An international team of geoscientists has discovered an unusual geological formation that helps explain how an undersea earthquake off the coast of Sumatra in December 2004 spawned the deadliest tsunami in recorded history. Instead of the usual weak, loose sediments typically found above the type of geologic fault that caused the earthquake, the team found a thick plateau of hard, compacted sediments. Once the fault snapped, the rupture was able to spread from tens of kilometers below the seafloor to just a few kilometers below the seafloor, much farther than weak sediments would have permitted. The extra distance allowed it...