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.