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.

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.

An island that rose from the sea following an earthquake is pictured off Pakistan's Gwadar coastline in the Arabian Sea. - Photo by Reuters

In this photo released by the Pakistani Government, Pakistani men walk on an island that appeared 2 kilometres off the coastline of Gwadar. - Photo by AFP

In this photo released by the Pakistani Government, Pakistani men walk on an island that appeared 2 kilometres off the coastline of Gwadar. - Photo by AFP

In this photo released by the Pakistani Government, Pakistani men walk on an island that appeared 2 kilometres off the coastline of Gwadar. - Photo by AFP

In this photo released by the Pakistani Government, Pakistani men walk on an island that appeared 2 kilometres off the coastline of Gwadar. - Photo by AFP

n this photo released by the Gwadar local government office on Wednesday, Sept 25, 2013, people walk on an island that reportedly emerged off the Gwadar coastline in the Arabian Sea. - Photo by AP

‹›

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.

This article was originally published in the Australian journal, The Conversation, on the 26th of September, 2013.