Friday, 25 April 2014

The Robben Island ‘Earthquake’ of 7 April 1620.

Most accounts of the history of seismology in South Africa relate the earliest recorded Earthquake in the country as having occurred on 7 April 1620, as recorded by the French explorer Augustin de Beaulieu, who was becalmed near Robben Island, prior to reaching Table Bay on 15 April. De Beaulieu recorded hearing a pair of ‘startling thunderclaps like cannon shots’, which have subsequently been interpreted as evidence of an Earthquake with a Magnitude of about 4 somewhere in the Western Cape.

Robben Island, to the northwest of Cape Town. Google Maps.

In a paper published in the South African Journal of Science on 13 September 2012, Sharad Master of the Economic Geology Research Institute at the School of Geosciences at the University of the Witwatersrand disputes this interpretation of the events of 7 April 1620.

Masters observes that Earthquakes recorded in the Cape Town area have been reported by witnesses as subterranean rumbling accompanied by the sound of distant thunder, but notes that only the thunder was recorded by de Beaulieu, who, being on a ship at sea, was in a poor location to make accurate recordings of seismic activity. He further notes that de Beaulieu never claimed that this thunder was evidence of an Earthquake, rather this interpretation was made based upon de Beaulieu’s record by J Theron in 1974 in a paper published in the Seismological Series of the Geological Survey of South Africa, which seems to have been widely accepted without criticism to date.

Masters observes that the most likely explanation for the sounds is that they were thunderclaps, just as recorded, and that nothing can be inferred from de Beaulieu’s failure to observe the accompanying lightning. He further notes that there are other phenomena that could potentially cause such sounds, such as distant landslides or sonic booms generated by meteorites, and while these are not common occurrences, neither are large Earthquakes in the Western Cape, and that it is not therefore sensible to jump to the conclusion that the noises of 7 April 1690 were the result of an Earthquake. 

Augustin de Beaulieu’s ‘Fleet of Montmorency’, which put into Table Bay in 1620. From an anonymous 17th-century French engraving. Wikimedia Commons/Masters (2012).

Based upon this conclusion Masters argues that the earliest recorded Earthquake in South Africa is the 1690 event recorded in Cape Town, where actual Earth tremors were recorded by a number of witnesses.

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Thursday, 24 April 2014

Asteroid 2007 TV18 passes the Earth.

Asteroid 2007 TV18 passed by the Earth at a distance of 2 841 000 km (a little under 7.4 times the average distance between the Earth and the Moon), slightly after midnight on Friday 18 April 2014. There was no danger of the asteroid hitting us, though had it done so it would have presented a genuine threat. 2007 TV18 is estimated to be between 33 m and 100 m in diameter, and an object towards the upper end of this range would be predicted to be able to punch straight through the Earth's atmosphere and impact the ground. This would result in an explosion over 2000 times as large as the one caused by the Hiroshima bomb, and create a crater over a kilometer in diameter, as well as devastation over a much wider area and climatic effects that would last for several years.

The calculated orbit of 2007 TV18. JPL Small Body Database Browser.

2007 TV18 was discovered on 9 October 2007 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2007 TV18 implies that the asteroid was the 471st  object (object V18) discovered in the first half  of October 2007 (period 2007 T).

2007 TV18 has a 503 day orbital period and an eccentric orbit that takes it from 0.81 AU from the Sun (i.e. 81% of the average distance at which the Earth orbits the Sun) to 1.67 AU from the Sun (i.e. 167% of the average distance at which the Earth orbits the Sun, outside the orbit of the planet Mars). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer).

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The origin and diversification of Bushbabies.

Bushbabies (Galagidae) are African nocturnal Primates related to Lorises (Lorisidae). Like the Lorises they are solitary nocturnal foragers living in the tree canopy, but while Lorises are slow, deliberate animals, the Bushbabies often move rapidly through the forest, and are capable of leaping from tree-to-tree like Monkeys.  The group does not have a fantastic fossil record (common for small, forest dwelling animals), though two stem group Bushbabies (i.e. animals more closely related to modern Bushbabies than to modern Lorises) are known from the Late Eocene of Fayum in Egypt and crown group Bushbabies (animals that are considered to be more closely related to some modern Bushbabies than others) appear in the fossil record from the Late Miocene onwards.

The Greater Brown Bushbaby, Otolemur crassicaudatus. Wikipedia.

The classification of modern Bushbabies is also somewhat uncertain; the group was traditionally split into two genera (Euoticus and Galago) and five species, though in recent years three more genera have been erected (Otolemur, Galagoides, and Sciurocheirus) and the group divided into as many as twenty species (the exact number varying due to a lack of consensus among primatologists). Bushbabies have proved difficult creatures to study in the wild, as they are small, nocturnal animals living high up in dense forests. They are also morphologically conservative, i.e. different and distantly related species retain essentially the same bodyplan as this suits their lifestyle well. They also lack the distinctive colouration and markings seen in Monkeys and Apes, as nocturnal animals they apparently rely on scent and vocalization to differentiate one-another. Recent studies have attempted to use genetic data to classify the group, but this has also proved problematic, with studies of different gene groups leading to different classifications.

In a paper published in the journal BMC Evolutionary Biology on 2 April 2014, Luca Pozzi of the Center for the Study of Human Origins at New York University, the New York Consortium in Evolutionary Primatology and the Behavioral Ecology and Sociobiology Unit at the German Primate Center, Todd Disotell also of the Center for the Study of Human Origins at New York University and the the New York Consortium in Evolutionary Primatology and Judith Masters of the African Primate Initiative for Ecology and Speciation at the Department of Zoology and Entomology at the University of Fort Hare describe the results of a new genetic study of Bushbaby phylogeny, which looks at 27 different sets of genes, with the aim of untangling the current confusion about relationships within the group.

Firstly Pozzi et al. found that the Galagidae is a valid taxonomic unit, i.e. all the Bushbabies were more closely related to one-another than they were to anything else. However they did discover some evidence that they were more closely related to Asian Lorises than to African ones. If this is correct (and the evidence is not yet conclusive) then the Lorisidae is a paraphyletic group, and the two Loris groups would need to be placed in different families.

Within the Family Galagidae the genus Euoticus (Needle-clawed Bushbabies) was found to be the sister taxon to all other members of the group, i.e. this genus is thought to have split away from the others earliest, possibly as early as 33 million years ago, shortly after the Eocene-Oligocene boundary. 

The genus Galagoides (Pygmy Bushbabies) was found to be paraphyletic; the West African members of the genus split away from the other Bushbabies early in the group, around 20 million years ago in the Middle Miocene, but the East African members of the genus were found to be closely related to (but still separate from) the genus Galago, from which they split around 14 million years ago. Since the generic name Galagoides was first used to describe West African species, Pozzi et al. believe a new genus should be erected to describe the East African Pygmy Bushbabies. This will be done formally in a separate paper with Judith Masters as the lead author.

Family tree for the Galagidae with estimated divergence dates. Pozzi et al. (2014).

The Early Miocene is associated with a period of cooling in Eurasia and North America, and increased aridity in Africa, with many forest species shrinking into refugia in West and Central Africa. The presence of the earliest fossil Bushbabies in Egypt, and the splitting of the oldest two groups of modern Bushbabies from the rest of the group (Euoticus and western members of Galagoides) suggests that this pattern was also seen in the Galagidae, though as with many other African forest species no fossils are available from this time, as the forests of the West Central African Early Miocene have produced no known major fossil deposits (forests are generally considered poor environments for the preservation of fossils). Two fossil Bushbabies are known from the Middle Miocene of East Africa, Komba and Progalago, though the relationships of these are uncertain, and they are not thought to be ancestral to any modern Bushbabies.

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Annular Eclipse to be visible from Australia, Antarctica and the southern Indian Ocean, 29 April 2014.

An Annular Eclipse will occur on Tuesday 29 April 2014, starting at about 3.52 am GMT. It will be visible from Australia, much of Antarctica and the southern Indian Ocean, eastern Java and the islands of the Bali, Flores and Savu Seas, though a full Annular Eclipse will only be visible from a small area of Antarctica.

Map showing the areas from which the 29 April 2014 Annular Eclipse will be visible. A total Annular Eclipse will only be visible from the dark grey semi-circle on Antarctica. The full extent of the eclipse will be visible as a Partial Eclipse from the dark grey area; in the light grey area it will either begin before dawn or continue after sunset. HM Nautical Almanac Office.

An Annular Eclipse is a type of Solar Eclipse, that is to say an eclipse in which the Moon passes between the Earth and the Sun, but one which occurs while the moon is close to aphelion (when it is furthest from the Earth). The Moon has a variable orbit, getting considerably closer and further from the Earth at different times, which alters its size as seen from the Earth. Thus when it is at its furthest from the Earth it appears considerably smaller than the Sun so an eclipse occurring at this time will produce a ring of sunlight, rather than a period of darkness. A Partial Annular Eclipse resembles a regular Partial Eclipse, in that the light of the Sun will be partially blocked by the Moon passing in from of it, though the disk of the Moon will be smaller.

An Annular Eclipse on 20 May 2013, photographed from Middlegate, Nevada. Wikipedia.

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Truck swallowed by Singapore sinkhole.

A truck carrying sand for the construction industry was partially swallowed by a sinkhole in Singapore on Thursday 24 April 2014. The incident happened at about 8.30 am local time when the driver of the truck, named as a Mr Su, was driving south on the Upper Changi Road East towards the Tanah Merah Ferry Terminal. Part of the road collapsed beneath the rear of the vehicle, causing the front to leave the ground and trapping it. Nobody is said to have been hurt.

The truck trapped in the hole in the Upper Changi Road East, Singapore, on 24 April 2014. Joyce Fang/The Straits Times.

Sinkholes are generally caused by water eroding soft limestone or unconsolidated deposits from beneath, causing a hole that works its way upwards and eventually opening spectacularly at the surface. Where there are unconsolidated deposits at the surface they can infill from the sides, apparently swallowing objects at the surface, including people, without trace.

On this occasion the sinkhole appeared close to where a tunnel for the new Downtown MRT Line underground railway is being constructed, and it is thought likely that the sinkhole is linked to the tunneling, although a precise cause for the incident has yet to be determined.

The approximate location of the Upper Changi Road East Sinkhole. Google Maps.

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Magnitude 6.6 Earthquake off the southwest coast of Vancouver Island.

The Natural Resources Canada reported a Magnitude 6.6 Earthquake roughly 20 km off the southwest coast of Vancouver Island, British Columbia, at about 8.10 pm local time on Wednesday 23 April 2014 (about 3.10 am on Thursday 24 April, GMT). This is a large quake, and was felt across much of southern British Columbia and northwest Washington State, although there are no reports of any damage or casualties, and no tsunami warning was issued.

The approximate location of the 23 April 2014 Vancouver Island Earthquake. Google Maps.

Vancouver Island is located on the western margin of the North American Plate, close to the Challenger Trench, along which the Explorer and Juan de Fuca Plate are being subducted. These are two remnant parts of the ancient Farallon Plate, which formerly underlay part of the eastern Pacific, and which has now been largely subducted beneath North America. The Explorer an Juan de Fuca Plates pass under the North American Plate as they sink into the Earth, but this is not a smooth process, with the plates constantly sticking together then breaking apart as the tectonic stresses build up.

The subduction of the Juan de Fuca Plate beneath North America. United States Geological Survey.

Witness reports can help geologists to understand the processes going on in Earthquakes and the structures in the rocks that cause them. If you felt this quake you can report it to Natural Resources Canada here.

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Wednesday, 23 April 2014

Asteroid 2014 GJ45 passes the Earth.

Asteroid 2014 GJ45 passed by the Earth at a distance of 10 190 000 km (roughly 26.5 times the average distance between the Earth and the Moon) at about 11.35 am GMT on Thursday 17 April 2014. There was no danger of the asteroid hitting us, though were it to do so it would present a realistic threat. 2014 GJ45 is estimated to be between 36 m and 110 m in diameter, and an object towards the upper end of this range would be predicted to be capable of punching straight through the Earth's atmosphere, impacting the planet's surface a causing an explosion roughly 2500 times as large as the Hiroshima Bomb. Such an event would create a crater around 1.5 km wide as well as causing devastation over a much wider area, and resulting in climatic effects that would probably last for several years.

The calculated orbit of 2014 GJ45. JPL Small Body Database Browser.

2014 GJ45 was discovered on 7 April 2014 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2014 GJ45 implies that the asteroid was the 1134th  object (object J45) discovered in the first half  of April 2014 (period 2014 G).

2014 GJ45 has a 331 day orbital period, with an elliptical orbit tilted at an angle to the plain of the Solar System which takes in to 0.63 AU from the Sun (63% of the distance at which the Earth orbits the Sun, considerably inside the orbit of the planet Venus) and out to 1.25 AU (25% 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 August 2013 and the next predicted in April 2015. Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2014 GS1 spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.

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