Typically, up to five earthquakes are recorded monthly near the summit. Swarms of five to ten shallow earthquakes over two or three days take place from time to time, predominantly in the region of below the summit. These earthquakes are thought to be caused by the circulation of hot fluids beneath Mount Rainier. Presumably, hot springs and steam vents within Mount Rainier National Park are generated by such fluids. Seismic swarms (not initiated with a mainshock) are common features at volcanoes, and are rarely associated with eruptive activity. Rainier has had several such swarms; there were days-long swarms in 2002, 2004, and 2007, two of which (2002 and 2004) included M 3.2 earthquakes. A 2009 swarm produced the largest number of events of any swarm at Rainier since seismic monitoring began over two decades earlier. Yet another swarm was observed in 2011.