Natural Phenomenon: Seismic Activity in Solomon Islands
- Alternative Names
The Solomon Islands lies in one of the world's most active seismic areas, exceeded only by Japan. Earthquakes of widely varying severity occur throughout the islands and there are several volcanoes. As a consequence, tsunamis occur also, leaving areas uninhabitable for long periods and even decades. Over thousands of years, many Solomon Islanders have perished in these natural catastrophes, which have at times forced people to migrate to different islands. Early European visitors recorded evidence of these events, providing us a four hundred-year time depth. We can also draw on local oral histories of geologic calamities. The accounts that follow are only a sample.
Resident Commissioner C. M. Woodford (q.v.) recorded tsunamis along the north coasts of Nggela, Malaita and Makira after a January 1898 earthquake. They washed away coastal villages and exposed reefs. Ferasubua, the largest artificial island in Malaita's Lau Lagoon, was swept clean, though its population of four hundred escaped to the mainland. (Cecil Wilson, 'S.E. Mala', SCL, Apr. 1906, 37; AR 1898-1899, 11; BSIP Handbook 1911, 23) On 25 January 1925 the Catholic stone cathedral at Visale on Guadalcanal was destroyed by an earthquake that lasted between forty and fifty seconds. The shock was felt throughout the central Solomons and a tsunami struck some localities. (AR 1925-1926, 3; NS 14 Feb. 1968)
On 3 October 1931 severe earthquakes were felt in the central and eastern islands, which continued spasmodically for a month. A tsunami following an earthquake on Makira completely destroyed eighteen villages and killed forty-eight people. On Owa Riki (Santa Catalina) Island the quake badly damaged the reef. (Tedder 2008, 76) Christians thought judgement day had come and some others thought that their ancestors were angry. (AR 1931, 20) In June 1933 tremors and a violent earthquake occurred at Gizo over two days, and in July 1934 Vanikolo experienced an earthquake, its epicentre thought to have been to the southwest, and a withdrawal of the ocean but no tsunami. (AR 1934, 20) Lake Lees on Guadalcanal was formed at this time. In March 1935 a severe earthquake hit the district around Gizo and in December Tulagi experienced severe tremors, and landslides occurred on Savo (q.v.) and Guadalcanal. (AR 1935, 18; BSIP Advisory Council Minutes, 7 May 1935) In May 1939 a large earthquake struck the central Solomons, causing a tsunami which damaged western Guadalcanal, the Russell Islands and Isabel. Local villages and plantation wharves were swept away and coconut plantations inundated by salt water. The tsunami was between six and nine meters high when it crossed the northwest coast of Guadalcanal. At Hautabu near Maravovo the Melanesian Mission Press was damaged and water tanks were destroyed. (BSIP Advisory Council Minutes, 3 May 1939; SCL Oct. 1939) The Shortland Islands just south of Bougainville suffered numerous earthquakes and oral testimonies from there record that many years ago a twelve-metre tsunami devastated low areas and drowned most of their inhabitants. A similar event occurred in the Western Solomons in 2007 when an earthquake centred off Rendova generated a tsunami that devastated the surrounding area, killing many and destroying much property.
Guadalcanal experienced its most severe earthquake in living memory on 21 April 1977, measuring 9 to 10 on the Richter Magnitude Scale at Mercalli, 8 at Tetere and 7.1 at Honiara. The epicentre was thought to have been a few kilometres offshore from Avuavu in the south. Honiara was shaken and buildings collapsed, roads split and shelving was dislodged. In the Hong Kong and Shanghai Bank glass shattered and walls cracked. There was great devastation in the island's centre and the reef off Lauvi Lagoon rose some two metres out of the ocean. New hot springs formed and sulphur fumes rose from the ground at Hai Marao, just outside Avuavu. The whole of Guadalcanal, particularly along the south coast, was raised and titled slightly. A tsunami that followed struck the coast from Avuavu westward, and landslides and seas of mud destroyed some villages. Food relief had to be delivered to the area for months until people could grow new crops. (SND 22 Apr. 1977, 29 Apr. 1977, 6 May 1977; Webber 2011, 223-228) Other, small earthquakes occur with regularity, such as one in the Shortland Island in 1974, which caused a minor tsunami. (AR 1974, 61)Volcanoes
There are seven active, dormant or extinct volcanoes in the Solomon Islands: Coleman Seamount; Gallego; Kana Keoki; Kavachi; Nonda; Savo; and Tinakula.
The only volcano currently considered active is Tinakula, a small island north of Nendö Island, about twelve kilometres from Graciosa Bay in the Santa Cruz area in Tomotu Province (formerly Santa Cruz Province and the Eastern Outer Islands District): location 10.38º south and 165.80º east; elevation 851 metres. Tinakula is about two kilometres in diameter and rises abruptly from the sea. We know it was active more than four hundred years ago because this was recorded by Mendaña's expedition in 1595. Tinakula is the exposed summit of a massive stratovolcano that rises three to four kilometres from the sea floor and is continually smoking. The satellitic cone is located on the southeast side. In about 1840 an explosive eruption produced lava flows that swept all sides of the island and killed its inhabitants. In 1951, Polynesians from Nukapu and Nupani resettled the island, the population peaking at 130. Volcanic activity decreased after an earthquake in February 1954 and the people survived a January 1966 eruption, which produced a large lava flow down the north side into the sea. (NS 7 Feb. 1966; Allan 1990, pt. 2, 136) The 160 former Nupani residents were evacuated to Santa Cruz after a September 1971 eruption when lava poured down the western side for two months. Some of them chose to return to Nupani and others stayed on Santa Cruz. (AR 1971, 5; NS 15 Sept. 1971, 15 Oct. 1971; Hughes 1972) In the late 1980s, two families from Nupani began another attempt at settlement. The volcano emits small eruptions approximately every hour in a plume of ash and rocks. Most large eruptions have originated from a core built up within the large breached crater.
Other volcanoes are in a sulpheritic stage. The best known of these is Savo Island, visible from Honiara across Iron Bottom Sound: location 9.13º south and 159.82º east; elevation 485 metres (1,591 feet). Savo is a forested andesitic-to-dacitic stratavolcano with a shallow, elliptical summit crater. Lava domes occur on the crater floor and its northeast rim, and there are older domes on the volcano's flanks. Pyroclastic flows and mudflows have travelled down valleys from the summit crater and formed debris fans along the coast. Thermal areas can be found in the summit crater and on the south to southeast flanks, and appear in offshore areas as steaming ground, hot springs and small geysers. The boulder and sand beaches and even the surf exhibit higher than atmospheric heat. Megapodes abound, using the heated sand to incubate their eggs. Thermally heated streams descend from the central lava dome, and steaming hot streams that descend from the crater are close to boiling near the top.
Savo was seen in eruption by the Mendaña expedition in 1568 and it erupted again in 1630-1640. Naturalist Henry B. Guppy in 1882 recorded from oral traditions that there was a long series of eruptions in the 1830s and 1840s, with the most significant probably in about 1847. The entire population perished during one eruption and another time a few survived. One eruption extended the island on its northern side. (Petterson et al. 2003)
During the late 1910s, severe earthquakes frequently shook Savo and several times the entire population was on the point of abandoning the island. The volcano is now quiescent but will erupt again at some time and is potentially dangerous, particularly to nearby Honiara, the capital city on Guadalcanal.
The newest volcano is Kavachi, a submarine volcano about thirty kilometres south of Vangunu in Western Province: location 9.02º south and 157.95º east; elevation minus twenty metres. Monitoring equipment placed on the volcano automatically transmits data on changes in its surface to geologists in Honiara. Kavachi, one of the most active submarine volcanoes in the southwest Pacific, is sometimes called Rejo te Kvachi (Kavachi's Oven). It sits on the site of subduction of the Indo-Australian plate beneath the Pacific plate. The shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to one kilometre in length many times since its first recorded eruption in 1939, and lava flows have been observed on the surfaces of these occasional islands. Residents of nearby Vangunu and Nggatokae (Gatokae) islands have reported seeing fire on the water in the area prior to 1939, which may indicate earlier eruptions. This area experienced a severe earthquake in March 1950 and the volcano erupted in 1951 astern of a trading vessel, which had just sailed over it. A small island emerged in 1952 that was gradually eroded away. Reports at this time suggest there were two volcanoes close together, although this may refer to the nearby Coleman Seamount (see below). Eruptions resumed in 1958 when a yellow cloud arose 1,300 metres in a few minutes, and continued spasmodically from 1959 until 1965, when a small island appeared once more. In January 1966, Kavachi ceased erupting and was eroded to below sea level, although it became active again in March, still below sea level. It rose from the sea again in 1968, sending smoke and ash approximately four hundred metres into the atmosphere. Early in 1970, Kavachi was reported to be fifteen to thirty metres high, but it disappeared soon after; in October it was shooting water high into the air and showing signs of re-emerging. (NS 28 Feb. 1970, 31 Oct. 1970)
The roughly conical volcano rises through water 1.1 to 1.2 kilometres deep on the north and even deeper on the south. During the periods when Kavachi has been observed there have been frequent submarine and occasional sub-aerial eruptions producing phreatomagmatic explosions that eject steam, ash and incandescent bombs above the sea surface. In November 1974 there were underwater eruptions every four minutes. (PIM July 1952, Dec. 1952; NS 28 Feb. 1958, 30 Nov. 1958, Dec. 1959, Sept. 1961, Feb. 1962, 15 Jan. 1964, June 1964, 21 Dec. 1965, 21 Jan. 1966, 7 Feb. 1966, 7 Apr. 1966; AR 1974, 61)
There may have been another young submarine volcano close by to the north of Kavachi, named Coleman Seamount or Cook Volcano: location 8.83º south and 157.17º east. Following earth tremors, a new submarine volcano was reported in 1964 about ten kilometres from Munda, which eventually subsided. The University of Hawai'i research vessel Moana Wave located the volcano south of the convergent boundary between the Indo-Australian and Pacific plates during a 1985-1986 voyage. But because it has disappeared there is now some doubt as to its existence.
There are four other volcanoes in the Solomons. Gallego is a series of volcanics from the Pliocene-to-Quaternary era comprising a group of steeply dissected cones coving a large area on northwest Guadalcanal: location 9.35º south and 159.73º east; elevation one thousand metres (3,281 feet). Local traditions mention a volcanic eruption. Mt. Esperance within the group may have been active during the past two thousand years, although the oral testimony could refer to nearby Savo Island. Kana Keoki Seamount lies southwest of Rendova Island along the Gizo Ridge, south of the convergent margin between the Indo-Australian and Pacific plates. It rises to within seven hundred metres of the surface from a depth of 3,700 metres. Nonda is the youngest of the volcanic features on Vella Lavella Island, which date from the Pliocene to Quaternary eras: location 7.67º south and 156.60º east; elevation 760 metres (2,493 feet). Nonda is a lava dome located within a well-preserved crater, and although there is no knowledge of any eruptions, in 1959 local villagers reported 'smoke' and explosive activity in the vicinity at the time of a major earthquake. The Paraso thermal area on Vella Lavella displays solfataras, hot springs, blowholes and boiling mud pools.
Simbo (Eddystone) Island in Western Province is actually two islands: one small one known as Nusa Simbo separated by a lagoon from a larger island known locally as Mandegugusu but as Simbo in the rest of the Solomon Islands. Mandegugusu has an active volcano known as Ove: location 8.292º south and 156.52º east; elevation 335 metres. Simbo contains three truncated andesitic volcanic centres. The southern half of the island is thermally active and contains fault-related fumarolic areas and hot springs near saltwater Lake Ove along the western coast near Mt. Patukio, which has a steep-walled summit crater. The lake's waters are a mixture of reds and yellows with a trace of sulphur constant in the air. There is also an undersea geyser near the island's shore, which at periodic intervals blows a steam column to the surface through the surf. Oral history suggests that at the end of the nineteenth century or early in the twentieth century there was an explosive enlargement of Ngusunu crater along the southwest coast. A newspaper report from 1899 records an eruption on Simbo, and 1950s oral testimony suggests that this 1899 event forced the evacuation of villages beside Lake Ove, which is adjacent to the Ngusunu crater. (Western Mail [Perth, W.A.], 14 Oct. 1899, 74; Webber 2011, 69-69)
Extinct volcanic cones exist on several islands-Kolombangara, Rendova, Nggatokae (Gatokae), Murray, Tikopia Vanikolo and Utupua. (NS 31 Jan. 1970; Global Volcanism Program, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C.: http://www.volcano.si.edu/world/; Grover 1956; Fisher 1954-1956; PIM July 1952, Dec. 1952; NS 30 Sept. 1968, 30 Oct. 1969; BSIP Handbook 1923, 22-23; Hviding 2005b, xxv)
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