Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Mike Stickney, Director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology.
The greater Yellowstone region has experienced dramatic deformation. Within Yellowstone Caldera, average uplift rates measured by leveling, GPS, and InSAR are generally in the range of 10–30 mm/yr. While uplift rates measured in mm/yr may seem small, if such uplift continued through geologic time—say 1,000,000 years—the uplifted land surface would be more than 10 km (6.2 miles) high—considerably higher than Mount Everest! Clearly such uplift cannot persist for long periods of time. Indeed, deformation measurements indicate equally significant subsidence of the caldera during historical times—the ground is moving both up and down. Prehistoric shorelines around Yellowstone Lake attest to the rise and fall of the caldera during the past several thousand years, with the cumulative motion being downward by tens of meters (yards). Researchers attribute this rise and fall of the land surface to accumulation and draining of fluids—magma, gas, and water—beneath the surface.
Studies have also documented rapid deformation outside of Yellowstone Caldera, particularly in the Hebgen Lake region. During 1973–1987, a trilateration survey, which measures horizontal deformation, revealed 8 mm/yr of NNE-SSW-directed stretching, or extension. The measured extension rate was consistent over the 14-year period of measurements and concentrated in a zone that roughly coincides with the epicentral area of the 1959 M7.3 Hebgen Lake earthquake. The authors of the article reporting the trilateration results suggested that the deformation was related to an extension of the magmatic system of Yellowstone Caldera, given that the deformation zone was associated with seismicity that trended at least 100 km from the Sour Creek resurgent dome in the northeastern part of the caldera westward to the southern Madison Valley. But does such a magmatic system really exist? There don’t seem to be other lines of evidence supporting this hypothesis. For example, there are no recent volcanic eruptive centers or thermal springs west of the border of Yellowstone National Park in the area of Hebgen Lake. The zone is seismically active, but the earthquake swarms that typify much of the seismic activity within the Park are not common west of Hebgen Lake.
Instead of a magmatic intrusion, could the deformation in the area of Hebgen Lake be related to the aftermath of the 1959 earthquake? One intriguing line of evidence is the relation between ongoing seismicity and the deformation that accompanied the 1959 earthquake. Following that event—one of the two largest earthquakes ever recorded in the Intermountain West of the United States (the other occurred in Nevada in 1915)—USGS geologists used a leveling survey along Highway 287 (which runs along the north shore of Hebgen Lake) and warped shorelines along Hebgen, Cliff, and Wade lakes to construct a map of the earthquake deformation. The map shows dramatic subsidence (up to 21 feet!) in the northwestern part of Hebgen Lake, with subsidence decreasing southward away from the two primary faults (Hebgen and Red Canyon) that ruptured in 1959. Overlaying a map of accurately located earthquake epicenters on the deformation reveals that the vast majority earthquakes located west of Yellowstone Park since 1990, when the Yellowstone and Montana seismic networks jointly began to provide excellent coverage of this area, fall within the zone of 1959 subsidence. The zone of seismicity extends eastward into Yellowstone National Park, and earthquake-related subsidence was noted near the Norris Geyser Basin, so it is possible that the zone of recent seismicity corresponds with the zone of 1959 subsidence between Hebgen Lake and Norris Geyser Basin—the most seismically active area of Yellowstone National Park.
The most intense seismicity west of Yellowstone extends about 10 km west of the Park boundary to the eastern edge of the main body of Hebgen Lake, which is also approximately the longitude of the 1959 earthquake epicenter and the eastern tip of the Hebgen fault. Minor seismicity characterizes the main body of Hebgen Lake and adjoining southern Madison Range to the west. The area between the Madison fault and Cliff and Wade lakes is another hotspot for recent seismicity. Most of this seismicity in the southern Madison Valley lies within the 1-foot subsidence contour from the 1959 deformation, but a distinct band of seismicity extends northward in the Madison Range, mostly to the east of the west-dipping Madison fault. Finally, west of Cliff and Wade lakes, seismicity continues westward more than 350 km along a trend that extends through southwest Montana into central Idaho.
The correlation of the most recent epicenters in the Hebgen Lake region with the zone of 1959 earthquake subsidence suggests that this seismicity may represent aftershocks of the 1959 Hebgen Lake earthquake. A growing body of evidence from other historical earthquakes indicates that large-magnitude seismic events that occur on low-slip-rate faults with long recurrence intervals may have aftershock sequences that last for many decades or even centuries. The deformation measured during 1973–1987 might also be related to the earthquake. If future surveys show that the extension rate is slowly decreasing over time, that would suggest that post-earthquake deformation can last for decades to centuries, just like aftershock sequences. Continued monitoring of the Hebgen Lake basin/southern Madison Valley with improved seismic and GPS instrumentation will provide important data for determining whether we are still witnessing aftershocks and extension from a major earthquake that occurred there more than six decades ago, and will help to better understand this zone of persistent seismicity in the Yellowstone region.
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