As the death toll rises from Nepal’s magnitude 7.8 earthquake, new research from the University of Washington shows that a similar fault off the Pacific Northwest coast is so weak and slippery in places that minuscule tidal forces produced by the sun and moon are enough to trigger tremors. The quakes involved are so tiny they’re imperceptible to people. But they’re part of a recently discovered process called slow slip that ratchets up pressure on the offshore fault called the Cascadia Subduction Zone — slightly boosting the risk of a megaquake and tsunami. The next slow-slip event is expected to begin sometime this fall or winter. The Cascadia fault and the one responsible for Nepal’s devastation are both boundaries where tectonic plates collide. In Nepal, the Indian subcontinent is being forced under Central Asia, while in the Northwest, the seafloor is diving — or subducting — under North America. Even though the faults aren’t identical, geologists say the Cascadia analysis could help improve the broader understanding of when and how such plate boundaries rupture, generating some of the world’s most powerful quakes.
For the study published Monday in the journal Nature Geoscience, UW geophysicist Heidi Houston analyzed tens of thousands of faint tremors under Washington’s Olympic Peninsula and Vancouver Island between 2007 and 2012. The tiny quakes occur when tectonic plates slip slowly past each other 20 miles or more underground. These slow-slip events last for several weeks and can propagate 100 miles or more north and south. The pattern repeats roughly every 12 to 14 months. Each slow-slip event releases as much energy as a magnitude 6.5 quake, but the motion is so slow it’s detectable only on sensitive seismometers and GPS stations, Houston explained. MORE