 |
Scenary of the Wind River area illustrating that the
surface is covered with sedimentary rocks and
supporting the observation that there is no
active volcanism within about 200 km. From the BBC
article by Matt Walker cited in the text. |
Today the BBC has a nice
article by Matt Walker pointing out an article by T.J. Craig and R. Heyburn in Earth and Planetary Science Letters (425, pp. 12-23, 2015) on a very deep earthquake in the Wind River range of Wyoming. The authors point out that while earthquakes in the mantle of the oceanic lithosphere are common, well-documented and well-constrained earthquakes in continental mantle are rare, partly because not only must the depth of the earthquake be constrained, but the depth of the Moho also has to be known. In 2013 there was a M4.8 earthquake in the Wind River Range of central Wyoming, a region that is normally relatively quiet seismically. Only once in the past ~60 years has a M5 earthquake been recorded, and most quakes do not even exceed M4. This earthquake, and one single aftershock, were initially recorded to be between 70-80 km depth.
The (very mathematical) analysis in this paper constrains the depth of the earthquake to 75 km (plus or minus 8 km), and makes it the second deepest earthquake now identified under a stable continental region. The depth of the Moho in this area is well constrained to be between 42-50 km, so the earthquake occurred well within the mantle, probably >20 km below the base of the crust. The only two other comparable earthquakes that the authors know of are the 1979 Randolphe, Utah, quake at 90 km, and the 2000 Arafura Sea earthquake at 61 km.
What caused this earthquake? The authors mention the possibility that the quake may result from the migration of fluids within the mantle. Such activity is known to cause microseismic activity at great depths in volcanic regions. However, the Wind River range is more than 200 km from the nearest volcanic region, the hot spot of Yellowstone. They also argue that since the Wind River earthquake ruptured an area of about 1,000,000 square meters, this area is much larger than would be expected from fluid-related origin. They cannot rule out this possibility, but prefer an explanation that the earthquake resulted from brittle fracture due to tectonically-derived stresses.
The results of a video-probe that descended into Old Faithful in 1991 are available on YouTube here (copyrighted video). My colleague in this project was the incredible experimentalist, Jim
Westphal of Caltech, we were aided by long-time collaborator and
Yellowstone Park geologist, Rick Hutchinson, and the efforts of the U.S. Geological Survey and the National Park Service this year made it possible to get the best possible quality video on-line. The challenges in building this probe were numerous because so little was known about the geometry of the conduit at the time. Literature reports included estimates that the conduit was over 100' long, but the most reliable estimates were smaller than that. We knew that the conduit would be dark and filled with hot steam and we knew that the probe would have to pass through a constriction only 4" in dimension, but otherwise we were descending into the unknown. In the early 1990's, the cumbersome video equipment of previous decades had finally become small enough that we could design an ice-cooled, self-illuminating, system to lower through the constriction into the conduit. You can see the camera and the video system as well as the conduit down to about 40' in the video. Enjoy!
