By Simon Redfern, University of Cambridge
A magnitude 7.7 earthquake hit south-central Pakistan on Tuesday, Sept. 23th. 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.
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 shown as a potential lurking tsunami threat. It has history. A tsunami of 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 4000 people along the Makran coast. It was the second worst tsunami event in the Indian Ocean, after the more recent 2004 Boxing Day Sumatra earthquake.
Part of the reason for the tsunami threat at Makran is the build up of huge piles of submarine sediments. As the Arabian oceanic plate is being subducted beneath the Eurasian continental plate, moving northward towards Iran and Pakistan, ocean floor sediment rocks get scraped off the top of the oceanic crust and stuck onto sea floor at the base of the coast. Over geological time, it has created one of the largest wedges of sediments on Earth, more than 7km thick in places. Earthquakes can make landslips in the wedge, but bizarrely sometimes they can cause 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, decomposed to gases like methane. When shaken up on Tuesday, these sediments seem to have erupted in a “mud volcano”, driven by the ease of burping methane from the depths. An island of mud rose above the sea, emitting gas that could be set light. 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, washed away by the oceans 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, that pass like ships in the night.
Simon Redfern does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.