Mount St. Helens, Washington
Summary of Mount St. Helens Seismic Activity,
September 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 September 1996. Twenty-three earthquakes were located beneath the volcano during the month. This is the highest monthly total so far this year, but is well within the range considered to be a safe, background level. A swarm of 6 earthquakes occurred on September 18, including a magnitude 2.2 event. A magnitude 2.4 earthquake on September 21 was one of the two largest earthquakes so far this year (tie).

Graphic

Caption: Cross-sectional view showing the locations of earthquakes beneath Mount St. Helens during September 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 October 1, 1995 to September 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.