Seismicity at Mount Hood is primarily characterized by fairly intense swarms of micro-earthquakes which typically occur about 4 to 7 km south of the summit. Over three quarters of the roughly 250 earthquakes which have been located at Mount Hood during the 1990s have occured in these swarms. These swarms usually last from a few hours to a couple of days and have had as many as 50 well located earthquakes and as few as 4. The largest event in a swarm usually does not occur at the beginning (such as a main shock-after shock sequence. The largest event in the roughly 15 sequences during the 1990s have varied from M=1.6 to M=3.5.

All of the earthquakes in the Mount Hood sequences have characteristic waveforms similar to tectonic earthquakes. Swarms of these types of events are fairly common on many volcanos. These are in contrast to low-frequency volcanic earthquakes which have a very different character and are often indicators of a possible eruption. The Mount Hood events likely are occurring on a fault or fault system located to the south of the volcano and represent either its reaction to regional tectonic stresses or possibly deep-seated volcanic stress below the region. Volcanic history and details of Mount Hood is summarized by pages at the Cascade Volcano Observatory.