Mount St. Helens
Mount St. Helens is the most historically active volcano in the Cascade range, having produced four major explosive eruptions since 1479, and dozens more smaller eruptions, including pyroclastic flows, lava flows and domes, and lahars. It is located approximately 80 km NE of Portland, OR.
Mount St. Helens is best known for its large explosive eruption, summit collapse and directed blast of May 18, 1980, which was the most expensive and deadly volcanic event in United States history. The volcano continued erupting during 1981-1986, producing a lava dome within the 1980 crater. Mount St. Helens last erupted in 2004-2008, when it produced a series of spectacular lava spines with a cumulative volume of almost 100 million cubic meters. Tremor and millions of small earthquakes accompanied both of the recent eruptions.
The Cascades Volcano Observatory and the PNSN cooperatively operate 21 seismometers on or near Mount St. Helens. On average, we locate 4 to 14 earthquakes within 10 km of the volcano each week. Mount St. Helens and the nearby St. Helens Seismic Zone occasionally produce swarms of many small earthquakes, but these are not usually directly associated with magmatic activity. An additional challenge on Mount St. Helens is that small rockfalls commonly occur on the steep crater rim.
Seismicity at Mount St. Helens is quite variable depending on the volcanoe's eruptive state. Most earthquakes locate at shallow depth directly under the volcano; however there is a St. Helens Seismic Zone (SHZ) that extends north and south of the volcano on which some earthquakes take place. Seismicity changes character just before eruptions begin. There is usually an increase in both number and size of earthquakes as well as a change in the character of the seismograms. Such changes were detected and recognized as precursors to most of the two dozen or so eruptions during the past 30+ years including a two month precursory sequence before the cataclislmic eruption of May 18, 1980. On average (not during eruptions or their precursors) we locate about 17 earthquakes a month at Mount St. Helens. Most of these events are in the depth range of 1-4 km. During eruptions earthquakes are even shallower. There have been a few times during the past thirty years that there have been many earthquakes in the depth range 4-9 km. We interpret these events to be in response to stresses generated by magna recharge into the main crustal magma system. Such recharge seems to have taken place during the periods: 1989-1991, late in 1995 and in 1997.
Decadal Seismicity (2002 - 2012)
Below you will find an epicenter map and time-depth plots for Mount St. Helens for the decade approximately 2002 - 2012 and a time-depth plot for the 18 years between 1980 1998. These show the typical or background levels of earthquakes under the volcano and the intense activity associated with eruptions and magma recharge. From Sep. 2004 to late 2007 the volcano was in mild, continuous eruption and produced over a million earthquakes, far more than could be located using traditional methods. Samples of events were analized and located and shown in the following figures. After the eruption quit (beginning of 2008) the seismicity decreased to normal background levels and since then all detected events have been analyzed and are shown in these figures.
Since the spring of 1980 there are a good number of seismograph stations located on (8) or very near (11) the volcano such that the regional seismic network can reliably detect and locate earthquakes of magnitdue 0.0 and larger in this area. Even smaller earthquakes 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.