This magnitude 3.1 event occurred less than 2 degrees away from Corvallis (CHOR). Note that the event was not visible on the unfiltered helicorder record. However by applying a filter to reduce the amplitude of the low frequency background noise, and increasing the gain, the event became clearly visible.
Helicorder record. Gain 30. No filter.
Helicorder record. Gain 300. Filter out frequencies lower than 1.5 Hz.
No filtering. Scale set to +- 100 counts.
High pass filter with 1 Hz corner applied 3 times. Scale set to +- 10 counts.