Whole Lot of Shaking Going On
Think about this: You are standing on the curb, waiting for the light to change so you can cross the street. A car drives up blaring music from super speakers. The driver rolls up the window. Now all you can hear are the lower range notes of the drum or the bass keeping the beat. Somehow the higher range notes or sounds that have higher frequencies (or shorter wavelengths) get dampened and do not reach your ears, while the lower range notes with their longer wavelengths do.

Earthquakes Generate Sound Waves
So what does this have to do with earthquakes and seismometers? It has to do with the fact that earthquakes create sound waves that travel through the earth. These sound waves shake the earth at a range of frequencies. The reason we do not hear them is that the frequency is lower than our ears can detect. Instruments called seismometers, however, can pick up and record the various vibrations, just like the car radio can pick up and transmit different sound frequencies. The seismometer can translate vibration frequencies and make a picture (seismograph) on paper or a computer screen.

Sources of Earthquakes
These instruments record ground motions from very small earthquakes that may be caused by hydrothermal or magmatic events or by faulting due to the divergence of the Pacific and Juan de Fuca tectonic plates. There are several major tectonic features within a few hundred kilometers of the Endeavour segment (the Juan de Fuca Ridge, active transform faults, the Explorer Plate and the subduction of the Juan de Fuca plate beneath North America) that are seismically active Another type of earthquake we expect to see in the seismometer data are teleseisms - large earthquakes that occur around the world.