The following statement was issued today by the U.S. Geological Survey’s Cascades Volcano Observatory and the University of Washington Seismology Laboratory (Pacific Northwest Seismograph Network):
Since about 2 a.m. PDT on the morning of Sept. 23, an earthquake swarm has been occurring at Mount St. Helens.
Through 5 p.m. on Sept. 23, about 200 small (less than magnitude 1) earthquakes have been located at Mount Saint Helens, and many smaller events have also been recorded through this morning. The earthquakes are occurring at shallow depths (less than 1 kilometer, or one-half mile), mostly under the lava dome that formed between 1980 and 1986.
Such earthquakes are common at Mount St. Helens, but a swarm with this many earthquakes has not been recorded for several years, most recently on November 3-4, 2001. The cause of such shallow swarms is uncertain, but may reflect increased ground water levels with the onset of autumn rain. The probability of small steam explosions that hurl rocks a few hundred meters (yards) may also be increased during periods with increased shallow earthquakes.
Prior to the 2001 swarm, the last period of increased earthquake activity at Mount St. Helens occurred in the spring and summer of 1998 when hundreds of earthquakes per month, most smaller than magnitude 2, were detected at depths as great as 9 kilometers (6 miles). An intrusion of magma (or molten rock) deep under the volcano and resulting increased gas pressure in the conduit that leads to the lava dome likely caused this increase in earthquakes.
The current swarm is different in that the events are typically much smaller and shallower. We see no evidence that an intrusion of magma similar to that of 1998 is underway.
We continue to monitor the situation closely and will issue additional updates as warranted.
Daily updates of earthquake data and other information can be found on the Internet at:
http://vulcan.wr.usgs.gov/Volcanoes/MSH/CurrentActivity (CVO Menu – Monthly Summaries)
http://www.pnsn.org/HELENS/welcome.html (University of Washington – Seismic Updates)