Water waves can have literally seismic impacts | Science News for Students

Water waves can have literally seismic impacts

Seismic waves spawned by lake waves could help scientists spot risks of earthquakes
Jan 12, 2018 — 6:45 am EST
lake waves

Some of the energy from waves on large lakes (such as Lake Ontario, here) gets transformed into seismic energy. Researchers are thinking about harnessing that energy in some places to map nearby earthquake fault zones.

Duentsch-Photo-Connect/iStockphoto

NEW ORLEANS, La. — The waves on large lakes carry a lot of energy. Some of that energy can penetrate the bottom and shore of the lake, creating seismic waves. These can shake the ground for kilometers (miles) around, a new study finds. Scientists now believe that recording those seismic waves could give them a load of useful data.

For instance, such data could help map underground features — such as faults —that point to possible earthquake risks. Or, scientists might use those waves to quickly tell whether lakes in remote, cloudy regions have frozen over.

Kevin Koper is a seismologist at the University of Utah in Salt Lake City. Several studies, he notes, have shown that lake waves can shake the ground nearby. But his team’s new study of six large lakes in North America and China has just turned up something interesting. Seismic waves triggered by those lake waves can shake the ground up to 30 kilometers (18.5 miles) away.

Seismic tremors are similar to the rolling waves on bodies of water. And in the new lake study, they passed by vibration-detecting instruments — seismometers (Sighs-MAH-meh-turz) — at a frequency of once every 0.5 to 2 seconds, Koper now reports.

“We didn’t expect that at all,” he says. The reason: At those particular frequencies, rock will typically absorb the waves pretty quickly. In fact, that was a big clue that the seismic waves had been generated by lake waves, he notes. He and his team couldn’t identify any other nearby sources of seismic energy at those frequencies.

Koper presented his team’s observations on December 13, here, at the fall meeting of the American Geophysical Union.

Mysteries abound

Lake Ontario waves
Waves on large lakes send part of their energy into the ground as seismic waves. Scientists might tap that seismic energy to gauge whether some largely inaccessible lakes are ice-covered.
SYSS Mouse/Wikipedia Commons (CC BY-SA 3.0)

The researchers studied lakes having a range of sizes. Lake Ontario is one of North America’s five Great Lakes. It covers about 19,000 square kilometers (7,300 square miles). Canada’s Great Slave Lake covers an area more than 40 percent larger. Wyoming’s Yellowstone Lake covers only 350 square kilometers (135 square miles). The other three lakes, all in China, each cover just 210 to 300 square kilometers (80 to 120 square miles). Despite these size differences, the distances traveled by the seismic waves triggered at each lake were about the same. Why that should be is a mystery, says Koper.

His group also hasn’t figured out yet how the lake waves transfer some of their energy into Earth’s crust. The seismic waves may develop, he says, when surf pounds the shore. Or maybe large waves in open water transfer some of their energy to the lake floor. This coming summer, the researchers plan to install a seismometer on the bottom of Yellowstone Lake. “Maybe the data that instrument gathers will help answer that question,” Koper says.

In the meantime, he and his team have been hatching ideas about how to make use of a lake’s seismic waves. One notion, he says, would be to map below-ground features near large lakes. This could help researchers spot faults that might signal a region is at risk for earthquakes.

The way they’d do it would be very similar to the idea behind computerized tomography (Toh-MOG-rah-fee). It’s the process at work in the CT scanners that doctors use. These devices beam X-rays into a targeted part of the body from many angles. A computer then assembles the data they collect into a three-dimensional views of some internal tissue, such as the brain. This lets doctors look at the body part from any angle. They even can divide the 3D image into a large number of slices that look just like two-dimensional X-ray images.

But while medical X-rays are powerful, seismic waves spreading from lakes are quite faint. To amplify those signals, Koper says, his team could simply add together a lot of data gathered over months. (Photographers often use a similar technique to take pictures at night. They will leave a camera’s shutter open for an extended time. That lets the camera collect a lot of dim light to create a picture that ultimately looks sharp and well-defined.)

Seismic-wave scans also could map other things as well, suggests Rick Aster. He’s a seismologist at Colorado State University in Fort Collins. For instance, researchers might map out any large masses of molten rock beneath volcanoes.

“Every time we find a new source of seismic energy, we’ve found a way to exploit it,” he says.

Seismic waves near lakes — or their absence — might even help environmental scientists, Koper says. For instance, those waves could provide a new way to monitor the ice cover on remote lakes in polar regions. (These are places where the effects of climate warming have been most exaggerated.)

Such regions often are cloudy in the spring and fall — exactly when lakes are thawing or freezing. Satellite cameras can scan such sites, but they may not get useful images through the clouds. Detecting seismic waves of the right frequencies with lakeside instruments might provide a good gauge that a lake has not yet frozen. When the ground later quiets, notes Koper, this might signal that the lake is now capped with ice.

Power Words

(for more about Power Words, click here)

amplify     To increase in number, volume or other measure of responsiveness.

angle     The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

computerized tomography     (CT, for short). A special kind of X-ray scanning technology that produces cross-sectional views of the inside of a bone or body.

earthquake     A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

Earth’s crust     The outermost layer of Earth. It is relatively cold and brittle.

fault     In geology, a fracture along which there is movement of part of Earth’s lithosphere.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

Great Lakes     A system of five interconnected lakes — Superior, Michigan, Huron, Erie and Ontario — the Great Lakes constitute the largest freshwater source in the world (based on surface area). They hold an estimated 6 quadrillion gallons of water, or about a fifth of the world's fresh surface water. To give some perspective on that amount, the lakes' water would, if spread evenly, cover the 48 touching U.S. states to a depth of about 2.9 meters (9.5 feet) deep.

molten     A word describing something that is melted, such as the liquid rock that makes up lava.

monitor     To test, sample or watch something, especially on a regular or ongoing basis.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

satellite     A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.

scanner     A machine that runs some sort of light (which includes anything from X-rays to infrared energy) over a person or object to get a succession of images. When a computer brings these images together, they can provide a motion picture of something or can offer a three-dimensional view through the target. Such systems are often used to see inside the human body or solid objects without breaching their surface.

seismic wave     A wave traveling through the ground produced by an earthquake or some other means.

seismology    The science concerned with earthquakes and related phenomena. People who work in this field are known as seismologists.

seismometer     (also known as a seismograph ) An instrument that detects and measures tremors (known as seismic waves) as they pass through Earth.

tissue     Made of cells, any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

X-ray     A type of radiation analogous to gamma rays, but having somewhat lower energy.

Citation

Meeting:​​ ​K.D. Koper, Y. Xu and R. Burlacu. Presentation S34A-06: Lakes as a source of short-period (0.5-2 sec) microseisms. American Geophysical Union Fall meeting. December 13, 2017. New Orleans, La.