The 2004 Tasman Sea earthquake, also known as the 2004 Macquarie Island earthquake, was a very large

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

8.1 earthquake which struck a remote area of the southern

Tasman Sea The Tasman Sea (Māori: ''Te Tai-o-Rēhua'', ) is a marginal sea of the South Pacific Ocean, situated between Australia and New Zealand. It measures about across and about from north to south. The sea was named after the Dutch explorer Abe ...

in the early morning hours of Friday 24 December 2004. Its epicentre was located roughly northwest of the

Auckland Islands The Auckland Islands (Māori: ''Motu Maha'' "Many islands" or ''Maungahuka'' "Snowy mountains") are an archipelago of New Zealand, lying south of the South Island. The main Auckland Island, occupying , is surrounded by smaller Adams Island, ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

, and roughly north of

Macquarie Island Macquarie Island is an island in the southwestern Pacific Ocean, about halfway between New Zealand and Antarctica. Regionally part of Oceania and politically a part of Tasmania, Australia, since 1900, it became a Tasmanian State Reserve in 197 ...

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. Shaking was reportedly felt as far as

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

and the

South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...

. The event was a complex intraplate earthquake within the

Australian Plate The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when India broke ...

, which generated a small

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explo ...

Tectonic setting

The region to the south of New Zealand involves a complex boundary between the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

and the

. Immediately to the south of the South Island is the

Puysegur Trench The deepStern, Robert J. (2004)Subduction initiation: spontaneous and induced Earth and Planetary Science Letters 226, 275–292. . Puysegur Trench is a deep cleft in the floor of the south Tasman Sea formed by the subduction of the Indo-Austral ...

, where the Australian Plate is

subducting Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

obliquely under the Pacific Plate. To the south of this, the

Macquarie Fault Zone The Macquarie Fault Zone is a major right lateral-moving transform fault along the seafloor of the south Pacific Ocean which runs from New Zealand southwestward to the Macquarie Triple Junction. It is also the tectonic plate boundary between the ...

is a dextral (right lateral)

transform boundary A transform fault or transform boundary, is a fault along a plate boundary where the motion is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a subductio ...

. Further south, the Australian Plate is subducting underneath the Pacific Plate once again at the

Hjort Trench The Hjort Trench is a linear topographic depression south of Macquarie Island in the southwest Pacific Ocean. Geologically, the depression is considered to be the seafloor expression of an ocean-ocean subduction zone, where the Australian plate ...

. These rapid transitions between boundary types are the result of the boundary being located close to the Australia-Pacific pole of rotation. The Macquarie Fault Zone was originally a

divergent boundary In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent b ...

. At divergent boundaries, the spreading ridges are offset from each other and separated by small sections of transform faulting.

Fracture zones Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

within the tectonic plates are created as continuations of these transform faults. Roughly 29 million years ago, the Australia-Pacific pole of rotation began to quickly migrate to the southeast, resulting in the boundary transitioning from a divergent boundary to a transform boundary over the span of about 4 million years. As a result of this transition, the fracture zones of the original divergent boundary have separated from each other and have curved to become more tangential to the plate boundary as they get closer to it. The area of fracturing within the Australian Plate is known interchangeably as the Macquarie Block or the Puysegur Block.

Earthquake

The earthquake occurred at a shallow depth and had a

moment magnitude The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

of 8.1, with its epicentre being located roughly to the west of the boundary between the Australian and Pacific Plates. The earthquake was a sinistral (left lateral)

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

event, with the rupture being in length. The rupture lasted roughly 100 seconds. Roughly 30 seconds into the rupture, a second subevent was observed to have occurred to the west of the main rupture, which lasted roughly 60 seconds. The rupture occurred on one of the many ancient fracture zones within the Macquarie Block, known as Fracture Zone 5 or the Lhuwa Fracture Zone. This fracture zone curves sharply as it approaches the plate boundary, and the epicentre of the earthquake was located where the fracture zone curves the sharpest. As a result, the fracture zone ruptured bilaterally; roughly ruptured to the south of the epicentre and roughly ruptured to the northwest of the epicentre. It is hypothesised that at the

, the thicker continental crust of New Zealand is acting as a buttress which impedes the movement of the Australian Plate. This results in the steep angle of subduction observed at the trench, and transfers stresses down into the Macquarie Block farther south, where the ancient fracture zones are pre-existing areas of crustal weakness. The stress that is imparted into the Macquarie Block is released through the rupture of these weak fracture zones, and this is believed to have been the cause behind this earthquake.

Aftershocks

The

USGS The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, a ...

recorded 42 aftershocks greater than magnitude 4.0 within the year after the earthquake. The largest of these was a magnitude 6.1 earthquake on 3 January 2005.

Other events

On Tuesday 23 May 1989, a magnitude 8.0 earthquake struck the plate boundary farther south near

. After this event, a large section of the southern end of the Macquarie Block was reactivated, with many smaller earthquakes occurring to the northwest of the plate boundary along one of the fracture zones. However, the fracture zone that was reactivated by the 1989 event was not the same fracture zone which ruptured in the 2004 event. Only 58 hours after the earthquake, the much larger magnitude 9.1-9.3

2004 Indian Ocean earthquake An earthquake and a tsunami, known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake, occurred at 07:58:53 local time (UTC+7) on 26 December 2004, with an epicentre off the west coast of northern Suma ...

occurred. The two events are believed to be unrelated. As a result of the Indian Ocean earthquake, little attention was immediately given into research on the Tasman Sea earthquake. The interfering seismic activity that resulted from the Indian Ocean event and its aftershocks made it difficult to reliably determine the epicentre locations of many of the aftershocks of the Tasman Sea event.

Impact

The earthquake was felt over a large area. In

, the shaking was prolonged and was strong enough to wake people up from their sleep at the southern end of the

, but no damage was reported. In

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

, local police received a handful of calls after the earthquake, with reports of shaking houses and swinging light fittings. On the remote

, all 22 people working at

Macquarie Island Station The Macquarie Island Station is a permanent Australian subantarctic research base on Macquarie Island, commonly called Macca, situated in the Southern Ocean and located approximately halfway between mainland Australia and Antarctica, managed b ...

reported they slept through the earthquake, only discovering it had happened upon waking up in the morning. There were no reports of damage on the island.

Tsunami

The earthquake generated a small

, which reached a height of in

Jackson Bay Jackson Bay / Okahu is a gently curving 24 km bay on the southern West Coast of New Zealand's South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the ...

, in Spring Bay,

Tasmania ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

, and in

Port Kembla A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ha ...

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

. There were no eyewitnesses to the tsunami and it caused no damage.

References

{{Earthquakes in 2004 2004 earthquakes 2004 tsunamis Earthquakes in Australia December 2004 events in New Zealand Earthquakes in New Zealand December 2004 events in Australia 2004 disasters in Oceania 2004 disasters in New Zealand 2004 disasters in Australia

Tectonic setting

Earthquake

Aftershocks

Other events

Impact

Tsunami

See also

References