The Sierra Nevada Yo-yo
The Grumpy Geophysicist has spent a lot of time (er, almost 30 years) worrying about the origin of the Sierra Nevada. While sometime later we may get into some of the Sierran controversies, today we’ll consider whether the Sierra is going up today.
You might think this is something pretty straightforward to measure; leveling lines, for instance go back nearly a century. But there are systematic errors in such traditional geodesy that make it hard to resolve the kinds of signals (maybe 1-2 mm/yr uplift at the crest would be about the highest we might expect). So space-based approaches are in order. And the use of such tools suggested that the Sierra was going up; the most recent such paper was a 2012 Geology paper by Bill Hammond and coworkers at UNR that concluded that the Sierra was going up between 1 and 2 mm/yr. This seemed to suggest that the range was still rising before our eyes.
But there were some funny things in the patterns seen in that paper:
First, there are few points in the Sierra and almost none are near the crest, which, if modern uplift mimicked the longterm uplift, should be the most rapidly rising part of the range. Second, it seems (and is most obvious in the smoothed plot at right) that the high rates of uplift were in the low western parts of the range. Given the noise in the signals (uncertainties were near 1mm/yr) it was the average that mattered, but one piece of slight of hand in this paper (that GG missed when reviewing it) was that a profile of the GPS uplift seemed to show the westward tilt we might expect–except that this was an artifact caused by projecting points far to the south into the profile in the north.
Well a new paper addresses this seeming discrepancy and, in retrospect, this was an obvious problem that should have been considered before (a nice article in Ars Technica discusses this). Colin Amos of Western Washington was lead on this paper and Bill Hammond is a coauthor. The basic gist of the paper is that removal of water from the San Joaquin Valley will drive uplift both in the valley and the surroundings. This is because rock behaves elastically. Like a spring that was pressed down, when you release some of the pressure, the spring expands and rises. And so you should expect uplift on the sides of the valley, which is the pattern in the Hammond et al. paper from 2012. Even better, there is a cyclicity to the uplift that matches times when water is withdrawn.
Does this mean the Sierra isn’t going up tectonically? Maybe, but there is still more to do. This new paper adds a few new GPS sites in the Sierra that are going down, which would certainly seem to dampen any uplift story. But there remain inconsistent elements to all this. The modeled profile in the Amos paper seems to fit the data, but in map view there are places going up where little water has been withdrawn (some farther north are not shown in the Amos et al. paper); GG suspects that a map view calculation would not necessarily fit the data nearly so nicely. Although very consistent with the story here, the seasonal cycles will probably be scrutinized because such seasonal cycles are present in many GPS records because of seasonal changes in the atmosphere. Being sure that the cycles are ground motion and not atmospheric may take some work (properly removing seasonal signals remains a major challenge for GPS work). The paper also didn’t consider the substantial amounts of oil withdrawn from this region. Other work that remains to be done is to figure out the viscous response of the mantle to this change in load (the paper is limited to an elastic solution). It might be possible to determine the viscosity of the upper mantle or lower crust from these observations.
While the main focus of the news coverage is on the relationship of this process to earthquakes near the San Andreas fault, there is an interesting sidelight. GG was coauthor on a paper with Jeff Unruh of Lettis Consultants International and Egill Hauksson at Caltech on seismicity in the San Joaquin Valley and its tectonic implications. We argued that a change in the orientation of faulting in the valley was caused by a rebound of thick crust now being freed from attachment to a dense body descending in the mantle below. Could this mechanism do the same thing? Well, maybe. The elastic solution presented in the Supplementary Materials indicates that the vertical normal stress would decrease more than the horizontal stress, consistent with the sense of deformation being observed. However, the changes in orientations of the horizontal stresses doesn’t appear on the face of things to match up and the largest effect doesn’t seem to be where the most water was removed. But water withdraawal would certainly complicate the stress field. Ah well, we wouldn’t want everything solved now or there wouldn’t be anything to do, would there?