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Q:

What fault lines pass under Mt. Saint Helens?

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Mt. Saint Helens is located on the St. Helens Fault Zone (SHZ). This is a strike-slip fault. Right at Mount St. Helens there is a gap and a step in the SHZ. This step causes the crust to pull apart inside the gap, creating a zone of weakness where volcanic material can more easily reach the surface. It will help you to understand this if you draw some pictures of a step in a strike-slip fault, with arrows to show the direction ov movement. Many volcanos are found in similar circumstances. The St. Helens Fault Zone was not discovered until after the eruption of Mt. St. Helens (1980). In 1981 a magnitude 5+ earthquake on the SHZ had thousands of aftershocks which "lit up" the fault. • Q: What is the best web site for Volcano information? A: The best site for information about volcanoes are at the Cascade Volcanic Observatory (CVO). In addition, the University of Washington has a volcanoes web page. • Q: There seems to be a great deal of activity on the webicorders, but there are not ... more
geophys.washington.edu
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