The oceanic floor offshore the coast of Cascadia is thrust beneath North America at a rate of 5 cm/yr. There is currently very little seismicity along this subduction zone. The modeling of records from continuously recording GPS station reveals that there in fact is intense activity there. The plate interface is locked at shallow depth (shaded area). The rupture of that locked patch could produce a devastating tsunamigenic earthquakes as happened in 1700. Further downdip, beneath the coastal area the data show recurring slow slip events (blue color) which unzip the plate interface generating tremors (black dots). These slow slip events are found to be analogous to regular earthquakes. The elastic strain released by the largest events are equivalent to M~7.5 earthquakes, but they slip so slowly that they generate hardly perceptible seismic waves.
Credit: Eric Vantroyen, Sylvain Michel, Adriano Gualandi and Jean-Philippe Avouac