 the region was shaken by the Weber earthquakes which were a series of three earthquakes with two larger than M6. This was followed by a M5.5 earthquake in 1992. More recently the M6.2 Eketahuna earthquake occurred to the west of the Pongaroa quake.
The M5.7 earthquake likely occurred on the interface between the subducting Pacific Plate and the overriding Australian Plate. The earthquake was a thrust event indicating that the Pacific Plate moved deeper and to the west. The maximum recorded ground shaking was an acceleration of about 0.1g, compared with the 2014 Eketahuna earthquake which had a maximum of 0.26g.
What do we expect will happen next?
Based on the earthquakes so far and our knowledge of the tectonics, including the Hikurangi subduction fault, we have compiled the following scenarios that could occur over the following weeks.
GNS Science will continue to monitor the seismic and geodetic activity in the region.
Last Updated at 3:10 9m 13/10/15 - we will continue to update this story with further details.
The greater Rotorua-Taupo area is an active volcanic environment, characterised by geothermal areas, volcanoes, faults and frequent small earthquakes. Often the earthquakes occur as short lived cluster that are known a swarms. Swarms are very common in the Rotorua-Taupo area. Larger ones will last weeks to months and include many hundreds of earthquakes. Larger and prolonged earthquake swarms are a common part of volcanic unrest. Overnight there was a small swarm in the Waimangu-Rotomahana area. The GeoNet seismometers recorded over 60 events and 20 are large enough to be located. The magnitudes range from 1.2 to 2.6, while depths ranged between 5 and 7.5 kilometres. The events were too small to be widely felt or cause any damage. No felt reports have been received by GeoNet. Only weak shaking would have been experienced by the local population.
Swarms are often characterised by no one main or large event, with many of the events being about the same size. Fourteen of the events in this swarm range M1.2 to M2.0 and six are larger than M2.0. The largest is M 2.6 and was the last event in the sequence. There are also many events too small to be located. This swarm is very similar to one on 15 January this year, when we located 19 events and the largest was M2.5.
A recent scientific paper by Dr Stephen Bannister and colleges has reviewed the earthquake swarm activity associated with the Waimangu-Rotomahana-Mt Tarawera geothermal field since 1990. They conclude sporadic swarms are common in this area and they typically only last 4-8 hours. The earthquakes are usually located in a 6 km long lineation that trends NE-SW from Lake Rotomahana and are related to the geothermal processes in the area. Last night’s earthquakes occurred in the same area.
The rapid detection and location of these small events demonstrates the capability of GeoNet to record this type of earthquake activity in these active areas. This helps under pin the volcano monitoring and allows the volcanologists to quickly assess the unrest, which in this case is very minor. In the last year there have been 204 locatable earthquakes in this area, which is normal.
This last week has seen a spurt of small earthquakes north of Taupo. The larger events, which are located close to Wairakei were widely felt by the local community. The largest event with a magnitude of 3.7, occurred on 3 July at 11.58pm at a depth of 7km. We have received over 520 felt reports, which range from weak to strong. Light shaking (Generally noticed indoors but not outside; Light sleepers may be awakened; Vibration may be likened to the passing of heavy traffic, or to the jolt of a heavy object falling or striking the building) was reported by 501 people.
We have located 69 small earthquakes in this area north of Taupo in the last year. Nine of these were in the last week. These events are under the Wairakei-Tauhara geothermal fields and it is common to record earthquakes in this area.
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)