The following site from a volcano in Kamchatka has some good examples of various volcanic type disturbances on a seismograph look like.

http://village.infoweb.ne.jp/~hyamasat/karymsky/events.html

A while back somebody was having the same concerns for activity showing up on one of the mountains in the Cascades. Turns out that it was wind and falling rocks etc. that was causing this activity. I don't know what the topography is at Lake Pillsbury so I can't really say anything other then the type of activity showing up on the seismograph for Lake Pillsbury is similar to the activity that had shown up on that Cascades mountain seismograph. -- Dennis