Roger Groom, Mt Tabor Middle School, developed this activity while working with Becca Walker at UNAVCO in Boulder, Colorado. Through this activity, students can learn about an exciting discovery made possible by invention of high-precision GPS receivers and deployment of these receivers across the Pacific Northwest. This activity can be used to illustrate how invention of new technologies can lead to new scientific discoveries that would have been impossible without the new instruments; a good lesson in how science works.
Some background: The friction between a subducting and an overriding plate of a subduction zone changes with depth. At shallow depths from the surface to 20 km depth, friction is high and the subducting boundary remains locked between very large earthquakes that occur every few decades or centuries. Deeper than about 40 km, friction on the subduction zone is very low and the subducting plate slides into the mantle without major earthquakes on the interface between the two converging plates. In some subduction zones, there is a transitional behavior called “episodic tremor and slip” (ETS) that takes place at intermediate depths of 20 to 40 km. ETS events on the Cascadia subduction zone occur when the Juan de Fuca Plate slips a centimeter or two farther beneath the North American Plate over a time interval from a few days to about two weeks in duration. This “slow slip” is accompanied by release of seismic waves (tremor) that are much longer in duration than seismic waves released by standard earthquakes. Because this process increases the stress on the locked shallow portion of the subduction zone, the probability of a great subduction zone earthquake may be higher during ETS events than at other times so this discovery has important implications for earthquake risk.