DESCRIPTION:
Ground-Crack and Thrust-Fault Measurements

MSH81_USGS_scientists_measure_radial_crack_base_dome_05-12-81.jpg
The U.S. Geological Survey established both periodic and continuous 24-hour monitoring programs at Mount St. Helens to study and predict eruptions. In this slide, geologists used a steel tape to measure the distance across a crack on the crater floor. Widening of cracks was an indication that magma was rising and deforming the area, leading to an eruption. These cracks were generally radial to the dome, like spokes of a wheel.
USGS Photograph taken on May 12, 1981, by Lyn Topinka.
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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.
[medium size] ... [large size] ... [TIF Format, 24 M]

Ground-Crack Measurements

New ground cracks appeared on the crater floor from several days to 2 to 4 weeks before all the 1980 to 1982 eruptions. The cracks, commonly tens of meters long and tens of centimeters wide, extended outwards from the dome like spokes from the hub of a wheel. Incandescent rock was visible in some cracks, and temperatures of escaping gas were measured as high as 840 degrees C. Measured with a steel tape, the cracks commonly showed continual widening that accelerated before eruptions. Such accelerated movement was used to predict several eruptions in 1981 and 1982.

Thrust-Fault Measurements

From 1980 to 1982, parts of the crater floor 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.

Movement along the faults was monitored by repeated measurements between benchmarks on either side of the fault by using a steel tape and by leveling between points on the upper and lower plates. The rate of movement accelerated before eruptions, and this provided the most consistent and reliable relatively long-term (1-3 weeks) predictive tool at Mount St. helens during 1981 and 1982. Rockfalls from the dome and crater walls have buried some major thrust faults under rock debris, others have been overridden by the dome itself; but new thrust faults occasionally appear, as in October 1983.

In mid-September 1980, cracks started forming in the crater floor radially from the vent, which was plugged by a small dome that formed in August. Pierces of reinforcing rod (rebar) were driven into the ground on both sides of the cracks to form "crack stations," and the distances between rebar stakes were measured with a steel tape. ... prior to the October 1980 explosive episode, the cracks widened at an accelerating rate in response to a rising magma body. ...

Thrust faults in the crater floor were first observed during the December 1980 dome-building episode. 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 ...

Monitoring of cracks and thrust faults is relatively simple and inexpensive, requiring only rebar and a sturdy 25-meter steel tape measure. We cut rebar into stakes 40-50 cm long and drive them into the ground, leaving enough of the rebar exposed to provide ground clearance when the tape is stretched between them. ...

... Three rebars forming a triangle spanning the crack or thrust fault is typical ... We set up a crack station by placing one rebar (designated #1) on one side of the crack (or on the upper plate of a thrust fault) and two on the other side (#2 and #3), numbered in clockwise fashion. By measuring distances between 1 and 2 and between 1 and 3, widening or closing of the crack or thrust fault can be monitored. The 2-3 distance measurement serves as a baseline and check on measuring precision (1-2 mm). With the combined three measurements, vector displacements can be calculated for rebar 1 relative to rebars 2 and 3. For the index point for both ends of the tape, we generally center-punch the top of the rebar and use this mark. One person holds the zero mark of the tape over the center mark of one rebar, and the tape is stretched tight and read over the center mark of the other rebar.

Measurements of change in the width of cracks on the crater floor of Mount St. Helens, Washington, were a major tool used in predicting dome-building episodes during 1980-1982. These measurements were made by field crews using a steel tape measure to determine the distance between lengths of reinforcing rod driven into the ground on opposite sides of cracks. In 1981, a telemetered, electronic tape measure was installed on the crater floor to monitor crack movement continuously. However, electronic and physical problems kept this and a second displacement meter from being viable predictive tools. In 1985, using new techniques and instrumentation, a third displacement meter was installed on the dome, and it successfully monitored the widening of a crack days before a dome-building episode. By 1989, there were two displacement meters of this type monitoring cracks on the dome.

Measurements of change in the width of cracks on the crater floor of Mount St. Helens, Washington, were a major tool used in predicting dome-building episodes during 1980-1982. These measurements were made by field crews using a steel tape measure to determine the distance between lengths of reinforcing rod driven into the ground on opposite sides of cracks. In 1981, a telemetered, electronic tape measure was installed on the crater floor to monitor crack movement continuously. However, electronic and physical problems kept this and a second displacement meter from being viable predictive tools. In 1985, using new techniques and instrumentation, a third displacement meter was installed on the dome, and it successfully monitored the widening of a crack days before a dome-building episode. By 1989, there were two displacement meters of this type monitoring cracks on the dome.

Beginning in September 1980, cracks that extended radially from the Mount St. Helens dome formed in the crater floor. As magma ascended into the dome the crater floor deformed, creating segments or blocks bounded by cracks that either widened or, less commonly, narrowed before and during extrusive episodes ... The rate of widening or narrowing of the cracks accelerated before extrusion occurred. Initially, a steel tape was used to measure changes in distance between reinforcing rod (rebar) stakes driven into the ground on either side of selected cracks (crack stations). This method became a reliable tool for predicting dome-building episodes at Mount St. Helens.

To monitor crack movement continuously, electronic tape measures (displacement meters) were installed to monitor displacements between rebar stakes. Measurements from the displacement meters are made every 10 minutes and telemetered to the Cascades Volcano Observatory (CVO).

... In June 1981, the first displacement meter was installed on the east crater floor. The displacement meter and electronics were housed inside a fiberglass box that was connected to a plastic pipe 10 cm in diameter and 12 m long. The displacement-meter wire (stainless steel) was strung through the plastic pipe to a fencepost on the other side of the crack. At this time, many of the cracks were hot and emitted steam and gas; where the wire crossed the crack, the plastic pipe was encased in a larger steel pipe for added protection.

A separate fiberglass box contained the telemetry unit. Measurements were telemetered to CVO at 10-minute intervals. ...

Following the March 1982 episode, the radial cracks on the crater floor were buried by pumice deposits ... Consequently no measurements of cracks were made until 1985, when we started to establish geodetic stations on the dome itself ... At this time a commercial displacement meter was purchased and installed next to an existing crack station. This third installation was environmentally secure, and both the displacement meter and taped distances showed similar trends preceding the May 1985 episode. Except for minor modifications, this is the system configuration used in 1990.

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