Mount St. Helens was quiet between January 1 and September 30, 1995. No explosion or emission of gas and ash occurred from the lava dome. Seismic activity is still low, but the number of small magnitude (less than magnitude 1) earthquakes beneath the crater has increased slowly but steadily from less than 10 events per month in January to about 100 events per month in September.
This increase in seismic activity is very small compared to the activity that preceded each of the explosive and dome-building eruptions between 1980 and 1986. During these eruptions, earthquake activity was clearly associated with the rise of magma (molten rock) into the volcano and its eruption at the surface. The dome-building eruptions were preceded and accompanied by intense shallow earthquake activity, located less than 2 miles (3.2 km) beneath the crater. In contrast, the recent earthquakes were smaller and originated at depths between about 1 and 6 miles (1.5 - 10 km) deep. There is no evidence to suggest that they indicate an upward rise of magma.
The current seismic activity closely resembles seismicity that began in late 1987 and occurred before and during a series of small gas explosions from the dome in 1989-1991. These explosions, though relatively small, were large enough to hurl dome rocks as large as 1 foot in diameter at least 0.5 miles (0.8 km) from the dome and produce ash plumes as high as about 20,000 feet (3.8 miles, 6.1 km) above sea level. Because the 1989-1991 steam explosions were not preceded by any specific short-term warning, the similarity of the current seismicity to that of the earlier episode raises our concern that future small explosions from the dome could again occur without additional warning.
Our experience with the 1989-1991 series of gas explosions from the dome as well as explosions during the years of dome growth suggests that they would produce hazards primarily within the crater, to a lesser degree in the stream channels leading from the crater, and to an even smaller degree on the upper flanks of the volcano. These hazards could include the impact of dome rocks ejected from the dome and rapidly moving hot-rock avalanches (known as pyroclastic flows) sweeping the crater floor. During the explosion on February 5,1991, a small pyroclastic flow reached the north edge of the crater. Heat from a rock avalanche or pyroclastic flow could also generate a lahar (flowing mixture of rock, mud, and water) in the crater and in channels leading from the crater. Also, gas explosions could generate dilute but visible ash plumes perhaps as high as 20,000 feet above the volcano and light ashfall as far as about 100 miles (160 km) downwind from Mount St. Helens.
The recent earthquakes originate at depths between about 1 and 6 miles (1.5 - 10 km) directly beneath the crater. This same zone of seismic activity became active in late 1987, about 2 years before the 1989-91 steam explosions began, and it marks the approximate location of the magma conduit system leading from the volcano's magma reservoir to the lava dome. Detailed study of the 1987-1991 seismicity and the 1989-91 steam explosions from the dome suggests that the two phenomena occurred in response to an increase in pressure in the conduit system.
One possible cause for the pressure increase is that volcanic gas (primarily water in gaseous form) became concentrated along the volcano's magma conduit system. The concentration of gas along the conduit was likely a consequence of the progressive cooling and crystallization of magma in the volcano's magma reservoir and conduit system -- as magma cools, mineral crystals grow and gas in the remaining molten rock becomes concentrated. If the gas cannot escape easily to the surface, it accumulates along the conduit, which leads to increased pressure. This increased pressure would likely lead to increased fracturing of rock immediately surrounding the conduit system (causing increased seismicity), as well as to intermittent sudden release of gas at the dome's surface. In addition, downward growth of cracks and fractures in the dome during and immediately after periods of intense precipitation could trigger gas explosions when such fractures intersect pressurized areas within or beneath the dome. Many but not all of the explosions in 1989-1991 followed periods of heavy rainfall (see enclosed fact sheet). Another possible cause for the pressure increase is intrusion of new magma into the lower depths of the conduit system. There is no evidence, however, that any new magma has moved to near the surface during 1995.
Regardless of the cause, it seems likely that the recent small change in seismicity reflects a renewed increase in pressure along the magma conduit system.
[Graphic,13K,GIF]
Map and cross-section of Mount St. Helens showing the location and depth of
earthquakes beneath the volcano that occurred between January 1 and September
30, 1995. In this graph, 0 depth is referenced to 1.5 km (about 1 mile)
below the
current summit of Mount St. Helens (8,363 feet above sea level). The width of
the cross-section is the same as the length of the A-A' line in map-view.
Note that
most of these earthquakes are smaller than magnitude 1. The earthquakes occur
in and around the magma conduit system that leads from the magma reservoir to
the lava dome.
[Graphic,23K,GIF]
Graph showing relationship between time and depth of earthquakes beneath Mount
St. Helens between January 1986 and September 1995. Solid arrows indicate the
onset of dome-building eruptions on May 8, 1986, and October 21, 1986. Note
the high concentration of earthquake activity between a depth of about 0 and 1
mile that accompanied these events. Dashed arrows between 1989 and 1991
correspond to six gas explosions from the dome. Note the slow increase in
earthquake activity between 1 and 6 miles (1.5 - 10 km)
beneath the crater beginning 2 years
before the first explosion, and a similar increase in earthquake activity
between
January and September 1995. In both graphs, 0 depth is referenced to 1.5 km or
1 mile below the current summit of Mount St. Helens (8,363 feet above sea
level).
[Graphic,10K,GIF]
Graph showing relationship between time and depth of earthquakes beneath Mount
St. Helens between January and September 1995. Note the increase in the
number of earthquakes between about 1 and 6 miles (1.5 - 10 km)
beneath the crater. Most of
the earthquakes are smaller than magnitude 1.0. The largest earthquake
(magnitude 2.3) occurred on July 4, 1995.
