April 6, 2012

Spectacular video of Devil’s Hole seiche

Posted by Austin Elliott

This video is a must-watch. A fortunate National Park Service biologist and a Scientific American reporter interviewing him were standing at a deep natural pool in Death Valley when it began to be dramatically rocked by an earthquake that had occurred 2,000 miles away.

Pond? Not if there's an earthquake on the other side of the planet.

Devil’s Hole is a deep, shaded, natural pool near Death Valley, in Nevada, that is home to some very specialized species of fish and is thus heavily studied. It’s also just the right size to be excited by low frequency “teleseismic” waves from distant earthquakes.

When large earthquakes happen anywhere on the planet, the seismic waves they emit ripple around the globe, detected in most places far from their epicenters only by extremely sensitive instruments. For instance, when the 7.4 earthquake struck the coast of Mexico a few weeks ago, it excited surface waves that raced dramatically beneath our feet in the U.S. imperceptibly to us. By the time seismic waves reach these great distance they are both weak and extremely low-frequency. Sensitive seismometers, however, easily pick up their signal.

Seismogram from central California (Monarch Peak) showing a 7.4 earthquake in southern Mexico. Note that during the strongest "shaking" the wave crests are ~15 seconds apart. Too slow for us to feel... just right for a small, sloshing pond.

Look at this seismic record from a station in central California on March 20. The M7.4 earthquake dominates the record even though this station is nearly 2,000 miles from the epicenter. The reason people in California weren’t knocked off their feet was that at that distance the seismic waves have a frequency far below anything we could expect to “feel”. If you look at the strongest waves recorded (mostly in red), you’ll see that there are about four peaks per minute, meaning that each wave takes about 15 seconds to pass. What you’re seeing is the ground oscillate back and forth every 7.5 seconds.

Count that out: wayyyy too slow to feel. If, however, you’re a small body of water of just the right size, this is the perfect amount of time to make you slosh over your banks. Think about being in a bathtub: if you shake really fast, the water splashes a little; if you slide very slowly, the water moves smoothly around you; if you swish back and forth at just the right medium speed, however, you can surpass the speed with which water can spill around you, and you end up making it slosh wildly. That’s basically what’s happening in the video above at Devil’s Hole.

Update April 8, 2012: The effect apparent in the video may not be a result of transverse seismic waves causing resonant sloshing, but may stem from compressive waves squeezing the aquifer and causing water level to fluctuate. Similar phenomena are further described on this USGS info page, and the theoretical background is laid out in this review paper in Science.

In the video, the undulation of water level starts around 10-15 minutes after the quake’s origin time. The videographer checks the time at ~18:13, and he’s referring to Coordinated Universal Time. The Quake started in Mexico at 18:02.

It’s really a quite spectacular demonstration of the passage of these very low frequency waves of motion in the rock. You can’t deny they’re happening when the pool beneath you is overflowing its banks!

There’s a nice first-hand narrative of the lucky event here, with a different video from an alternate perspective:

http://www.scientificamerican.com/article.cfm?id=earthquake-at-devils-hole