DESCRIPTION:
Thrust Faults

MSH81_USGS_scientists_measure_thrust_fault_05-12-81.jpg
Two USGS scientists measure thrust fault at base of Mount St. Helens' dome.
USGS Photograph taken on May 12, 1981, by Lyn Topinka.
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From 1980 to 1982, parts of the crater floor (of Mount St. Helens) became slightly wrinkled several weeks before eruptions. A few wrinkles developed into thrust faults, a low-angle fracture, along which rocks above the fracture are pushed over rocks below the fractures. By summer 1981, a complex system of thrust blocks had disrupted much of the southwestern part of the crater floor. The thrust faults formed as rising magma forcefully ruptured the crater floor, shoving parts of it upward and outward from the vent toward the rigid crater walls. Before the August 18, 1982, eruption, the leading edge of one thrust fault grew from less than 30 centimeters to roughly 5 meters high a few days before the eruption.

Thrust faults in the crater floor were first observed during the December 1980 dome-building episode (of Mount St. Helens). Thrust stations, established across the leading edge of the upper plate, were monitored in the same way as were the crack stations. As the upper plate of the thrust overrode the lower plate, measured distances shortened. The rate of shortening increased prior to dome-building episodes, just as did the rate of widening of cracks. The changing rates were used to predict the start of the next eruptive episode ... We also leveled between rebar stakes and determined that the upper plate was being uplifted as it thrust forward. ...

During the March 19, 1982, eruptive episode ... all cracks and thrust faults on the crater floor were buried by pumice or stripped away, thereby ending one major phase of monitoring ...

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