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Our seismic stations record anthropogenic (human-made) explosions nearly every day. In other regions, such as the East Coast, these signals are more common than natural seismicity. In Alaska, these blasts are most frequently recorded near established mining projects, such as Usibelli Coal Mine near Healy and Fort Knox Gold Mine near Fairbanks. Occasionally we record blasts from Red Dog Mine in Northwest Alaska as well.
How do we tell the difference between a regular tectonic earthquake and an explosion? First, we look at the waveforms. Mine/quarry blasts have fairly distinct seismogram patterns that look different from those of a tectonic earthquake. P-waves (the first type of seismic wave to arrive) from mine/quarry blasts tend to be more emersive (showing gradual build-up) and do not have distinct, impulsive (sudden) onsets like typical tectonic earthquakes. S-waves (the slower, secondary type of seismic signal to arrive) generated by mine/quarry blasts lack short frequencies and have smaller amplitudes than regular earthquakes. Then we look at the location. Since nearly all of the explosions we locate in Alaska are mine/quarry blasts, signals that originate near these sites are potential candidates.