What's Really Shaking

When the earth moves under your feet, the cause isn't always an earthquake. Many types of phenomena, from the shudders of a glacier to the detonation of a nuclear bomb, can move mountains and get entire regions rocking. Berkeley professor of earth and planetary science Douglas Dreger analyzes the seismic signatures left behind by these and other exotic, earthquake-like events. His work is aiding fields as diverse as glaciology, mine rescues, and nuclear treaty enforcement.

The 1996 eruption of Iceland's Bardarbunga volcano. Douglas Dreger was able to interpret its seismic rumblings to deduce the internal workings of the eruption. Image credit: Oddur Sigurdsson, Iceland Geological Survey

Dreger takes full advantage of the data from modern digital broadband seismometers. These instruments are sensitive enough to register the slightest earth quivers as well as the strongest quaking in each of three directions. Dreger filters such digital seismograms to reveal far more than shaking magnitude. Using a technique called moment tensor inversion, he can distinguish true earthquakes from mine implosions, volcanic eruptions, and nuclear tests.

Dreger ground-truths his techniques by examining known earth movement phenomena. An example of this are his studies of seismograms produced during nuclear tests conducted in Nevada. A first glance at the seismograms suggests that the motion of the first waves to arrive is the same in every direction-a pattern consistent with an explosion. But by analyzing only the low-frequency waves, Dreger, William Walter of Lawrence Livermore National Laboratory, and colleague and graduate student Sean Ford were able to show that the tremors include directed, transverse components called Love waves. "That means it was not a clean explosion, that it might have triggered a small earthquake, often referred to as tectonic release," Dreger says.

Equally informative but more tragic was the 2007 Crandall Canyon Mine collapse. Prior to the accident, workers in the final stages of removing a coal seam in Utah had been retreat mining-digging out the earthen columns supporting the mine's ceiling, waiting for the ceiling to fall, and collecting the coal rubble. After several columns had been removed at once, a large section of the ceiling collapsed. Its fall triggered the domino-like failure of dozens of adjacent columns, trapping and killing six miners. A subsequent collapse killed three rescuers days later.

Dreger and colleagues reconstructed the details of the collapse from seismic data. They found that 80 percent of the energy expended in the magnitude 3.9 event was due to the downward movement of the mine ceiling. The remaining 20 percent involved the transverse shaking associated with earthquakes, likely due to sympathetic faulting in the roof of the mine. "We can't say there wasn't an earthquake just before the collapse, but it would have to have been so small as to not be detected by our seismographs, less than a magnitude 1," Dreger says.

Based on this analysis, Dreger developed a means to help characterize future mining accidents that could improve accident assessments and save lives. "With this type of analysis we can calculate the collapse area. In the future, such analyses if done rapidly could help to assess the scope of a mining disaster and facilitate rescue efforts."

The jerky movements of glacier ice resemble the explosive shudders of earthquakes, only in miniature. Hundreds of thousands of icequakes may ripple through the frozen reaches of a single glacier every year. Together with glaciology student Fabian Walter, Dreger has uncovered new mechanisms that help explain how glacial ice moves forward.

Seismometers within the Gornergletscher of the Swiss Alps had recorded a class of icequakes that only occurred near the glacier's base at night. Dreger's methods identified the movement of a nearly horizontal fault accompanied by an expansion within the glacier's ice. "We think the ice is moving en masse when water melted during the day reduces friction at the base of the glacier. As it starts to refreeze at night, the traction at the base starts to grow. Because there's inertia in that ice mass, we think the upper section eventually tears away from the lower," Dreger says. "The tearing causes it to pop open, but it probably doesn't stay open very long."

Dreger and Walter are continuing to analyze shaking data from the glacier to uncover new types of glacier movements. "This is, pardon the pun, just the tip of the iceberg as far as the objectives of these seismic studies," Dreger says.

Source: Boston University