First publishing together in 1967, George Plafker, Rubin, and their colleagues did painstaking fieldwork after the magnitude 9.2 Alaskan earthquake in 1964, covering hundreds of kilometers of Alaskan shoreline in small boats, helicopters, and float-equipped aircraft after the 1964 quake helped to launch a new field of megathrust earthquake geology, which used observations of the placement and dating of intertidal organisms such as acorn barnacles, mussels and rockweed to determine the amounts of vertical change in land relative to sea level near subduction zones. Plafker and his colleagues determined that the massive Alaskan quake was caused by rupture along a deeply buried fault in a subduction zone where the Pacific tectonic plate thrusts north below the North American plate. Earlier accounts of the Alaskan earthquake had suggested that the quake took place as slip along a vertical fault, as the Pacific plate rotated counter-clockwise against the North American plate. These studies by Plafker, Rubin and colleagues were very important evidence for the existence of subduction processes during the early debates of plate tectonics. See for a geological and historical summary.