DIY Hypothesis Testing
[8:40 MST 8/15/15–updated to add some clarification and replace the figure with a more interpretable one].
OK, not quite do it yourself, but it is looking like in the coming year armchair geophysics nerds can take their own read on an interpretation of vertical uplift rates in the Sierra Nevada.
The background is this: We are now accumulating enough of a recording history of permanent GPS monuments that vertical uplift rates might be emerging from the noise. (Vertical rates are far harder to measure than horizontal ones because (1) there are no GPS signals coming from below a station and (2) atmospheric/ionospheric effects tend to be more significant in the vertical and (3) vertical rates of mm/yr are much lower than cm/yr rates typical of horizontal motions). In the Sierra Nevada, a Geology paper by Hammond et al. in 2012 argued that the vertical signals in the Sierra were now out of the noise and showed that the range is currently rising in the 1-2 mm/yr range. A 2014 paper in Nature by Amos et al. reinterpreted much of the same data quite differently, arguing that what was being seen was an elastic expansion of the lithosphere as water was being pumped out of aquifers in the Central Valley just to the west.
[Addendum as some of this was unclear. The elastic response is like that of a spring. Imagine you have a mattress with a weight on it–say a heavy suitcase. When you pick up the bag, the top of the mattress rises up as the springs in it are no longer compressed. Also the areas around the bag rise up some. This elastic response is different from the slower response of inflowing asthenosphere that produces post-glacial rebound such as is seen around Hudson Bay.]
It is possible (indeed, likely) that both processes are active, but untangling the two isn’t trivial. First, the Amos et al. paper only uses a solution for elastic response of an infinite half space to a change in a load, a solution that has a free parameter (where you pick 0 uplift to be), so removing that solution from the observations to see if anything else is present is troublesome (basically, you can get what you want by picking a different 0 level). Second, the loading/unloading by pumping isn’t terribly well characterized at sub basin levels you’d really like–global datasets like those produced by the GRACE satellite constrain the overall change in the water budget but have very poor lateral resolution. And third, the vertical signals remain very noisy and subject to seasonal issues. So looking at the variations in motion over the last 10 years or so hasn’t really made it terribly clear which process is most dominant.
However things can get a lot clearer if current long-term weather (short term climate) forecasts bear out. Measurements of temperature anomalies in the Pacific have been pointing to an El Niño event for some time, and in the past couple of months the strength of this event appears to be considerable, perhaps it will be the strongest in the past 60 years. Usually such strong El Niño events result in heavy winter precipitation in California, which presumably would result in recharge of aquifers instead of depletion. Thus, if Amos et al. are correct, the Sierra should end up going down after a year with such heavy precipitation [it is like putting that suitcase back on the mattress]; if things are purely tectonic, the trends should continue upward.
This is a “DIY” event because Bill Hammond and his gang at UNR kindly have made a nice interactive system you can bring up to see the GPS data. The station GG is watching most is the Plate Boundary Observatory’s P567 (plotted below); this is fairly close to the San Joaquin Valley but is also in an area suspected of experiencing a lot of recent tectonic uplift (per Saleeby et al., 2013). Now a word of caution: snow loading in the Sierra also causes an elastic response, so the Sierra going down a lot as the winter progresses is to be expected, as is a rebound as the snow melts and water moves to the west. You can also see this station had a couple of odd quirks in early 2010 and 2012. It will be a year-to-year comparison that will be most clear. Happy armchair interpreting!