Mount Rainier is a stratovolcano that started to grow about half a million years ago and has erupted as recently as 1,100 years ago. It is located approximately 90 km SE of Seattle, WA.
The primary hazards associated with Mount Rainier's eruptions are debris and mud flows. In most cases, these flows were triggered during times of eruptions, both as shallow earthquakes shook loose unstable portions of the upper volcano and as hot lava and rock debris melted glacier ice. Mud flows have swept down all of the river valleys that head on the volcano, including an event about 500 years ago that was not associated with any known eruption. Some mud flows have traveled as far as Puget Sound, and much of the lowland between Seattle and Tacoma is formed of pre-historic debris from Mount Rainier..
The Cascades Volcano Observatory and the PNSN cooperatively operate 11 seismometers on or near Mount Rainier. In an average month, 1-5 well-located, high-frequency earthquakes are recorded near the summit of Mt. Rainier. In addition, small swarms of 5-10 earthquakes over a 2-3-day time period sometimes occur. All of these earthquakes are shallow, with most locating near sea level (~4 km below the summit), and are interpreted by Moran (1997) to be occurring in response to stresses associated with the circulation of hot fluids beneath Mount Rainier. These fluids are thought to be the source for the hot springs and steam vents found at the summit and at various points within Mount Rainier National Park (e.g. Frank, 1995). An additional challenge on Mount Rainier is that its glaciers also produce small swarms of icequakes which have some characteristics similar to volcanic earthquakes.
A common source of earthquakes near Mount Rainier is the "West Rainier Seismic Zone", a difuse zone of activity running roughly north-south along the west edge of the park. Earthquakes in this zone are thought to be primarily due to regional tectonic stresses rather than to volcanic sources.
Seismicity 2001 - 2011
Seismicity at Mount Rainier is characterized by swarms of micro-earthquakes at shallow depth under the summit, larger and deeper events in the West Rainier Seismic Zone (WRSZ) just out side of the Park to the west (not shown on figures below) and very shallow seismic events due to glacier motion (not shown on figures below). Most earthquakes directly under the volcano occur at depths of 1-5 km below the surface; however, because of the difficulty of determining an accurate depth for very shallow events some of them have a fixed depth at the surface. Some of these shallow earthquakes occured in swarms lasting from a few hours to a few of days. The largest event in a swarm sometimes occurs in the middle of the swarm though many follow a standard main shock-after shock pattern. The largest events in the past 20 years were M=3.2 events in 2002 and 2004, although there was a M=4.5 earthquake in 2006 located about 10 km east of the summit. The average rate of events directly under the volcano for the past 20 years is about 7 located events per month but this rate is highly variable.
Decadal Seismicity (2001 - 2011)
Below you will find an epicenter map and time-depth plot for Mount Rainier for the decade, approximately 2001 - 2011. These show the typical or background levels of earthquakes under the volcano. There are a good number of seismograph stations located on (4) or very near (6) the volcano such that the regional seismic network can reliably detect and locate earthquakes of magnitdue 0.5 and larger in this area. Even smaller earthquakes, as well as glacier-quakes can sometimes be detected and located though many also are missed.
- Red dots represent events occuring in the last month.
- Green dots represent events occuring in the last year, but more than a month ago.
- Circles represent older events.
- Triangles represent PNSN seismic stations. Station names are listed below each triangle.
- Black star indicates approximate position of Mt. Adams summit.
- Magnitude and distance scales appear at lower right.