Seismic Energy From Waterfalls in Yellowstone National Park

Abstract:

We surveyed continuous seismic data recorded at two seismic stations in Yellowstone National Park that are located near to Yellowstone National Park's Upper and Lower Falls. Lower Falls is the largest waterfall in Yellowstone, with an estimated flow rate of 70 cubic meters per second, falling an estimated 94 meters, while the Upper Falls has a flow of 70 cubic meters per second, jetting over a 21 meter gap downward 33 meters.

A study based on a deployment of seismometers in Yellowstone in September and October of 1972 had found a predominant 2 Hz signal associated with the Lower Falls, with the signal remaining above background noise within 6 km of the falls in every direction but the south.

Station YUF is a three-component, broadband seismometer operated by the University of Utah Seismograph Stations that is located approximately 1.5 km southwest of the Lower Falls, while station B206 is a three-component, short-period, borehole seismometer operated by the Plate Boundary Observatory, located roughly 1.2 km west of the Lower Falls. We computed power spectral densities (PSD) for all available hour-long segments of continuous data from the vertical components of YUF and B206 beginning September 22, 2006 and July 10, 2008, respectively. Yearly spectrograms were used to visualize the PSDs.

Both stations showed spectral peaks in the double-frequency microseismic band, with stronger amplitudes in winter than in summer, presumably generated mainly by storms in the North Pacific. Both also showed strong peaks near a period of 1 s, but with the opposite seasonal dependence. This 1 s peak signal broadens in frequency during the summer, from 1 to 5 Hz, as well as uniformly increasing in power across this band. This short-period noise was compared to discharge measurements of the Yellowstone River made at the Yellowstone Lake outlet, about 18.5 km upstream from the Upper Falls. For periods of 0.5-2.0 s the correlation coefficient between the seismic energy and the river discharge rate was as high as .97. We are currently extending the analysis to polarization of the three-component seismic data to determine the relative contribution of the two falls.