The 2018 Fiji earthquakes occurred on August 19, at 00:19:40 UTC and on September 6 15:49 UTC. The epicenters were located close to the Fijian island

Lakeba Lakeba (pronounced ) is an island in Fiji’s Southern Lau Archipelago; the provincial capital of Lau is located here. The island is the tenth largest in Fiji, with a land area of nearly 60 square kilometers.Steadman (2006) It is fertile and wel ...

, and around 270 km from the small town of

Levuka Levuka () is a town on the eastern coast of the Fijian island of Ovalau, in Lomaiviti Province, in the Eastern Division of Fiji. Prior to 1877, it was the capital of Fiji. At the census in 2007, the last to date, Levuka town had a population ...

on Ovalau. The first earthquake registered a magnitude of 8.2, and is the largest earthquake of 2018. It had a focal depth of 600 km, making it the second largest earthquake ever recorded at a depth greater than 300 km; a tie with the

1994 Bolivia earthquake The 1994 Bolivia earthquake occurred on June 9, 1994. The epicenter was located in a sparsely populated region in the Amazon jungle, about 200 miles from La Paz. The Harvard CMT Project assigned it a focal depth of 647 km and a magnitude ...

, and behind the 2013 Okhotsk Sea earthquake. The initial earthquake was caused by a

normal fault 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 tecton ...

below the South Pacific Ocean. An 7.9 event struck the islands again on September 6 at a depth of 670 km; this earthquake was a mainshock of its own. Both earthquakes may be considered a doublet event.

Tectonic evolution

Initially, the Pacific Plate and the Indo-Australian Plate subducted, resulting in the heavier

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 ...

to move beneath the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

. The

subduction 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 ...

of the two plates resulted in the formation of a volcanic

island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

called the Vityaz Arc. Twelve million years later, a similar event occurred. The Pacific plate converged towards the Indo-Australian plate at a north westward motion. Similarly, another volcanic island arc was formed near Fiji. The north westward motion continued to occur creating lots of stress on the plate boundary. Consequently, the Vityaz Arc was broken and the Fiji fracture zone was created. Recently, the movement of the plates created a subduction zone on the New Hebrides

Trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from ero ...

. However, the Indo-Australian plate was pulled below the Pacific plate. The motion of these plates created a new volcanic island arc called the New Hebrides Arc. Eventually, the New Hebrides Arc and the Fiji islands started to diverge, which ultimately created the North Fiji Basin.

Background

A notable earthquake in Fiji's history was one that occurred off the coast of Suva in 1953, as well as the 1979 Taveuni earthquake. The Suva earthquake of 1953 measured 6.8, and was located off the southeastern coast of

Viti Levu Viti Levu (pronounced ) is the largest island in the Republic of Fiji. It is the site of the nation's capital, Suva, and home to a large majority of Fiji's population. Geology Fiji lies in a tectonically complex area between the Australian P ...

. The earthquake collapsed a

coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

platform and ultimately caused a tsunami. The earthquake, combined with the tsunami, took the lives of eight people and severely damaged a lot of architecture. The latter earthquake occurred in 1979 measuring 6.9. While this event did not claim any lives, it caused major damage to buildings and triggered a major landslide on the island of

Qamea Qamea (pronounced ) is one of three islets lying to the east of Thurston Point on the island of Taveuni, Fiji, the others being Matagi and Laucala. Geography Qamea lies some 2.5 kilometers east of Thurston Point and covers an area of 34 square ...

Earthquakes

Deep earthquakes are commonly defined as events occurring at a depth of 300 km or greater. These earthquakes occur within subducting slabs that are descending into the mantle. As these plates bend or flex, faults break out to accommodate the deformation and along with it, occasionally producing earthquakes. Normally, deep earthquakes such as this one will not have too many aftershocks, however there are cases in which a deep earthquake produces much more than it should as if it were a shallow earthquake like Tonga in 1994.

August 19

Initially reported as an 8.0 earthquake, it was later revised by the

United States Geological Survey 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, ...

to an 8.2. Due to it being a deep-focus event, no damage was reported but it was widely felt. The undersea quake was too deep to trigger a large tsunami according to the US Pacific Tsunami Warning Center. Small "tsunami" waves were spotted along the coast, although it was too deep to generate more than a negligible tsunami. More than 250 aftershocks were recorded 34 days after the mainshock. The focal mechanism solution suggest the shock had a complex rupture process, and fast rupture velocity of 4.1 km/s. The rupture was observed along multiple faults in a mainly north-northeast-striking fault for a length of 102 to 152 km.

September 6

Eighteen days after the 8.2 event, another earthquake struck closer to the island of

. The 7.9 earthquake was a downgrade from an initial magnitude of 8.1. The earthquake had a complex combination of strike-slip and dip-slip during rupture within the slab. It was an unusual event as this portion of the downgoing slab is rather aseismic; not capable of producing earthquakes. The depth of this event was slightly deeper than the first quake, and the rupture velocity was much slower, at 2.5 km/s. Coulomb stress transfer from the 8.2 quake may have triggered the 7.9 earthquake. However, stress from the first quake itself was not sufficient to generate the latter event. The 7.9 event may have been dynamically triggered by the 8.2 quake but the 18-day interval period was longer than usual as triggered quakes usually nucleate within a few hours to days.

References

External links

* * {{Earthquakes in 2018 2018 earthquakes Earthquakes in Fiji Earthquakes in Tonga Doublet earthquakes 2018 in Fiji August 2018 events in Oceania