Thursday 26 September 2013

Magnitude 7.7 Earthquake in Balochistan; over 350 dead and new island created.

The United States Geological Survey recorded a Magnitude 7.7 Earthquake at a depth of 15 km in Baluchistan Province in southeast Pakistan, slightly before 4.30 pm local time (slightly before 11.20 am GMT) on Tuesday 24 September 2013. This is an extremely large and dangerous Earthquake, and was felt as far away as New Delhi and Dubai. Over 350 casualties have been recorded already, a number which is likely to rise sharply, and buildings have been destroyed across Arawan District, where the quake occurred. Villages close to the epicenter are said to have had around 90% of homes destroyed; building collapses are a particular danger in Baluchistan as many buildings are made of mud bricks, which can liquify in Earthquakes forming a fine dust which asphyxiates people trapped beneath fallen structures. There have been a number of aftershocks in the area, several of which have been Earthquakes in excess of Magnitude 5.

Villagers in Labach in Arawan District forced to sleep outside following the destruction of their homes by the 24 September 2013 Earthquake. Arshad Butt/AP.

The quake also lead to the formation of a new island off the coast of Gwadar, roughly 400 km to the southwest of the event's epicenter. This is roughly 18 m high and covers an area of approximately 30 by 76 m. Earthquakes on thrust belts can form hills and islands by forcing rock formations up over other rocks (this is how most mountains are formed, though not by single events), but on this occasion the new island is thought to be the result of a mud volcano, according to geologists from the Pakistan Navy. Mud volcanoes are not true volcanoes (caused by hot magma from deep within the Earth) but are the result of liquids or gasses being released suddenly from rocks into soft sediments, resulting in dramatic upwelling of mud and gas. In this gas the new island has been found to be venting methane in a number of places, leading to the conclusion that a sudden change of pressure caused methane ice buried in the mud to thaw, causing it to expand rapidly and raise sediment to form the new island. Frozen methane in marine sediments is quite common, as the gas freezes at quite high temperatures under pressure. Mud volcanoes are a fairly common occurrence in Baluchistan, though where they occur offshore, as is the case with this one, they are often eroded away by the sea fairly quickly.

Residents of Gwadar investigating the new island on 25 September 2013. Behram Balochi/AFP/Getty Images.

The quake occurred directly on the Makran Fault, along which the Arabian Plate to the south is being subducted beneath the Eurasian Plate to the north. The Arabian Plate is being pushed northward by the movement of the African Plate to the south, as well as expansion beneath the Red Sea caused by the formation of new oceanic crust. To the west the continental portion of the Arabian Plate is in contact with the Eurasian Plate, causing folding and uplift that is leading to the formation of the Zagros Mountains. In the east oceanic crust attached to the Arabian Plate is being subducted beneath the Eurasian Plate, causing uplift on the Iranian Plateau, which underlies Baluchistan, eastern Iran and the southwest of Afghanistan. This subduction is not a smooth process, with the rocks constantly sticking together then breaking apart as the pressure builds up. 

The approximate location of the 25 September 2013 Baluchistan Earthquake. Google Maps.

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