(Portland, OR)  --  A recent swarm of tremors is being tracked in the Pacific Northwest.  The Pacific Northwest Seismic Network began tracking and mapping small earthquakes along the Cascadia Subduction Zone recently.  The tremor swarm is centered around the Central Oregon Coast and all the way up toward Canada.  Experts say recent activity is so subtle that most people won't feel it.  Experts warn it could be a precursor to a major quake.