Wednesday, 20 September 2017

Magnitude 7.1 Earthquake in Puebla State, Mexico, kills at least 225 people.

The United States Geological Survey recorded a Magnitude 7.1 Earthquake at a depth of 51 km, approximately 55 km to the south of the city of Peubla in the Mexican State of the same name, slightly before 1.15 pm local time (slightly before 6.15 pm GMT) on Tuesday 19 September 2017. The event has caused extensive destruction in the states of Puebla and Morales as well as the Mexico City Federal District, with extensive building collapses in urban areas and 226 confirmed deaths at the time of writing. The event was felt as far away as Austin in Texas and San Salvador in El Salvador.

Collapsed building in Mexico City following the 19 September 2017 Earthquake. Daniel Cardenas/Anadulu/Getty Images.

Mexico is located on the southernmost part of the North American Plate. To the south, along the Middle American Trench, which lies off the southern coast off Mexico, the Cocos Plate is being subducted under the North American Plate, passing under southern Mexico as it sinks into the Earth. This is not a smooth process, and the plates frequently stick together then break apart as the pressure builds up, causing Earthquakes on the process.

The approximate location of the 19 September 2017 Mexico Earthquake. USGS.

The Cocos Plate is thought to have formed about 23 million years ago, when the Farallon Plate, an ancient tectonic plate underlying the East Pacific, split in two, forming the Cocos Plate to the north and the Nazca Plate to the south. Then, roughly 10 million years ago, the northwesternmost part of the Cocos Plate split of to form the Rivera Plate, south of Beja California.

 The position of the Cocos, Nazca and Rivera Plates. MCEER/University at Buffalo.

In a paper published in the Journal of Geophysical Research, in 2012, a team led by Igor Stubailo of the Department of Earth and Space Sciences at the University of California Los Angeles, published a model of the subduction zone beneath Mexico using data from seismic monitoring stations belonging to the Mesoamerican Seismic Experiment, the Network of Autonomously Recording Seismographs, the USArray, Mapping the Rivera Subduction Zone and the Mexican Servicio Sismologico Nacional.

Partially collapsed school in Mexico City following the 19 September 2017 Earthquake. EPA/Shutterstock.

The seismic monitoring stations were able to monitor not just Earthquakes in Mexico, but also Earthquakes in other parts of the world, monitoring the rate at which compression waves from these quakes moved through the rocks beneath Mexico, and how the structure of the rocks altered the movement of these waves.

Collapsed building in the city of Jojutla in Morales State, Mexico, following the 19 September 2017 Earthquake. Carlos Rodriguez/AP.

Based upon the results from these monitoring stations, Stubailo et al. came to the conclusion that the Cocos Plate was split into two beneath Mexico, and that the two plates are subducting at different angles, one steep and one shallow. Since the rate at which a plate melts reflects its depth within the Earth, the steeper angled plate melts much closer to the subduction zone than the shallower angled plate, splitting the Trans-Mexican Volcanic Belt into sections above the different segments of the Cocos Plate, and causing it to apparently curve away from the subduction zone.

  Top the new model of the Cocos Plate beneath Mexico, split into two sections (A & B) subducting at differing angles. (C) Represents the Rivera Plate, subducting at a steeper angle than either section of the Cocos Plate. The Split between the two has been named the Orozco Fracture Zone (OFZ) which is shown extended across the Cocos Plate; in theory this might in future split the Cocos Plate into two segments (though not on any human timescale). Bottom Left, the position of the segments on a map of Mexico. Darker area is the Trans-Mexican Volcanic Belt, orange circles are volcanoes, brown triangles are seismic monitoring stations, yellow stars are major cities. Bottom Right, an alternative model showing the subducting plate twisted but not split. This did not fit the data. Stubailo et al. (2012).

The two major Earthquakes that have happened in Mexico this September, on the seventh and nineteenth of the month, are thought to relate to this fragmenting of the Cocos Plate, with the events thought likely to have been caused by tearing along one of these splits.

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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Tuesday, 19 September 2017

Family evacuated after Florida home swallowed by sinkhole

A family have been forced to evacuate their home after is was swallowed by a sinkhole in the city of Apopka in Orange County, Florida, on Tuesday 19 September 2017.  The hole was first noticed slightly after 8.00 am local time, and rapidly expanded to around 7.5 by 3 meters, causing the house to partially collapse into it. It is not clear if the hole has reached its maximum size.

Home in Apopko, Florida, partially engulfed by sinkhole on 19 September 2017. WESH.

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.

 The approximate location of the 19 September 2017 Apopka sinkhole. Google Maps.

The precise cause of this sinkhole is unclear, but is thought likely to have been caused by high rainfall associated with Hurricane Irma acting on soft limestone deposits in the area. Many parts of Florida are particularly prone to sinkholes, due to the porous limestone that underlies much of the state. This is eroded over time by acid in rainwater (most rainwater is slightly acidic, though pollution can make this worse), and can collapse suddenly, causing overlying sediments to collapse into the hole and a sinkhole to open up. This can be triggered by human activity, such as pumping water out (which causes the water to flow, facilitating acid dissolution of the limestone), but is essentially a natural process. 

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Monday, 18 September 2017

Asteroid 2017 RV2 passes the Earth.

Asteroid 2017 RV2 passed by the Earth at a distance of about 832 100 km (2.17 times the average distance between the Earth and the Moon, or 0.56% of the distance between the Earth and the Sun), slightly before 2.25 am GMT on Tuesday 12 September 2017. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2017 RV2 has an estimated equivalent diameter of 11-34 m (i.e. it is estimated that a spherical object with the same volume would be 11-34 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) in the atmosphere between 30 and 15 km above the ground, with only fragmentary material reaching the Earth's surface.

The calculated orbit of 2017 RV2  Minor Planet Center.

2017 RV2 was discovered on 14 September 2017 (two days after its closest approach to the Earth) by the Atlas MLO Telescope at Mauna Loa Observatory in Hawaii. The designation 2017 RV2 implies that the asteroid was the 71st object (object V2) discovered in the first half of September 2017 (period 2017 R). 

2017 RV2 has a 691 day orbital period and an eccentric orbit tilted at an angle of 0.21° to the plane of the Solar System, which takes it from 0.98 AU from the Sun (i.e. 98% of he average distance at which the Earth orbits the Sun, slightly inside the orbit of Venus) to 2.08 AU from the Sun (i.e. 208% of the average distance at which the Earth orbits the Sun, considerably more than the distance at which the planet Mars orbits). 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). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in June 2002 and the next predicted in June 2019. 

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Cholera kills 44 in Borno State, Nigeria.

Forty four people are known to have died, and another 2300 possible cases have been reported, in an outbreak of Cholera in Borno State, Nigeria, since the first cases were reported on 16 August 2017. The UN Office for the Coordination of Humanitarian Affairs in Nigeria estimates that 3.7 million people are at immediate threat, particularly the large number of internally displaced people living in temporary camps in the state, due to the ongoing conflict between Nigerian Government troops and Boko Haram Islamic Militants, and is working to improve sanitation in these camps, in addition to setting up four specialist treatment centres for severe cases and seven rehydration centres for less severe cases.

A Cholera treatment centre in Borno State. Medecins Sans Frontieres.

Cholera is caused by the Bacterium Vibrio cholerae, a Gram-negative, comma-shaped Gammaproteobacteria, related to other pathogenic Bacteria such as Yersinia pestis (Bubonic Plague), and Esherchia coli (food poisoning). The Bacteria produce proteins which can cause watery diarrhoea, which helps spread the disease, and can prove fatal in severe cases, as patients are killed by extreme dehydration.

SEM image of Vibrio cholerae Bacteria. Kim et al. (2000).

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Magnitude 5.8 Earthquake off the coast of Choapa Province, Chile.

The United States Geological Survey recorded a Magnitude 5.8 Earthquake at a depth of 10 km, 95 km offshore of the city of Illapel in the Chilean province of Choapa, slightly after 10.45 am local time (slightly after 1.45 pm GMT) on Monday 18 September 2017. There are no reports of any damage or injuries associated with this event, but people have reported feeling the event in the Santiago and Valparaiso areas.

The approximate location of the 18 September 2017 Antofagasta Earthquake. USGS.

Chile is located on the west coast of South America, which is also the convergent margin between the Nazca and South American Plates. The Nazca Plate is being subducted beneath the South American Plate and is sinking beneath the South American Plate. This is not a smooth process, the rocks of the two plates continuously stick together then, as the pressure builds up, break apart again, causing Earthquakes. As the Nazca Plate sinks deeper it is partially melted by the heat of the Earth's interior. Some of the melted material then rises up through the overlying South American Plate as magma, fuelling the volcanoes of the Chilean Andes.
 The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
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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Sunday, 17 September 2017

Maevia eureka: A new species of Jumping Spider from Miocene Chipas Amber.

Jumping Spiders, Salticidae, are the largest family of Spiders, with about 5800 described species in 600 genera, roughly 13% of all described species. They are noted for their exceptional vision, possibly the best of any Arthropods, which is used in hunting, with these Spiders being active hunters that stalk their prey before catching it with a final leap, as well as in mating, with many species having evolved elaborate courtship rituals. 

In a paper published in the journal Peer J on 25 July 2017, Francisco Riquelme of the Laboratorio de Sistemática Molecular at the Universidad Autónoma del Estado de Morelos, and Miguel Menéndez-Acuña of the Centro de Investigación en Biodiversidad y Conservación, also at the Universidad Autónoma del Estado de Morelos, describe a new species of Jumping Spider from a peace of Miocene amber from Chiapas State in Mexico.

The ambers of Chiapas State, Mexico, were laid down in the Miocene (and possibly Oligocene) in shallow marine environments. The amber is thought to be derived from resin secreted from a Leguminous tree of the genus Hymenaea, which lived in mangrove forests along the Caribbean shoreline. This amber is almost identical to amber produced in similar mangrove forests during the Miocene in the Dominican Republic and to a lesser extent other parts of the Caribbean, and a thriving, and sometimes illegal, trade in these fossils, combined with poor recording by commercial fossil collectors (particularly those acting illegally) means that fossils in Caribbean amber derived from private collections can be hard to connect to their point of origin.

The new species is placed in the extant genus Maevia, which contains a number of extant species from North America, and possibly South America and Australasia, but no previously described fossil species, and given the specific name eureka, refering to the 'eureka moment' that occured when the specimen was found, it having been dug out by habd during the excavation of a latrine for use during fieldwork. The species is described from a single female specimen, 3.6 mm in length and pale brown in colour, with white markings, preserved in a piece of almost clear amber.

Maevia eureka. Amber inclusion as seen in raw condition using regular light. (A) Dorsal view. (B) Ventral. Scale bar 2 mm. Riquelme & Menéndez-Acuña (2017).

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