How underwater earthquakes form.

Seismologists at Washington University have identified what is causing earthquakes beneath the Pacific Ocean. The mechanism is bookshelf faulting, which occurs when slivers of crust are separated by parallel faults like books on a shelf. These faults are fractures in the Earth's crust that allow the forces of plate tectonics to slide the slivers past one another. In bookshelf faulting, looking down on the slivers as they rotate is similar to looking at books as they fall when a bookend is removed.

The earthquake hotbeds are mid-ocean ridges--elevated portions of the sea floor that are among the most seismically active regions in the world. They are formed from molten rock welling up from the Earth's mantle, then cooling to form new oceanic crust. The sliding movement of the Earth's dozen or so tectonic plates helps form the ridge.

Laura Reiser Wetzel, a doctoral student in earth and planetary sciences; Douglas A. Wiens, associate professor of earth and planetary sciences; and Martin C. Kleinrock of the Department of Geology and Geophysics, Woods Hole Oceanographic Institute, analyzed waves from 27 large earthquakes along Pacific mid-ocean ridges. They combined seismic analysis, giving them earthquake location and type of faulting, with sonar imaging, which creates high-resolution images of oceanic features. Among other formations, the sonar imaging revealed structures called propagating rifts--tears parallel to the mid-ocean ridge formed because one side elongates like a crack in a windshield, while the other side dies off. It is between these propagating rifts that bookshelf faulting occurs, the researchers say.

The finding sheds new light on earthquake studies in general as well as settling a long seismological controversy over the primary faulting mechanism along mid-ocean ridges. Previously, seismologists had thought that earthquakes along...