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 1st November
2015.
|
Poster created on the 29th January 2016 for the West Christchurch Sequence of the previous day. |
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 4th
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, 4th January 2016
A moderate earthquake struck at 7:30pm on the 4th
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, 5th 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.
The second major earthquake struck at 12:57pm on the 5th
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.
The third major earthquake, and largest in January, struck
at 1:14pm on the 11th 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.
A moderate earthquake struck at 2:47pm on the 12th
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.
A moderate earthquake struck at 1:22am on the 13th
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 4th
January Taupo and 12th 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.
A moderate earthquake struck at 11:51am on the 21st
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 4th
January Taupo, 12th January Whanganui and 13th 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.
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.
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
28th 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, 29th January 2016
The fifth major earthquake struck at 3:20pm on the 29th
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.