Tuesday, 7 April 2020

Magnitude 5.2 Earthquake between Tobago and Grenada.

The United States Geological Survey recorded a Magnitude 5.2 Earthquake at a depth of 31.0 km roughly half way between the islands of Tobago and Grenada,  slightly after 11.25 pm local time on Monday 6 April 2020 (slightly after 3.25 am on Tuesday 7 April GMT). There are no reports of any casualties or damage associated with this event, though the quake was felt over a large area, with people reporting feeling it across Tobago, Grenada, Trinidad, and St Vincent and the Grenadines.

The approximate location of the 6 April 2020 Tobago and Grenada Earthquake. USGS.

The Lesser Antilles Islands, including Grenada and Tobago are located at the eastern fringe of the Caribbean Tectonic Plate. The Atlantic Plate (strictly speaking, an extension of the South American Plate which runs to the northeast of the Caribbean) is being subducted beneath this, and as it sinks into the Earth, is melted by the heat of the planets interior. Some of the melted material then rises up through the overlying Caribbean Plate as magma, fuelling the volcanoes of the Lesser Antilles Volcanic Arc. The subduction of the Atlantic Plate beneath the Caribbean Plate is not a smooth process, with the two plates constantly sticking together then breaking apart as the tectonic pressure builds up, causing Earthquakes in the process, though since the boundary between the two plates is some way to the east of the islands, Earthquakes in the Lesser Antilles tend to be both deep and offshore, which lessens their destructive potential.

 The subduction of the Atlantic Plate beneath the Caribbean Plate fuels the volcanoes of the Lesser Antilles Volcanic Arc. George Pararas-Carayannis.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The University of the West Indies is interested in hearing from people who have felt Earthquake events in the region; if you felt such a quake then you can report it here. 

See also...

Follow Sciency Thoughts on Facebook.

Cyclone Harold makes landfall in Vanuatu, killing at least six people.

Six people have been confirmed dead and 21 more are missing after Cyclone Harold made landfall on the Espiritu Santo, the largest island of the Republic of Vanuatu, on Monday 6 April 2020. The storm is described as a Catagory 5 Cyclone, which brought with it sustained wind speeds of up to 235 km per hour (a sustained wind speed is a speed that is sustained for over a minute), making it the most severe storm ever to hit Espiritu Santo, and the worst storm to hit Vanuatu in five years. After passing over Espiritu Santo the island gained strength as it moved to the southeast, hitting the island of Pentacost with sustained winds of 270 km per hour later in the day.

Damage caused by Cyclone Harold on the island of Espiritu Santo, Vanuatu. Oscar Delai Umuumulovo/IRFCAsiaPacific/Twitter.

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 path and strength of Cyclone Harold. Thick line indicates the past path of the storm (till 6.00 am GMT on Tuesday 7 April 2020), while the thin line indicates the predicted future path of the storm, and the dotted circles the margin of error at twelve and twenty four, thirty six, forty eight, and seventy two hours ahead. Colour indicated the severity of the storm. Tropical Storm Risk.

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.

Flooding associated with Cylcne Harold on Espiritu Santu Island. Oscar Delai Umuumulovo/IRFCAsiaPacific/Twitter.

See also...

Follow Sciency Thoughts on Facebook.

Attackers kill four in raid on gold mine in the Democratic Republic of Congo.

Four people, reported to be three Chinese nationals and a Congolese citizen, have been killed in an attack on a gold mine in Ituri Province in the northeast of the Democratic Republic of Congo on Saturday 4 April 2020. A second Congolese national is described as having been seriously injured in the incident, at the Ndaki Mine in the village of Sumbabho in Irumu District. The identity of the attackers is unclear at this time, but it has been reported that a quantity of gold was removed during the raid.

The approximate location of the Ndaki Gold Mine. Google Maps.

The Democratic Republic of Congo is one of Africa's poorest nations, and like many other countries has granted concessions to mining companies in areas where small-scale artisanal mining has traditionally helped to supplement the incomes of subsistence farmers. The country has suffered years of political instability and has extremely poor infrastructure, is heavily indebted and has a chronic corruption problem, which means that little of the wealth generated from the industry makes it into government coffers, and that which does is unlikely to be spent on development or other projects likely to benefit the population. These problems are made worse by a series of conflicts in the area, which plays host to rebel groups opposed to the Congolese government and militiamen that fled the 1994 conflict in Rwanda and have never returned. In addition, the area has suffered incursions from several neighbouring countries, mostly with an official purpose of defending borders against Congo's instability, but with a strong interest in the minng industry. Militia groups linked to neighbouring states also operate in the area, and again are said to be involved in illegal mining.

The situation has been brought under control to some extent in the mineral-rich provinces in the southeast of the country, but the situation in Ituri Province has deteriorated recently, possibly due to the ongoing conflict in nearby South Sudan, with the Canadian mining company Banro closing its ooperations in the province and the Chinese Embassy in Kinshasa recommending against travel to the area.

See also...

Follow Sciency Thoughts on Facebook.

Monday, 6 April 2020

Magnitude 2.2 Earthquake near Brackley in Northamptonshire, England.

The British Geological Survey recorded a Magnitude 2.2 Earthquake at a depth of 4 km, about 1 km to the southeast of the town of Brackley in Northamptonshire, England, slightly before 4.30 pm British Summertime (slightly before 3.30 pm GMT) on Saturday 4 April 2020. There are no reports of any damage or casualties associated with this event, and nor would any be expected of an Earthquake of this size, but it may have been felt locally.

The approximate location of the 4 April 2020 Northamptonshire Earthquake. Google Maps.

Earthquakes become more common as you travel north and west in Great Britain, with the west coast of Scotland being the most quake-prone part of the island and the northwest of Wales being more prone  to quakes than the rest of Wales or most of England. However, while quakes in southern England are less frequent, they are often larger than events in the north, as tectonic pressures tend to build up for longer periods of time between events, so that when they occur more pressure is released.

The precise cause of Earthquakes in the UK can be hard to determine; the country is not close to any obvious single cause of such activity such as a plate margin, but is subject to tectonic pressures from several different sources, with most quakes probably being the result of the interplay between these forces.

Britain is being pushed to the east by the expansion of the Atlantic Ocean and to the north by the impact of Africa into Europe from the south. It is also affected by lesser areas of tectonic spreading beneath the North Sea, Rhine Valley and Bay of Biscay. Finally the country is subject to glacial rebound; until about 10 000 years ago much of the north of the country was covered by a thick layer of glacial ice (this is believed to have been thickest on the west coast of Scotland), pushing the rocks of the British lithosphere down into the underlying mantle. This ice is now gone, and the rocks are springing (slowly) back into their original position, causing the occasional Earthquake in the process. 

(Top) Simplified diagram showing principle of glacial rebound. Wikipedia. (Bottom) Map showing the rate of glacial rebound in various parts of the UK. Note that some parts of England and Wales show negative values, these areas are being pushed down slightly by uplift in Scotland, as the entire landmass is quite rigid and acts a bit like a see-saw. Climate North East.
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. If you felt this quake, or were in the area but did not (which is also useful information) then you can report it to the British Geological Survey here.  

See also...

Follow Sciency Thoughts on Facebook.

Asteroid 2011 GM44 passes the Earth.

Asteroid 2011 GM44 passed by the Earth at a distance of about 11 663 000 km (30.4 times the average distance between the Earth and the Moon, or 7.80% of the distance between the Earth and the Sun), at about 7.30 am GMT on Monday 30 March 2020. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. 2011 GM44 has an estimated equivalent diameter of 220-680 m (i.e. it is estimated that a spherical object with the same volume would be 220-680 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be 23 000-825 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 3-10 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.

 Image of 2011 GM44 taken on 5 April 2011. The image is a composite of eleven 20 second esposures taken with the Tzec Maun Foundation's Takahashi TOA-150 f/7.3 refractor telescope at the Siding Spring Observatory. The dotted lines are stars, which have moved between exposures, with the single point at the centre of the image, indicated by the two lines, being the asteroid. Giovanni Sostero and Ernesto Guido/Comets & Asteroids - Small Bodies of the Solar System.

2011 GM44 was discovered on 5 April 2011 at Siding Spring Observatory in Australia. The designation 2011 GM44 implies that it was the 1068th asteroid (asteroid M44 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that M44 = 12 + (24 X 44) = 1068) discovered in the first half of May 2011 (period 2011 G).

 The calculated orbit of 2011 GM44. JPL Small Body Database.

2011 GM44 has a 252 day orbital period, with an elliptical orbit tilted at an angle of 49.3° to the plain of the Solar System which takes in to 0.38 AU from the Sun (38% of the distance at which the Earth orbits the Sun, and roughly the distance at which Mercury orbits the Sun) and out to 1.19 AU (19% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in March 2018 and the next predicted in April 2022. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2011 GM44 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (2011 GM44 is also classified as a Potentially Hazardous Asteroid. The asteroid also has occasional close encounters with the planet Mercury, with the last having happened in April 2008, and the next predicted for March 2024.

See also...

Follow Sciency Thoughts on Facebook.