The 1999 Ambrym earthquake occurred on November 26 at with a moment magnitude of 7.4 and a maximum Mercalli intensity of VII (''Very strong''). The

back arc The back-arc region is the area behind a volcanic arc. In island volcanic arcs, it consists of back-arc basins of oceanic crust with abyssal depths, which may be separated by remnant arcs, similar to island arcs. In continental arcs, the back-ar ...

thrust event occurred within the

Vanuatu Vanuatu ( or ; ), officially the Republic of Vanuatu (french: link=no, République de Vanuatu; bi, Ripablik blong Vanuatu), is an island country located in the South Pacific Ocean. The archipelago, which is of volcanic origin, is east of no ...

archipelago, just to the south of the

volcanic island Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

Ambrym Ambrym is a volcanic island in Malampa Province in the archipelago of Vanuatu. Volcanic activity on the island includes lava lakes in two craters near the summit. Etymology Ambrym (also known as ''Ambrin'', ''"ham rim"'' in the Ranon language ...

. Vanuatu, which was previously known as New Hebrides, is subject to volcanic and earthquake activity because it lies on an active and destructive plate boundary called the New Hebrides Subduction Zone. While the National Geophysical Data Center classified the total damage as moderate, a destructive local

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 ...

did result in some deaths, with at least five killed and up to 100 injured. This was one of very few events along the New Hebrides Subduction Zone that has been studied. Scientists from technical institutes in the region submitted papers to the scientific journals about the event, and while not all data on each of the aspects of the event are in agreement, what is certain is that the d'Entrecasteaux Ridge is producing unusual effects where it infringes into the

subduction zone 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 ...

via 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 ...

Tectonic setting

The primary tectonic feature of the island chain is the New Hebrides Subduction Zone, the convergent boundary of the Indo-Australian and

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 ...

s. Along the

Wadati–Benioff zone A Wadati–Benioff zone (also Benioff–Wadati zone or Benioff zone or Benioff seismic zone) is a planar zone of seismicity corresponding with the down-going slab in a subduction zone. Differential motion along the zone produces numerous earthqu ...

, earthquake activity has been observed as shallow, intermediate, and deep-focus events at depths of up to . Volcanic activity is also present along this north-northwest trending and northeast- dipping oceanic trench. While much of the island arc experiences intermediate-depth earthquakes along a Wadati–Benioff zone that dips steeply at 70°, the area adjacent to the

d'Entrecasteaux Ridge The d'Entrecasteaux () Ridge (DER) is a double oceanic ridge in the south-west Pacific Ocean, north of New Caledonia and west of Vanuatu Islands. It forms the northern extension of the New Caledonia–Loyalty Islands arc, and is now actively su ...

does not. There is a corresponding gap in seismicity that occurs below where it intrudes into the subduction zone from the west. According to the NUVEL-1 global relative plate motion model, convergence is occurring at roughly per year. The uncertainty, which also affects the Tonga arc, is due to the influence of spreading at the

North Fiji Basin The North Fiji Basin (NFB) is an oceanic basin west of Fiji in the south-west Pacific Ocean. It is an actively spreading back-arc basin delimited by the Fiji islands to the east, the inactive Vitiaz Trench to the north, the Vanuatu/New Hebrides ...

. Of the 58 M7 or greater events that occurred between 1909 and 2001, few were studied.

Earthquake

Scientists with the Institut de recherche pour le développement in

New Caledonia ) , anthem = "" , image_map = New Caledonia on the globe (small islands magnified) (Polynesia centered).svg , map_alt = Location of New Caledonia , map_caption = Location of New Caledonia , mapsize = 290px , subdivision_type = Sovereign st ...

investigated the extent of the aftershock zone and estimated that the rupture area was about , but an inversion of

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

-based displacement data showed a smaller rupture area of . It was also described as an

intraplate In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny (mountain building). Anorogenic magmatism occurs, for example, at mid-ocean ridges, hotspots and continental rifts. This contrast ...

event that occurred away from the east-dipping subduction interface on a west-dipping fault in an area with an uncertain type of convergence (either subduction or crustal thickening). The

Harvard Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

Centroid Moment Tensor project lists the slip vector as 67°, indicating that the mechanism was mostly thrust, with a significant amount of left-lateral strike-slip motion. Their submission for other fault parameters showed that the north-striking fault dipped shallowly at 30°. USGS Shakemap - 1999 Ambrym earthquake

Damage

Earthquake damage was limited to northern

Pentecost Island Pentecost Island is one of the 83 islands that make up the South Pacific nation of Vanuatu. It lies due north of capital Port Vila. Pentecost Island is known as in French and in Bislama. The island was known in its native languages by ...

, but the tsunami destroyed the village of Baie Martelli (population 300) on the southern tip of Pentecost. The loss of life there was minimal and could be attributed to well-educated people with regard to tsunami hazard, that even at the late hour, the villagers were awake because of a wedding celebration, and because of quick access to a safe area. Eyewitnesses described the initial phase of the tsunami as negative (withdrawal). This wave action came within 10 minutes of the earthquake and was followed shortly after by two larger waves that were separated by about 15 minutes. These waves destroyed all the buildings that were made of woven grass and

corrugated metal Corrugated galvanised iron or steel, colloquially corrugated iron (near universal), wriggly tin (taken from UK military slang), pailing (in Caribbean English), corrugated sheet metal (in North America) and occasionally abbreviated CGI is a bu ...

roofs. The maximum height of the waves above

sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...

was . To the west of the epicenter on the island of

Malakula Malakula Island, also spelled Malekula, is the second-largest island in the nation of Vanuatu, formerly the New Hebrides, in Melanesia, a region of the Pacific Ocean. Location Malakula is separated from the islands of Espiritu Santo and Malo by ...

, a moderate runup of just caused the wooden cargo ship ''Halimon'' to sink while it was at anchor in of water. The ship was loaded with 18 tons of

copra Copra (from ) is the dried, white flesh of the coconut from which coconut oil is extracted. Traditionally, the coconuts are sun-dried, especially for export, before the oil, also known as copra oil, is pressed out. The oil extracted from copr ...

(dried coconut meat). While the crew were asleep on board, the initial withdrawal of the tsunami caused the vessel to settle onto the bottom of the bay and list severely. Later, the first positive tsunami wave entered the harbor and swamped the vessel. Investigators were reminded of a similar situation from the much more destructive tsunami that followed the

1868 Arica earthquake The 1868 Arica earthquake occurred on 13 August 1868, near Arica, then part of Peru, now part of Chile, at 21:30 UTC. It had an estimated magnitude between 8.5 and 9.3. A tsunami (or multiple tsunamis) in the Pacific Ocean was produced by t ...

in Chile that affected multiple vessels in a similar fashion.

Tsunami

A wave height was recorded at the southern tip of Pentecost at the village of Baie Martelli. There, all simple structures were destroyed, except for a church. While 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 ...

placed the epicenter to the south of Ambrym island (very close to the ISC-GEM coordinates on the map), the Harvard epicenter is to the north (somewhat between Pentecost and Ambrym). The northerly location could account for the greatest runup that was observed on the south shore of Pentecost and the north shore of Ambrym. The wave heights decreased with distance from that location, but scientists lacked a source for the unexpected runups ( to ) that were seen well to the south on the island of Efate.

References

External links

M7.5 - Vanuatu
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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, ...