Magnitude 4.0 – ILLINOIS

Magnitude 4.0, Illinois - Apr 20, 2008

A final seismogram from the series of southern Illinois earthquakes, this one occurred a couple of days after the main event.  Compare this one with the last two to get a feel for the logarithmic change in measured amplitude with magnitude.  The main event at magnitude 5.2 briefly went off scale while this one at magnitude 4.0 while visible is much smaller in measured amplitude.

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