Since 1985, Mount Rainier seismicity has been monitored by the Pacific Northwest Seismic Network (PNSN) and CVO via a network of seismic stations located within 20 km (12 mi) of Rainier. Mount Rainier is one of the most seismically active volcanoes in the Washington and Oregon Cascades. In an average month, the PNSN locates 3 to 4 earthquakes within 3 miles of the summit, including several events that have been felt within Mount Rainier National Park (MRNP). The largest recorded earthquake beneath Mount Rainier was a magnitude 3.9 in 1973, with magnitude > 3.0 events also occurring in 1976, 1990, 2002, and 2004. While earthquake swarms (defined as three or more earthquakes in a single day) are rare beneath Rainier, they do occur on occasion. The most notable swarm occurred between September 20th and 22nd 2009, when over 1000 earthquakes were detected. Depths of Rainier earthquakes are exclusively shallow, with most occurring 3 to 5 km (2 to 3 mi) below the summit (1 to 3 km, or 0.5 to 2 mi, below the base of the edifice). Very few earthquakes have occurred within the edifice itself. Scientists believe that earthquakes at Mount Rainier occur by hydrothermal fluids "lubricating" existing faults within basement rock underlying the Rainier edifice.
At the first sign of unusual earthquake activity, scientists from the USGS and other institutions will deploy additional instruments on and around Mount Rainier to monitor earthquakes, deformation, and other symptoms of volcanic unrest. The monitoring information will be used to assess the state of unrest and to issue appropriate advisories and warnings to emergency-response officials and the public.
Tectonic faults, associated with motion in the crust and not with volcanic processes, are another producer of earthquakes near Mount Rainier. The most active tectonic faulting region is the Western Rainier Seismic Zone (WRSZ), which extends north-south for approximately 65 km (40 mi) and runs along the western boundary of MRNP. In July of 1988 a magnitude 4.1 earthquake and magnitude 3.8 aftershock occurred in the northern part of the WRSZ, both of which were widely felt within the National Park and communities nearby. Additional tectonic faults produce fewer earthquakes, but on occasion have been the source for significant events. The largest earthquake ever recorded within MRNP occurred on one such fault, a magnitude 4.5 earthquake in October 2006 that was located approximately 12 km (7.5 mi) east of Mount Rainier and strongly felt within MRNP. In contrast to earthquakes located directly beneath Mount Rainier, such earthquakes are thought to be normal "tectonic" earthquakes caused by tectonic forces that also produce earthquakes throughout western Washington.
Glaciers are another significant producer of seismicity, with tiny glacier quakes often dominating records of stations located close to the summit (RCS, RCM, and STAR). Although such events can look like volcanic earthquakes, they are readily distinguished because they are only recorded on nearby stations (since they occur within ice or along the ice-rock interface, much of the seismic energy is absorbed by ice and doesn't make it to further-away stations). Occasionally much larger events are recorded across the Rainier network, including icefalls, rockfalls, and avalanches. One example is several spectacular ice/rockfalls off the Nisqually glacier in June 2011, the largest of which produced debris flows that were witnessed by a number of climbers. This video was filmed by some climbers not affiliated with the USGS, but it is a good documentation of a rockfall at Mount Rainier.