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

of Ambrym. 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 The United States National Geophysical Data Center (NGDC) provided scientific stewardship, products and services for geophysical data describing the solid earth, marine, and solar-terrestrial environment, as well as earth observations from spac ...

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 exp ...

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 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 Plates. Along the Wadati–Benioff zone, 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 Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...

. While much of the

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 ...

experiences intermediate-depth earthquakes along a Wadati–Benioff zone that dips steeply at 70°, the area adjacent to the d'Entrecasteaux Ridge 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. 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 The Institut de Recherche pour le Développement or IRD () is a French science and technology establishment under the joint supervision of the French Ministries of Higher Education and Research and Foreign Affairs. It operates internationally ...

in New Caledonia 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 satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...

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

was . To the west of the

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

on the island of Malakula, 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 co ...

(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 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

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-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 Efate (french: Éfaté) is an island in the Pacific Ocean which is part of the Shefa Province in Vanuatu. It is also known as Île Vate. Geography It is the most populous (approx. 66,000) island in Vanuatu. Efate's land area of makes it Vanu ...

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, ...

Unusual mechanism of the Ambrym and Pentecost
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Eurekalert! The American Association for the Advancement of Science (AAAS) is an American international non-profit organization with the stated goals of promoting cooperation among scientists, defending scientific freedom, encouraging scientific respons ...