When the earth moves, the Sanford Lab listens

Three research groups at the Sanford Underground Laboratory at Homestake detected recent earthquakes in Japan, Chile, Haiti and Mexico -- as well as last September's tsunami in the Pacific Ocean. The data illustrate how the Sanford Lab, as the nation's Deep Underground Science and Engineering Laboratory, will play a major role in future explorations into the structure of planet earth.

The data also underscore Sanford Lab's international role. "It's neat to have that connection here in Lead (S.D.) to events that are occurring around the world," Sanford Lab Science Liaison Director Jaret Heise told the Lead City Commission recently, during an update for local government officials. "It's remarkable data that allows us to monitor the earth and its changing events."

The three research groups are:

  • The Transparent Earth Group, which has sensors at four underground locations 2,000 feet underground. (The University of California, Berkeley and the South Dakota School of Mines andTechnology)
  • The Homestake Hydrostatic Water system, which as 18 sensor in three arrays 2,000 feet underground. (Fermi National Accelerator Laboratory, the University of Wisconsin and South Dakota School of Mines)
  • The Deep Underground Gravity Laboratory, or DUGL, which has sensors at the 300-foot level and 4,100-foot levels in the lab. (California Institute of Technology and the University of Minnesota)

Graduate student Jason Van Beek of South Dakota School of Mines and Technology, who works with the Tranparent Earth Group, provided the plot at the upper right, showing the disastrous earthquake in Chile on Feb. which shows the effects of the disastrous Chilean 8.8 earthquake on Feb. 26-27. The smaller blip to the left was a Magnitude 7.0 earthquake in Japan. The gentle rising and falling of the plot illustrates "earth tides," which, like ocean tides, are cause by gravitational effects of the moon and sun.

The plots above and at right are from an instrument called a "tiltmeter"  2,000 feet underground. (Click the plots and photos to make them larger.) "As the P and S waves move away from the earthquake's epicenter, they deform the rocks through which they are traveling," Van Beek explains. P waves, or primary waves, oscillate in the direction the wave travels. (Sound waves, for example, are P waves.) S waves, or secondary waves, travel much slower and oscillate perpendicularly to the direction of travel. "This tiltmeter records the changing angle of the ground relative to horizontal," Van Beek says. "In much the same way, a kayaker on the ocean could detect a wave traveling underneath by observing luggage sliding towards either the back or front of the boat."

Dr. Larry Stetler of South Dakota School of Mines and Technology and Jim Volk of Fermi National Accelerator Laboratory have set up kind of tiltmeter at the 2000 Level. Their 18 instruments are in three long arrays -- two a thousand feet long, and the third about 600 feet long. The tiltmeter instruments sit on cylindrical concrete pads, and they are connected by tubes -- one on the bottom filled with water and one on top filled with air. These arrays act as very sensitive bubble levels. They're experiment is set up to measure very small changes in the tilt of the 2000 Level as the lower levels of the Sanford Lab are dewatered. Though less sensitive then the Transparent Earth instruments to far off earthquakes, they did register the Chilean quake. "This plot shows the 24 hours (UTC) of Feb 27," Stetler exlains. "You can see the first arrival, and another 8 arrivals from a surface wave that went around the Earth a couple of times." (Stetler filtered out some data above to clarify the plot.)

The Deep Underground Gravity Laboratory (DUGL, "DOO-guhl) is an experiment to determine whether Sanford Lab is stable enough for gravity-wave detectors. Last September DUGL instruments on the 4,100-foot level clearly picked up the effects of the tsunami in the Pacific Ocean.

"We were underground at that time, installing two new stations," Dr. Jan Harms, a post-doctoral researcher from the University of Minnesota, reports. "When we came back to the surface late in the afternoon, to check remote access and data quality, I thought that we did something wrong, since the data from the new stations looked bad. Ground motion was much too strong, and the ground oscillations untypically slow."

However, Harms said other established instruments elsewhere in the lab recorded the same anomaly. "So obviously, we observed a real signal," Harms says. "My first thought was that we just had a blast in the mine." Sanford Lab crews are excavating a new cavern for the LUX and Majorana experiments, but Sanford Lab Science Liaison Director Jaret Heise assured Harms there had been no blasts that night.

"The news of the day then finally gave the answer," Harms said. "What we saw was the signal produced by the magnitude 8 earthquake near the Samoa Islands in the Pacific Ocean." In fact, the DUGL instruments continued to pick up vibrations from the tsunami itself, as the wave travelled around the world. " The seismic signal from ocean waves that we measure at all times with our seismic stations was increased by a factor 50," Harms said.