Landslide in the Solomon Islands kills at least four.

Four people have been confirmed dead and three more are missing following a landslide on the island of Guadalcanal in the Solomons on Thursday 12 March 2020. The event happened at Lambi on the west coast of the island amid heavy rains and flooding associated with a low-pressure system that swept across the islands. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

 The approximate location of the 12 March 2020 Guadalcanal landslide. Google Maps.

The Solomon Islands has a tropical climate, with a wet season that runs from November to April and a dry season that lasts from May to October. However, the dry season is not completely arid, typically still receiving about 100 mm of rain per month, and this year has been exceptionally wet, with some of the heaviest rain ever recorded in the islands falling in the past week, and local authorities are warning there is a danger of further landslips and flooding.

Damage to Takaboru Bridge on  Guadalcanal Island following days of heavy rain in the area. Loni Sitana/Facebook.

Low presure systems are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical weather systems peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides.

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Magnitude 5.5 Earthquake beneath Guadalcanal Island, the Solomons.

The United States Geological Survey detected a Magnitude 5.5 Earthquake at a depth of 31.5 km, beneath the island of Guadalcanal in the Solomons slightly after 0.40 am local time on Friday 3 January 2020 (slightly after 1.40 pm on Thursday 2 December, GMT).There are no reports of any damage or injuries associated with this event.

 The approximate location of the 3 December 2020 Guadacanal Earthquake. USGS.

Guadacanal Island lies on the southern part of the Pacific Plate, close to its boundary with the Australian Plate. The Australian Plate is moving northward relative to the Pacific Plate, and being subducted beneath it, along the Solomon Trench, which runs to the south of Guadacanal Island. This is not a smooth process, and the two plates continuously stick together and then break apart as the pressure builds up, creating shocks which we experience as Earthquakes.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

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Magnitude 5.8 Earthquake in the Solomon Islands.

The United States Geological Survey detected a Magnitude 5.8 Earthquake at a depth of 10 km, off the west coat the island of Guadalcanal in the Solomons slightly after 5.50 am local time on Friday 30 March 2018 (slightly after 6.50 pm on Thursday 29 March, GMT).There are no reports of any damage associated with this event, but some minor damage occurred on Guadalcanal.

The approximate location of the 30 March 2018 Guadacanal Earthquake. USGS.

The Guadacanal Island lies on the southern part of the Pacific Plate, close to its boundary with the Australian Plate. The Australian Plate is moving northward relative to the Pacific Plate, and being subducted beneath it, along the Solomon Trench, which runs to the south of Guadacanal Island. This is not a smooth process, and the two plates continuously stick together and then break apart as the pressure builds up, creating shocks which we experience as Earthquakes.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

 

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Neotrygon vali: A new species of Maskray from Guadalcanal Island, in the Solomon Archipelago.

Maskrays, Neotrygon spp., are a group of Stingrays, Dasyatidae, found in the Indian Ocean and west Pacific. They get their name from a distinctive coloured marking around the eyes, which resembles a mask, but have a number of other distinctive features, including short tails with well developed dorsal fins, small mouths with enlarged cuspid teeth, and large pectoral fins with a single row of thorns along their dorsal midline.

In a paper published on the bioRxiv beta database on 25 July 2017, Philippe Borsa of Ecologie marine tropicale des océans Pacifique et Indien at the Institut de recherche pour le développement, describes a new species of Masked Ray from Guadalcanal Island, in the Solomon Archipelago.

The new species is named Neotrygon vali, where 'vali' means 'Stingray' in Gela, one of the languages spoken on Guadalcanal. The new species is described from a female specimen 295 mm in length obtained from the Plaza fish market in Honiara om Guadalcanal Island. It is distinguished from other Maskrays by the darkness of its eye marking and the pattern of blue spots on its dorsal surface, and confirmed as a new species by DNA analysis.

Guadalcanal Maskray Neotrygon vali showing the pigmentation patterns that differentiate it from Neotrygon kuhlii from Vanikoro. Borsa (2017).

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