Sloshing Detected in Yellowstone Lake Helps to Locate Magma Storage Region
Several years ago, using sensitive new monitoring equipment located in shallow boreholes, scientific staff from UNAVCO, a member institution of the Yellowstone Volcano Observatory (YVO), detected an odd rhythmic signal near Yellowstone Lake. Further work demonstrated that this strain signal, with a 78-minute frequency, came from an intermittent seiche (pronounced "say-sh"), or standing wave, only an inch or two tall, within Yellowstone Lake (Plots below show seiche signal). These waves can be triggered by atmospheric phenomena such as high winds or barometric changes, and usually takes a few days to die off.
Fluid dynamic models show that the frequencies of the standing waves relate directly to the shape of the Lake (see "What is a Seiche?" at right). Multiple separate waves are present, representing oscillations in water flow among the multiple lake basins. Most surprising to the geophysicists was that the signal of these small waves along the lake's surface could be recognized 30 km (19 mi) away from the lake at a distant strainmeter (instrument locations in map above).
USGS postdoctoral fellow Karen Luttrell, together with colleagues from UNAVCO and elsewhere, recently published an article in Geophysical Research Letters revealing how the presence of magma beneath the ground at Yellowstone allows the seiche signal to travel further than it would in the earth's crust under normal (magma-free) conditions (cartoon below illustrates amplification of seiche signal). In essence, instead of behaving like a solid (elastic behavior) like most of Earth's crust, regions beneath Yellowstone can slowly flow (viscoelastic behavior) and transfer strain across greater distance. The authors estimate that magma is present starting at 3—6 km (2—4 mi) beneath the ground surface, and that the magma is mostly crystallized but is still partly molten. The new results are consistent with and complementary to results from seismic and other studies (Smith et al., 2009) . YVO scientists continue to monitor the Yellowstone volcano area for any signs of unrest.