Mount St. Helens, Washington:
Summary of Mount St. Helens Eruptive Activity,
October 1 - November 30, 1995


This summary was compiled by the U.S. Geological Survey and University of Washington Geophysics Program

Summary of activity:

Mount St. Helens was quiet between October 1 and November 30, 1995. No explosion or emission of gas and ash occurred from the lava dome. Seismic activity beneath the crater declined slightly from about 90 earthquakes per month in October to about 40 earthquakes per month in November. Rates of deformation on the lava dome have not changed from the slow, steady rates measured in the past 3 years.


Caption Graph showing relationship between time and depth of earthquakes beneath Mount St. Helens between January and Novmber 1995. Note the increase in the number of earthquakes per month from less than 10 in January to about 100 in September. Earthquake activity declined in late October to about 40 earthquakes per month in November. Most of the earthquakes are smaller than magnitude 1.0. The largest earthquake (magnitude 2.3) occurred on July 4, 1995.



Summary of Crater Observations:

In response to the increase in seismicity between January 1 and September 30, 1995, USGS scientists worked in the crater to determine whether the long-term rates of deformation on the dome had changed. The scientists surveyed 13 benchmarks on the lava dome and installed two tiltmeters and a displacement meter on the top, west side of the dome. The surveys showed that no increase in deformation of the dome occurred as a consequence of the recent earthquake activity, but the northwest side of the dome continues to move downward very slowly as it has since a series of small explosions occurred between 1989 and 1991. In contrast to the current period of quiet, significant deformation of the dome was routinely measured prior to the onset of lava extrusion during the dome-building eruptive episodes between 1980 and 1986.

Data from the tiltmeters and displacement-meter show only slight changes that we attribute to environmental conditions, such as temperature and snow depth, not renewed volcanic activity. The two tiltmeters and displacement meter are located near LITTLE VILLAGE, a survey benchmark on the lava flow that was extruded on the west side of the dome in September 1984. This site was selected for both types of instruments because of its accessibility and its history of responding well to the intrusion of magma into the dome. For example, prior to the most recent eruption in October 1986, the west side of the dome moved outward several meters, an existing graben structure subsided abaout 2 meters, and several new fractures appeared and widened. The new displacement meter spans one of these fractures and it can measure the closing or opening of the fracture by as little as about 1millimeter. The high-precision tiltmeter is capable of detecting changes in angle of the ground surface of less than 1/10,000 of a degree over a range of about 1 degree; the low-precision tiltmeter can detect changes of less than 1/1,000th of a degree over a range of about 60 degress. The real-time transmission of this data means that information on deformation of the dome will be available during the winter snow season, a time when access to the crater is very limited, to compare with future changes in seismic activity.

Summary of lahars:

Periods of intense rainfall in November generated several surges of water and rock debris, known as lahars, from the crater. All of the lahars were detected by the USGS real-time acoustic-flow network(AFM) and probably flowed into Spirit Lake. None were detected in the North Fork Toutle River. Such lahars are common during intense rainfall following the dry summer months. With sufficient rainfall, sediment that has accumulated in gulleys and channels in the crater during summer and fall is eroded and transported downstream. Small lahars like these remain in the stream channel leading to Spirit Lake or the Toutle River and pose no threat to people or structures downstream.