Mount St. Helens, Washington
Summary of Mount St. Helens Seismic Activity,
June 1996


The following information is provided by the U.S. Geological Survey's Cascades Volcano Observatory and by the University of Washington's Geophysics Program.

Earthquake activity beneath Mount St. Helens remained at a low level during June 1996. Ten earthquakes were recorded beneath the volcano during the month. None of these earthquakes exceeded magnitude 2.0. Monthly earthquake totals since January 1996 have been: January, 14; February, 13; March, 19; April, 16; May, 11; and June, 10.

During the night of June 9, a large seismic event from the volcano triggered an automated, 24-hour alarm system. The character of the signal suggested that the source was a rockfall from the crater wall. This interpretation was confirmed by a USGS crew working in the crater on June 11, who observed a large, fresh rockfall deposit that originated from the south crater wall. Rockfalls are a common occurrence in the crater during the summer, and generally do not indicate any increase in volcanic activity. Activity at the volcano remains at a safe, background level.

Caption: Cross-sectional view showing the locations of earthquakes beneath Mount St. Helens during June 1996. 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.

Graphic

Caption: Graph showing relationship between time and depth of earthquakes beneath Mount St. Helens from July 1, 1995 to June 30, 1996. 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.