NZ Anniversaries: 06/01/2015 Mw 5.6 Wilberforce Quake

The 2015 Wilberforce Earthquake

A simplified felt report map of the January 2015 Mw 5.6 Wilberforce Earthquake on the South Island of New Zealand (Source: GeoNet - Imagery: Google Earth)

Introduction

During 2019 I will be writing blog posts on the anniversaries of the major earthquakes to strike New Zealand since European colonisation began - the first quake chronologically occurred in 1848. Each quake will get a small piece on the cause of the earthquake and it's effects & impacts (both societal and environmental) - for some of these events there will be far more information owing to their recent occurrence, greater impact or greater potential impact if they were to recur in the future.

My aim with this exercise is to hopefully better communicate the seismic hazard in New Zealand to the public - some areas have surprisingly had significant and damaging earthquakes, but these and the risk they pose may not be known to many people besides academics.

Links to sources for the information shared are provided throughout the texts of these posts & the others I produce (including my 2019 >/=Ml 4.0 quakes & 2019 widely felt quakes pages).

Wilberforce

In the early morning of January 6th 2015, at 6:48am, a sharp earthquake rocked much of the South Island. It's origin was approximately 9 kilometres beneath the Southern Alps, in the uppermost streams of the Wilberforce River, Canterbury Province.  The quake most likely took no more than 5 seconds to rupture a small (~5km) fault, but in doing so it produced a moment magnitude 5.6 (local magnitude 6.0) earthquake. 

The earthquake was initially located beneath Lake Coleridge, which led to very confused reactions from people living there who had not experienced the quake to a severe intensity (MM5 or moderate intensity only). This was subsequently revised to it's correct position.

As can be seen in the pictured at the top of the article, the quake was felt from New Plymouth in the north down to Dunedin and Invercargill in the south; it was felt most strongly on the west coast of the South Island (from Fox Glacier up to Westport) and also in the westernmost & southernmost areas of the Canterbury Plain (Timaru, Temuka, Geraldine, Methven, Oxford).

A seismogram of the 2015 Wilberforce Earthquake from the Waitara Valley seismograph station (filtered 1-5 Hz - Source: GeoNet)

The earthquake occurred on a strike-slip fault - that is where two slabs of rock slide past one another laterally, with the fault which slipped being vertical. The below focal mechanism shows the two possible fault planes it could have occurred on - a NNW-SSE sinistral (left-lateral motion) fault or a WSW-ENE dextral (right-lateral motion) fault. GeoNet aftershock locations do not allow an easy analysis of which of these two fault strikes (alignments) the quake occurred on as they follow a NW-SE alignment themselves. The largest aftershocks (Mw 4.5 & Mw 4.6) occurred 6 and 11 hours after the mainshock - both of these were also strike-slip.

The focal mechanism for the 2015 Wilberforce Earthquake shows it was the result of strike-slip faulting (Source: GeoNet)

The primary impact from this quake was the number of landslides which occurred within the Southern Alps around the epicentre. An aerial survey by GNS identified 263 landslides, the second largest of which (25km away from the epicentre) temporarily dammed the Arahura River.

This earthquake has probably been forgotten by most people because it's societal impact was negligible, but this is the most recent example of a sizeable Southern Alps earthquake. The Southern Alps plays host to many different strike-slip and thrust faults which accommodate some of the relative plate motion of the obliquely colliding Australian & Pacific Plates - most of this is accommodated by the Alpine Fault.

Quakes have occurred on faults in this area in the past - the 1929 Arthur's Pass, 1994 Arthur's Pass and 1995 Cass earthquakes are largest to have occurred within the past 170 years and will be discussed further later in the year.

The earthquake was too small really to change the stress regime in the Southern Alps - small segments of the adjacent Alpine Fault were stressed towards failure and destressed away from failure, but the changes exerted were too small to really change the likelihood of an Alpine Fault quake in the short-term or long-term.

This article was written by J H Gurney at 18:00 GMT on the 5th January 2019 (07:00 NZDT on the 6th January 2019).

New Zealand Earthquakes of January 2016

New Zealand Seismic Summary – January 2016

The first month of 2016 has been relatively busy. GeoNet has recorded 1642 earthquakes of magnitude 1.0 or larger, 137 of these being magnitude 3.0 or greater. Of the latter total 33 events were reported felt by members of the public. No earthquake were recorded which had moment magnitudes larger than 5.0; the last such event to strike New Zealand was a moment magnitude 5.3 earthquake, 35 kilometres north-east of Taihape, on the 1^st November 2015.

Poster created on the 29th January 2016 for the West Christchurch Sequence of the previous day.

Mw 4.6, Pongaroa, 4^th January 2016

The first major felt earthquake struck at 1:07pm on the 4^th January, 20 kilometres east of Pongaroa on the North Island. The earthquake, with a local magnitude of 5.0 and moment magnitude of 4.6, struck at a focal depth of 30 kilometres and was the result of extensional normal faulting within the subducting Pacific Plate. This earthquake followed a moment magnitude 5.7 earthquake on the 12^th October 2015, only 5 kilometres to the west and may have been triggered by this prior earthquake due to stress changes within the subducting slab.

The earthquake had 15 aftershocks of local magnitudes 2.1 to 4.1 located in a broadly north-west to south-east area. The largest aftershock (3:10pm on the 4^th January) was given a moment magnitude of 4.2 and was probably a result of reverse faulting at a focal depth of 36 kilometres. This second moment tensor solution had far less confidence due to the seismic waves from the moment magnitude 6.7 earthquake in Manipur, India which struck earlier on the 4^th January.

GeoNet received 1,132 felt reports from the public for the 4^th January earthquake, with a maximum intensity of MM6 (strong shaking) received from Palmerston North. Reports were definitely received from Picton in the south to Ohakune in the north. Other felt reports are displayed on GeoNet’s map but I have less confidence in their validity.

Ml 3.2, Taupo, 4^th January 2016

A moderate earthquake struck at 7:30pm on the 4^th January, 25 kilometres north-west of Taupo. The earthquake, with a local magnitude of 3.2, had a depth of 10 kilometres. The location of this earthquake is fairly normal, with small swarms periodically striking to the north of Taupo at very shallow (less than 15 kilometres) depths.

This earthquake occurred within the Taupo Volcanic Zone, a southern extension of Lau-Havre Basin which runs from west of Fiji down to the Bay of Plenty, parallel to the Kermadec Trench. This back-arc basin is responsible for several submarine volcanoes (seamounts) which erupt periodically. White Island and the other Bay of Plenty islands are within this volcanic belt, as are the North Island volcanoes of Rotorua, Taupo, Tongariro and Ruapehu. It is this particular region which tends to have a large share of the shallow depth earthquakes on the North Island.

GeoNet received 104 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Taupo. Reports were definitely received from Taupo in the south to Atiamuri in the north.

Mw 3.8, Seddon, 5^th January 2016

A moderate earthquake struck at 9:04am on the 5^th January, 15 kilometres north-east of Seddon. The earthquake, with a local magnitude of 3.9 and moment magnitude of 3.8, had a focal depth of 25 kilometres and was a result of strike-slip faulting. This earthquake struck on the north-western fringe of the aftershock sequence to the July and August 2013 Cook Strait Sequence which included two moment magnitude 6.6 earthquakes.

The Cook Strait and Marlborough regions represent a transition from the Pacific Plate subducting beneath the North Island at the Hikurangi Trench to a transpressive regime involving numerous strike-slip and oblique-reverse faults. These have resulted in numerous earthquakes in the past 200 years, including the 1848 Marlborough Earthquake and the 1929 Murchison Earthquake, both of which had estimated magnitudes in excess of 7.5. These faults act a transfer region between the Hikurangi Trench and the 460 kilometres long Alpine Fault.

GeoNet received 195 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Seddon and Blenheim. Reports were definitely received from Richmond in the west to Lower Hutt in the east.

Ml 4.7, Opotiki, 5^th January 2016

The second major earthquake struck at 12:57pm on the 5^th January, 10 kilometres north-east of Opotiki. The earthquake, with a local magnitude of 4.7, had a depth of 71 kilometres and was almost certainly related to the subduction of the Pacific Plate beneath the Indo-Australian under the North Island. Unfortunately no moment tensor has yet been made available, so any further analysis of the source of this earthquake is impossible.

GeoNet received 328 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Whakatane, Opotiki and Opape. Reports were definitely received from Wellington in the south to Te Kaha in the north.

Mw 4.8, Rotorua, 11^th January 2016

The third major earthquake, and largest in January, struck at 1:14pm on the 11^th January, 30 kilometres east of Rotorua. The earthquake, with a local magnitude of 4.3 and a moment magnitude of 4.8, had a focal depth of 150 kilometres and was the result of oblique reverse faulting.

As with the 5^th January Opotiki earthquake this earthquake was the related to the subduction of the Pacific Plate beneath the Indo-Australian under the North Island and was most likely a result of slab tear within the subducting slab of the Pacific Plate. A similar but far larger earthquake struck the Cook Inlet in Southern Alaska on the 24^th January 2016, with a moment magnitude of 7.1 and a focal depth of 122 kilometres (GEOFON, GFZ Potsdam).

GeoNet received 766 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Wellington, Tawa, Hastings and Gisborne. Reports were definitely received from Blenheim in the south to Te Kaha in the north.

Ml 4.1, Whanganui, 12^th January 2016

A moderate earthquake struck at 2:47pm on the 12^th January, 30 kilometres south of Whanganui. The earthquake, with a local magnitude of 4.1, had a depth of 39 kilometres. This struck a very active seismic region beneath the North Taranaki Bight, which is believed to the southernmost tip of the extensional region which extends southwards from the Lau-Havre Basin via the Taupo Volcanic Zone. In this instance, as with the Lau-Havre Basin and the Kermadec Trench, this back-arc basin is parallel to the Hikurangi Trench east of the North Island.

GeoNet received 309 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Marton. Reports were definitely received from Stratford in the west to Wellington in the east.

Ml 3.8, Taumarunui, 13^th January 2016

A moderate earthquake struck at 1:22am on the 13^th January, 30 kilometres of Taumarunui. The earthquake, with a local magnitude of 3.8, had a depth of 15 kilometres. This earthquake, as with the 4^th January Taupo and 12^th January Whanganui earthquakes, was a result of the extensional regime related to the back-arc basin running from the South Taranaki Bight all the way up to Fiji. Earthquakes are common in this particular region, located just west of the volcanic peaks of Tongariro, Ngauruhoe and Ruapehu, running from Taumarunui in the north to Ohakune in the south.

GeoNet received 109 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Ohakune and Taumarunui. Reports were definitely received from Whanganui in the south to Tihoi in the north.

Ml 3.6, Whanganui, 21^st January 2016

A moderate earthquake struck at 11:51am on the 21^st January, 30 kilometres south-west of Whanganui. The earthquake, with a local magnitude of 3.6, had a depth of 13 kilometres. This earthquake, as with the 4^th January Taupo, 12^th January Whanganui and 13^th January Taumarunui earthquakes, was a result of the extensional regime related to the back-arc basin running from the South Taranaki Bight all the way up to Fiji. This earthquake almost certainly occurred on a one of the many wrench and normal faults dominating the North Taranaki Bight, as revealed since the 1970s by seismic tomography conducted by the petroleum industry.

GeoNet received 125 felt reports from the public for this earthquake, with a maximum intensity of MM5 (moderate shaking) received from Aramoho. Reports were definitely received from Wellington in the south to Taupo in the north.

Ml 3.3 & Ml 3.2, Thornton Swarm, Whakatane, 24^th January 2016

A small earthquake swarm struck between 9:17pm and 11:23pm on the 24^th January, 10 kilometres north-west of Whakatane. The 23 earthquakes of this swarm, with local magnitudes between 1.7 and 3.3, had depths of 1 to 11 kilometres. Of these 23 earthquakes, 11 had magnitudes of 2.5 or larger, with the largest two events of local magnitude 3.3 (9:32pm, 7 kilometres depth) and local magnitude 3.2 (9:46pm, 5 kilometres depth).

Such swarms occur fairly often, with a small swarm occurring almost annually. These are again related to the back-arc basin which runs from the South Taranaki Basin in the south to Fiji in the north, running through the Taupo Volcanic Zone and the Bay of Plenty. This particular swarm was not particularly impressive, mainly due to the largest earthquake only being local magnitude 3.3.

GeoNet received over 600 felt reports from the public for these earthquake, with maximum intensities of MM5 (moderate shaking) from Coastlands, Whakatane and Ohope. The magnitude 3.3 earthquake garnered 270 felt reports, whilst the magnitude 3.2 earthquake garnered 101 felt reports. Only three recorded earthquakes in the swarm were not reported as felt.

Mw 3.9, Christchurch, 28^th January 2016

The fourth major earthquake, and largest on the South Island in January, struck at 12:24pm on the 28^th January, 10 kilometres west of Christchurch. The earthquake, with a local magnitude of 4.1 and moment magnitude of 3.9, had a focal depth of 5 kilometres and was the result of right-lateral strike-slip faulting on an east-west striking fault. This earthquake was a major late aftershock to the September 4^th 2010 moment magnitude 7.2 Darfield Earthquake and struck at the far east of the aftershock zone to that earthquake. It was the largest earthquake to hit Christchurch since a moment magnitude 4.0 earthquake on the 26^th December 2014.

The earthquake had 4 aftershocks of local magnitudes 2.4 to 2.9. Three aftershocks had magnitudes of 2.8 to 2.9: a local magnitude 2.9 (12:46pm on the 28^th January) definitely had a maximum intensity of MM5 (moderate shaking) in the southwest of the city and was felt as far away as Duvauchelle in the south and Rangiora in the north (a maximum felt distance of 41 kilometres); a local magnitude 2.9 (9:00pm on the 28^th January) had a maximum intensity of MM5 (moderate shaking) in the southwest of the city and was felt as far away as Sumner in the east (a maximum felt distance of 21 kilometres); and a local magnitude 2.8 (11:47pm on the 29^th January) had a maximum intensity of MM4 (light intensity) and was felt as far away as Northwood in the north (a maximum felt distance of 17 kilometres).

GeoNet received 2,348 felt reports from the public for the 28^th January earthquake, with a maximum intensity of MM6 (strong shaking) received from Hornby, Halswell, Riccarton, Somerfield, Saint Albans and Christchurch (the southwest, west and centre of the city). Reports were definitely received from Akaroa to Rangiora, with possible reports from Greymouth (MM4) and Kaikoura (MM3).

Ml 4.1, Picton, 29^th January 2016

The fifth major earthquake struck at 3:20pm on the 29^th January, 30 kilometres east of Picton. The earthquake, with a local magnitude of 4.1, had a depth of 36 kilometres. Earthquakes in the western part of the Cook Strait are commonplace and if in excess of local magnitude 3.5 are generally well felt both in the Marlborough region of the South Island and the Wellington region of the North Island. This earthquake was no exception and was the strongest earthquake to strike near the capital.

GeoNet received 1,372 felt reports from the public for this earthquake, with maximum intensities of MM5 (moderate shaking) received from Linkwater, Kelburn, Wellington, Lyall Bay, Roseneath, Rongitai, Eastbourne and Days Bay (all around Wellington Harbour, both west and east coasts). Reports were definitely received from Richmond in the west to Masterton in the east.

Written by J H Gurney, 3:25am NZDT, 1st February 2016.

From Devon, United Kingdom.

Moderate Rotorua Quake Rocks North Island

At 1:14pm NZDT on the 11th January 2016 a local magnitude 4.6 (moment magnitude 4.8) earthquake struck 25 kilometres east of Rotorua in the north of the North Island, just south of the Bay of Plenty. The earthquake was given a focal depth of 150 kilometres, consistent with subduction related earthquakes beneath the Taupo Volcanic Zone. This earthquake was likely near the plate interface as the Pacific Plate subducts beneath the Indo-Australian Plate below the North Island. The earthquake was very well felt, with over 750 felt reports received by GeoNet, from Te Kaha in the north to Blenheim on the South Island. The highest intensities were received from eastern locations on the North Island, namely Te Aro & Tawa (Wellington area), Hastings and Mangapapa (Gisborne area), all at MM5 (moderate) intensity.

This is the first moderate felt earthquake in New Zealand since the local magnitude 4.7 earthquake near Opotiki on the 5th January. As with that earthquake, this earthquake occurred at an intermediate depth, although owing to the fact it was further west than the Opotiki event it struck at a greater depth. This is because the Pacific Plate is sinking beneath the North Island from off the east coast at the Hikurangi Trench.This it the second most felt earthquake so far this year after the local magnitude 5.1 Akitio earthquake on the 4th January (which received over 1100 felt reports).

Yesterday's Rotorua earthquake was a result of oblique reverse faulting on a vertical fault plane at a depth of 150 kilometres. Whether this puts it within the Pacific or Indo-Australian plates is impossible to say with certainty. Nevertheless, with a moment magnitude of 4.8 this is the largest New Zealand earthquake so far this year.

Written by J H Gurney, 12:05am NZDT, 12th January 2016.
From Devon, United Kingdom.

Deep Earthquake Strikes Bay of Plenty

At 12:57pm NZDT on the 5th January 2016 a local magnitude 4.7 earthquake struck 15 kilometres southeast of Opotiki in the north of the North Island, just south of the Bay of Plenty. The earthquake was given a focal depth of 78 kilometres, consistent with most quakes in this area (although shallow seismicity does occur as well). This certainly places it near the plate interface as the Pacific Plate dives very deep below New Zealand and the Indo-Australian Plate at this point. The earthquake was felt relatively well with more than 300 felt reports received by GeoNet, from Te Kaha in the north to Wellington in the south. The highest intensities were received from more epicentral population centres, Whakatane, Opotiki and Opape at MM5 (moderate).

This is the third earthquake so far this year in New Zealand which has had a magnitude greater than 4.0. The 4th January had two such events: a moment magnitude 4.6 (local magnitude 5.1) near Akitio which was the result of Pacific in-slab normal faulting and a moment magnitude 4.2 (local magnitude 4.0) near Akitio which was (with very low confidence from GeoNet*) was a result of Pacific in-slab reverse faulting at 36 kilometres depth. This latest event near Opotiki was likely oblique normal or normal faulting, although I could not say in which plate regardless of any focal depth given by a moment tensor solution.

*The Akitio Earthquakes on the 4th January 2016 occurred an hour after the moment magnitude 6.7 earthquake in Manipur Province, India - this earthquake killed at least 23 people. The low frequency body waves from the India Earthquake masked the local signal from the Akitio Earthquake, making any moment tensor solution questionable. I have been assured that the moment tensor for the local magnitude 5.1 earthquake is quite reliable, but owing to its smaller magnitude the later local magnitude 4.0 event has a far poorer confidence in the solution given (personal email, John Ristau, GeoNet).

Written by J H Gurney, 10:44pm NZDT, 5th January 2016.
From Hampshire, United Kingdom.

First Moderate Seddon Aftershock of 2016

At 9:04am NZDT on the 5th January 2016 a local magnitude 4.2 earthquake struck 15 kilometres east of Seddon. This earthquake is the first moderate aftershock to the Seddon Sequence of July and August 2013. Currently GeoNet has received more than 160 felt reports, primarily from Blenheim and Wellington, but also with reports from Picton and Seddon. The maximum felt intensity was MM5 (moderate) in Blenheim, a usual location for maximum intensities from Seddon earthquakes.

This earthquake has not yet been revised, so it's location needs to treated with caution. Currently it is located beneath Cape Campbell at a depth of 37 kilometres, placing it deep within the Pacific Plate. Whilst the location is very plausible, the depth is more questionable.

More to follow...

Written by J H Gurney, 9:35am NZDT, 5th January 2016.
From Hampshire, United Kingdom.

Moderate Earthquake Rattles North Island

At 1:07pm on the 4th January 2016 a local magnitude 5.1 earthquake struck 20 kilometres east of Pongaroa on the lower North Island. The earthquake was given a focal depth of 25 kilometres (as is stated), which indicates it occurred within the Pacific Plate. With a relatively deep focus (a shallow depth compared to others, but deep for the area in which it struck) this earthquake was unsurprisingly well felt from Picton in the south to Okahune in the north. The maximum intensity reported was MM6 in the Palmerston North area; this seems a common place for highest reported intensities from earthquakes in this part of the lower North Island. The earthquake has had to 15 aftershocks in the 12 hours after the mainshock between M1.7 and M4.0. From herein this event shall be referred to as Akitio II due to its proximity to the township and river of the same name. This earthquake is remarkable as being one which I "predicted" - I shall explain this later on.

On the 12th October 2015 a moment magnitude (Mw) 5.7 (local magnitude 5.8) earthquake struck 15 kilometres east of Pongaroa on the lower North Island. The earthquake was given a focal depth of 23 kilometres, which also indicates it occurring within the Pacific Plate. Over 6000 felt reports were received by GeoNet, with reports from Auckland down to Dunedin. The earthquake had an intensity of MM7 (damaging) in Tiraumea and MM6 intensities from Ongaonga (near Waipukurau) in the north to Ohau (near Levin) in the south. Over 900 of the felt reports received were from the Palmerston North area. A GeoNet moment tensor solution (given to me via email) indicated that this earthquake was in fact shallower (14 kilometres deep) and a result of either steeply-dipping reverse or shallowly-dipping oblique reverse faulting. This would indicate that the earthquake actually struck within the lower part of the overlying Indo-Australian Plate. The earthquake was followed by a relatively strong aftershock sequence, with two strong aftershocks striking within two hours of the mainshock (local magnitude 4.2 and 4.4). From herein this earthquake will be referred to as Akitio I as it was also very close to the Akitio township.

The area afflicted by these events is no stranger to moderate or even large earthquakes. In March 1934 a massive earthquake (moment magnitude 7.2-7.4) struck Pahiatua, injuring one and causing large damage to the lower North Island. In February 1990 a local magnitude 6.1 earthquake near the township of Weber was followed by a moment magnitude 6.4 earthquake about 10 kilometres northeast in May. Two smaller earthquakes (magnitude 5.5) struck in August 1990 and March 1992 as aftershocks to the February event. In January 2014 a moment magnitude 6.2 earthquake struck 20km east of Eketahuna and was followed by two earthquakes of magnitude 5.2 and 5.5 in September 2014.

The Weber Earthquakes are undoubtedly the most helpful regarding the current Akitio Earthquakes. The Weber Earthquakes began with a local magnitude 6.1 earthquake about 10 kilometres north of Akitio I in February at 1990 (Weber I). This event, with a depth of approximately 20 kilometres, was a result of normal faulting on a steeply dipping fault within the subducting Pacific Plate. The May event (Weber II) occurred about 10 kilometres northeast of the February event and was shallower (about 10 kilometres deep), thus occurring within the overlying Indo-Australian Plate. This event was a result of oblique reverse faulting on a shallow fault, just as Akitio I likely was. This means that these sequences (the Weber Earthquakes of the 1990-1992 and the ongoing Akitio Earthquakes) are inextricably linked and similar in mechanism and cause. Stress increases caused by Weber I triggered the later Weber II earthquake.

Back to the fact I predicted yesterday's earthquake (Akitio II). As the aftershock sequence to Akitio I was occurring my mind was brought back to the Weber Earthquakes I described above. At the time I knew very little about the sequence other than its occurrence (I was born in 1996) so, owing to Akitio I occurring in the same area I decided to use it as a proxy for what might follow. The aftershock sequence, though strong enough, was not outstanding as past New Zealand sequences had been (such as Seddon in July and August 2013). This made me pose the question of whether further earthquakes of magnitude 5.0 or larger might occur; I surmised that it was more likely to be updip of Akitio I due to stress transfer perhaps preferentially following a line of weakness (the plate interface), thus coming to the location of yesterday's earthquake. Whether this is the exact cause of Akitio II or not is unclear, but stress transfer was certainly involved and this earthquake cannot be called an aftershock of Akitio I either (it is outside the initial aftershock zone from Akitio I and more than likely struck on a different fault, perhaps even a normal fault if indeed it occurred in the Pacific Plate).

All of this will need further analysis and exploration, but I believe this is most likely case for the earthquake of January 4th 2016. Further aftershocks to this earthquake may occur, but are unlikely to be too large. As always with any shallow New Zealand earthquake there is a very small chance that this could be a foreshock to a larger earthquake in the same area. The stress changes induced by Akitio II will have been smaller than those from Akitio I due to its smaller magnitude, but may increase stress on another fault where stress was already increased by Akitio I (or could just have easily nullified the earlier stress increase). A moment tensor from GeoNet will at least answer some of these questions.

Edit 11:08am NZDT 6th January: I have just received an email reply regarding the moment tensor solution for the Akitio II local magnitude 5.1 earthquake. This shows that the earthquake had a moment magnitude of 4.6 (a bit smaller than I suspected) and a focal depth of 30 kilometres. This places it within the subducting Pacific Plate and to correlate with what Weber I (February 1990) would infer it was the result of a moderately dipping normal fault, almost certainly the nodal plane dipping to the northwest at 52 degrees. This would explain why an earthquake of a smaller magnitude (4.6) was felt so widely - it was for the east coast of the lower North Island a relatively deep focus earthquake.

Written by J H Gurney, 2:26am NZDT, 5th January 2016.
Frkm Hampshire, United Kingdom.

Eketahuna Aftershocks Still Strong

At 5:35am NZDT on the 3rd January 2016 a local magnitude 3.7 earthquake struck about 5 kilometres south-east of Eketahuna in the Lower North Island. The earthquake was felt locally, with reports from Mauriceville, Eketahuna, Pahiatua and Palmerston North. Felt intensities did not exceed MM4 (surprising considering the proximity to Eketahuna and the depth being shallower than previous Eketahuna events at only 10km).

This earthquake is the first moderate earthquake in New Zealand this year. A M4.0 near St Arnaud did not actually occur, it being one of three mislocated earthquakes sourced from wave arrivals in New Zealand from a moment magnitude 5.6 earthquake near Raoul Island, ~1200 kilometres north-north-east of the North Island. There will be many more of these events this year - I simply refer to them as Kermadec earthquakes (along the Kermadec Trench, which marks the plate boundary between the Pacific and Indo-Australian Plates to the north of New Zealand (with the Pacific Plate diving, or subducting, westwards beneath the Indo-Australian Plate).

The Eketahuna earthquake is most interesting as it struck outside the aftershock zone from the January 2014 Mw 6.2 earthquake. It is also much shallower (the aftershock zone from the 2014 quake had depths of between 20 and 35 kilometres). This is likely a result of stess changes in the crust - when an earthquake strikes, the rock around the rupture zone from it is further stressed (or less stressful) as a result. This is what caused the February 2011 Christchurch Earthquake after the Darfield M7.0 Earthquake in September 2010 and the August 2013 Lake Grassmere Earthquake after the July 2013 M6.6 Cook Straits Earthquake.

Last Wednesday (30th December 2015) a magnitude 4.7 earthquake struck 20km west of Pongaroa, ~30km north-east of today's Eketahuna earthquake. This event was definitely a result of stress triggering by the 2014 earthquake and was very well felt with over 500 reports to GeoNet and a maximum intensity of MM5. It was also the right depth for it to possibly be related to the main Eketahuna earthquake at 22km (or a similar neighbouring fault). A moment tensor solution, which will show which type of fault was responsible for the 30th December event, should be released by GeoNet in the coming days and will confirm if it was indeed on the same fault (or give supporting evidence to such a hypothesis).

I expect many more earthquakes in New Zealand this year and this blog shall continue to report on them as they occur. Posts will only be made for well felt earthquakes or swarms and not for all, or for large events north or south of New Zealand (on the Kermadec Trench and Macquarie Ridge respectively).

For more information on my Eketahuna research, see here.

Written by J H Gurney, 10:16 NZDT, 3rd January 2016.
From Hampshire, United Kingdom.