Mount St. Helens, Washington
Summary of Mount St. Helens Eruptive Activity,
December 1995


Summary of activity: Mount St. Helens was quiet in December. No explosion or emission of gas and ash occurred from the lava dome, and no explosion-like seismic signals were detected. The number of small magnitude (mostly less than magnitude 1) earthquakes beneath the crater decreased from about 40 per month in November to about 25 per month in December. Rates of deformation on the lava dome have not changed from the slow, steady rates measured in the past 3 years.

Summary of Seismicity: Seismicity in December is similar to that during the first 6 months of 1995. Taken together, the gradual decrease in the number of earthquakes since September and the absence of any small explosions related to the earlier increase in seismicity (see earlier summaries below), even during recent periods of heavy precipitation, decrease our concern over the possibility of future small explosions from the dome. Such explosions are still possible, but the likelihood of their occurrence is no greater than during the first 6 months of 1995.

Graphic

Caption: Graph showing relationship between time and depth of earthquakes beneath Mount St. Helens from January 1 to December 31, 1995. Note the gradual increase in earthquake activity between 1 and 6 miles beneath the crater from January through September, and the subsequent decrease from mid-October through December. 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). Data and graph provided by the Geophysics Program, University of Washington.

Caption: Cross-sectional view showing the locations of earthquakes beneath Mount St. Helens during December 1995. The cross-section represents a vertical slice of the crust downward from line A-A' in the map view. The earthquakes occurred in and around the magma conduit system that leads from the magma reservoir to the dome. 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). Data and graph provided by the Geophysics Program, University of Washington.



Summary of Crater Observations: No visits were made to the crater in December. See summary for the period October 1 - November 30, 1995, for more information.