There is currently an ongoing earthquake swarm in the region immediately south of Mt. Lassen. The largest magnitude is a M 3.9 (USGS). The California Volcano Observatory (CalVO) is monitoring these earthquakes. The earthquakes do not appear to be related to volcanism, but most likely related to the fault systems that strike through that area. These faults are an extension of the Walker Lane fault system, which is a sister of the San Andreas plate boundary. The Walker Lane fault system is a right lateral (dextral) fault system that extends the east side of the Sierra Nevada Mountains. We do not currently know how this part of the plate boundary ends, if it feeds into Cascadia, etc. We have some ideas, but I will leave this a mystery as many of these ideas are unpublished and I do not want to scoop anyone. There are no USGS active faults mapped in this region, but there has been earthquake activity in the Lake Almanor area in the last year or so (see map below from 5/23/13). This region may be an extension of the Butt Creek fault zone (the red faults directly to the southeast of this swarm.
The M 3.9 earthquake has a focal mechanism that shows NE-SW extension.
Here is a primer for those who would like to understand focal mechanisms better (from the USGS.