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.