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Large earthquake causes mud island to rise from the sea Sun Dec 01, 2013 3:09 am
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Jesse Allen and Robert Simmon, EO-1/NASA
A magnitude 7.7 earthquake hit south-central Pakistan on Tuesday this week. Reports of hundreds of casualties highlight the awful scale of the tragedy, made more difficult for rescuers by the remote location of the quake, 270km north of Karachi.
One of the surprises waiting for people who arrived in the quake’s aftermath was a new island. Just offshore near the site of the earthquake, the island appears to be a large pile of mud, built by the distinctive conditions in the area of the fault.
The quake was caused by movement of the Earth on a fault in the crust at rather shallow depth, around 15km below the surface. The movement at the fracture was a rupturing, as the oceanic crust of the Arabian tectonic plate is dragged down, or “subducted,” beneath the Eurasian continental plate at Pakistan. It is part of what geologists term the “Makran subduction zone,” which extends parallel to the Indian Ocean coast south of Pakistan and Iran.
Earlier this year, the Makran subduction zone was found to be a potential lurking tsunami threat. It has history. A tsunami occurred there on November 28, 1945. Caused by a magnitude 8.1 earthquake, it triggered a landslide under the ocean that generated a 15m-high tsunami, resulting in the deaths of more than 4,000 people along the Makran coast. It was the second worst tsunami event in the Indian Ocean, after the more recent December 2004 Sumatra earthquake.
Part of the reason for the tsunami threat at Makran is the build up of huge piles of submarine sediments. The Arabian oceanic plate is being subducted beneath the Eurasian continental plate, moving northward towards Iran and Pakistan. The sediment that has built up on top of the oceanic plate gets scraped off by the overlying plate and stuck onto the seafloor at the base of the coast.
Over geological time, this process has created one of the largest wedges of sediments on Earth, more than 7km thick in places. Earthquakes can make landslips in the wedge, causing the sediments to tumble down into the deeper ocean. But bizarrely, sometimes they can produce islands, too.
Tuesday’s earthquake shook those offshore sediments to the south of Pakistan. They are mainly muds and sands, rich in the rotted remains of dead sea life that have fallen into the sediments over the millennia. Over time, this material has decomposed to gases like methane. When shaken up on Tuesday, these sediments seem to have erupted in a “mud volcano,” driven by the burping methane from the depths. An island of mud rose above the sea, emitting gas that could be set alight. In fact, locals who tried this had difficulty quenching the flame.
Mud volcanoes have been seen at the Makran subduction zone many times before. The 1945 earthquake triggered a number of mud volcanoes and offshore islands formed in the same region. More recently, one formed off Pakistan in November 2010. They all disappeared soon after, their soft material washed away by the ocean's waves and storms.
It is likely that this week’s new island will only make a temporary appearance, subsiding beneath the waves as the Earth settles back to another period of temporary quiescence. In the meantime, an opportunity remains to really find out more about the nature of these ephemeral islands, before this one passes like a ship in the night.]
Enlarge
Jesse Allen and Robert Simmon, EO-1/NASA
A magnitude 7.7 earthquake hit south-central Pakistan on Tuesday this week. Reports of hundreds of casualties highlight the awful scale of the tragedy, made more difficult for rescuers by the remote location of the quake, 270km north of Karachi.
One of the surprises waiting for people who arrived in the quake’s aftermath was a new island. Just offshore near the site of the earthquake, the island appears to be a large pile of mud, built by the distinctive conditions in the area of the fault.
The quake was caused by movement of the Earth on a fault in the crust at rather shallow depth, around 15km below the surface. The movement at the fracture was a rupturing, as the oceanic crust of the Arabian tectonic plate is dragged down, or “subducted,” beneath the Eurasian continental plate at Pakistan. It is part of what geologists term the “Makran subduction zone,” which extends parallel to the Indian Ocean coast south of Pakistan and Iran.
Earlier this year, the Makran subduction zone was found to be a potential lurking tsunami threat. It has history. A tsunami occurred there on November 28, 1945. Caused by a magnitude 8.1 earthquake, it triggered a landslide under the ocean that generated a 15m-high tsunami, resulting in the deaths of more than 4,000 people along the Makran coast. It was the second worst tsunami event in the Indian Ocean, after the more recent December 2004 Sumatra earthquake.
Part of the reason for the tsunami threat at Makran is the build up of huge piles of submarine sediments. The Arabian oceanic plate is being subducted beneath the Eurasian continental plate, moving northward towards Iran and Pakistan. The sediment that has built up on top of the oceanic plate gets scraped off by the overlying plate and stuck onto the seafloor at the base of the coast.
Over geological time, this process has created one of the largest wedges of sediments on Earth, more than 7km thick in places. Earthquakes can make landslips in the wedge, causing the sediments to tumble down into the deeper ocean. But bizarrely, sometimes they can produce islands, too.
Tuesday’s earthquake shook those offshore sediments to the south of Pakistan. They are mainly muds and sands, rich in the rotted remains of dead sea life that have fallen into the sediments over the millennia. Over time, this material has decomposed to gases like methane. When shaken up on Tuesday, these sediments seem to have erupted in a “mud volcano,” driven by the burping methane from the depths. An island of mud rose above the sea, emitting gas that could be set alight. In fact, locals who tried this had difficulty quenching the flame.
Mud volcanoes have been seen at the Makran subduction zone many times before. The 1945 earthquake triggered a number of mud volcanoes and offshore islands formed in the same region. More recently, one formed off Pakistan in November 2010. They all disappeared soon after, their soft material washed away by the ocean's waves and storms.
It is likely that this week’s new island will only make a temporary appearance, subsiding beneath the waves as the Earth settles back to another period of temporary quiescence. In the meantime, an opportunity remains to really find out more about the nature of these ephemeral islands, before this one passes like a ship in the night.]
