The Feb. 28 Seattle quake happened just that way, for example, triggering a magnitude 6.8 event, Zoback says. The earthquake started 30 miles deep below the crust, where the oceanic crust is diving underneath the continent. She describes the Juan de Fuca oceanic plate as bending and brittle, and occasionally it cracks. Such a crack caused causing the magnitude 6.8 earthquake. We have a lot of theories about how earthquakes happen, says Yale seismologist Jeffrey Park of Yale University.
None yet explain the data very well.
On the local scale, some scientists hypothesize that bits of rock along faults crumble intermittently, loosening tension between plates, like sandpaper suddenly worn smooth, Park says. The loosening allows parts of the crust deep in the earth to suddenly slide rapidly past each other, causing an earthquake. If they re correct, likely quakes will always be hard to predict because it means very small perturbations lead to big things, he says. Prediction efforts now center on using monitors to measure subtle changes in the Earth s surface that may precede a quake. N.Y. Researchers like McCaffrey continually find new facets to crustal movements that surprise experts.