|
Little seismic work was done at Mount St. Helens before 1980. We are not aware
of any record of earthquakes preceding or accompanying the eruptions early in the
last century. A catalog of felt earthquakes in Washington (Rasmussen, 1967)
lists a number of earthquakes in the early 1900's at Cougar, 18 km southwest of
the peak, but there is no way of knowing whether these shocks were related to
the volcano.
A microearthquake survey of Mount St. Helens and several other Cascade volcanoes was conducted in 1970 by Unger and Mils (1973). Two types of earthquakes were detected near the peak: one they interpreted to be similar to volcanic B-type earthquakes described by Minakami (1970), and the others they interpreted as more typically tectonic in character. The tectonic earthquakes were located just to the southwest and northeast of the peak at moderate depths of a few kilometers, while the volcanic earthquakes were located high on the cone itself at shallow depths. While Mount St. Helens has a higher earthquake rate than other Cascade volcanoes surveyed, Unger and Mills did not interpret their data to indicate that the mountain was showing signs of volcanic activity. Additional seismic studies were undertaken at Mount St. Helens in the early 1970's (Weaver and Malone, 1976, 1979). Several field experiments were undertaken to isolate low-frequency, B-type earthquakes. Arrays of portable seismic stations placed around an high up on the mountain revealed the mountain's glaciers as the sources of the low-frequency "earthquakes". These, in reality, were not volcanic earthquakes but vibrations produced as the glaciers slid in jerky movements down the rather steep sides of the mountain. The low-frequency emergent character of these events was due to a propagation effect resulting in attenuation of high frequencies. The recordings of true volcanic earthquakes during the past year -- (Web note: 1980) -- reaffirms this glacial interpretation of the earlier observations. |
Return to:
[Report Menu] ...
