Some instruments are at high elevations or remote locations, and fixing them takes longer than other, easier to access instruments. The PNSN has more than 150 stations. Temporary problems with a few stations at any given time will not interfere with our ability to identify and analyze seismic activity in the Pacific Northwest. Some types of faulting are driven by extension, others by compression. Animations show the various fault types.
Foreshocks are earthquakes which precede larger earthquakes in the same location.
As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time. Each step of one in magnitude is an increase of ten times the amount of ground motion amplitude, corresponding to thirty-two times the amount of 'elastic' energy in the form of seismic waves. So a magnitude 6 quake has over 1,000 times as much energy as a magnitude 4 quake, and a 100 fold increase in ground motion amplitude. Above magnitude 6.0, the ground motion amplitude can no longer increase, and the excess energy is expressed as a longer duration of shaking.