Sciency Thoughts: Magnitude 6.5 Earthquake off the west coast of Espiritu Santo Island, Vanuatu.

Sunday 5 March 2023

Magnitude 6.5 Earthquake off the west coast of Espiritu Santo Island, Vanuatu.

The United States Geological Survey recorded a Magnitude 6.5 Earthquake at a depth of 10.0 km, to the west of Espirtitu Santo Island, Vanuatu, slightly before 5.05 am local time on Saturday 4 March 2023 (slightly before 6.05 pm on Friday 3 March, GMT). There are no reports of any damage or casualties associated directly with this event, although the event happened directly after the island nation was hit by Cyclone Judy, on Thursday 2 March, and Cyclone Kevin, on Friday 3 March, both of which caused considerable damage, making it hard to assess the impact of the Earthquake. Cyclones are a common problem in Vanuatu, but the country has never previously been hit by two storms in such a short period of time.

The approximate location of the 4 March 2023 Vanuatu Earthquake. USGS.

Vanuatu is located on the southwestern fringe of the Pacific Plate, close to its boundary with the Australian Plate, which is being subducted along the New Hebrides Trench, to the west of the islands. The subducting Australian Plate passes under the islands of Vanuatu as it sinks into the Earth, causing Earthquakes as the plates stick together then break apart as the pressure builds up. As the plate sinks further it is partially melted by the heat of the friction combined with that of the Earth's interior. Some of the melted material then rises through the overlying Pacific Plate, feulling the volcanoes of Vanuatu.

Tropical storms, called Cyclones in the Indian Ocean and South Pacific, 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 storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

The formation of a tropical cyclone. Natural Disaster Management.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.