What About Great Salt Lake?
By Barry J. Solomon
Seiches (pronounced “sayshes”) are oscillations of enclosed bodies of water, similar to the sloshing of water in a bathtub. The term was first used in 19th century Switzerland to apply to standing waves set up on the surface of Lake Geneva by wind and changes in barometric pressure. However, seiches often occur following an earthquake. Although strong ground shaking most commonly causes earthquake seiches, more dramatic seiching motion may be produced by a permanent vertical ground displacement beneath a water body, such as may be caused by surface faulting or tectonic subsidence. These waves are sometimes called “surges” to differentiate them from much milder but otherwise similar oscillations of closed water bodies caused by ground shaking.
Recent interest in possible seiches in Great Salt Lake results from the potential for lakeshore flooding associated with historically high lake levels in the early 1980s. Possible seiches were first discussed in this context by the University of Utah Seismograph Stations in an analysis of earthquake-design considerations for the inter-island diking project, which described accounts of waves generated in Great Salt Lake by the 1909 magnitude 6+ Hansel Valley, Utah earthquake. Although felt reports placed the location of the 1909 earthquake about 9 miles northeast of the north lake shore, the accounts of waves generated by this event suggested that the earthquake might actually have occurred beneath the lake and caused displacement of the laμke bottom, generating a surge. The larger 1934 magnitude 6.6 Hansel Valley earthquake apparently did not generate similar lake waves, consistent with instrumental location of the earthquake epicenter and with surface faulting located just northeast of the north lake shore.
The height of the wave generated by the 1909 Hansel Valley earthquake was later estimated at more than 12 feet using elevations of Great Salt Lake and the Lucin Cutoff railroad trestle, which was overtopped by the wave. However, this estimate did not consider the cause of the wave. The estimate is now believed to be an example of the potential for waves generated by differential subsidence of the lake floor associated with one of several active faults underlying the lake. Thus, when describing the consequences of an earthquake with a specified magnitude and location (a scenario earthquake), the mechanism of wave generation is important to consider and depends upon the relative locations of the lake, earthquake epicenter, and surface fault rupture. In the case of our scenario, neither a surge from displacement of the lake bed nor a seiche from ground shaking will be significant.
Survey Notes, v. 35 no. 1, January 2003