People are often surprised at how sensitive our instruments are to earthquakes. This animation shows seismometers across Alaska as they respond to shaking from the magnitude 6.3 Skwentna earthquake on September 25. This earthquake was felt by people from Homer to Fairbanks. In Anchorage, it knocked items off shelves, damaged ceiling tiles, and cracked sheetrock, but no structural damage was reported.

The epicenter is marked with a red star. Each circle represents a seismic instrument in our network. The earthquake begins three seconds into the animation. Watch the circles darken as the waves travel outward from the epicenter. Notice also that they continue to flash or pulse, showing the ground continuing to shake in central Alaska after the waves reach distant places like Deadhorse, Atka, and the Red Dog Mine.

How sensitive are the instruments? The scale shows Peak Ground Acceleration (PGA) in units of micro-gravities (ug). Ground acceleration tells us how strongly the earth shook in a specific place. 1ug is equal to one-millionth of the acceleration you feel from gravity. At the most distant stations - and at all stations, as movement from the earthquake subsides - you can see our instruments recording PGA as weak as 1-5ug.