Science Spotlight

Station P056

Researcher: Chuck Meertens

Living through the Sylmar Earthquake was a pivotal moment as I've been spending the rest of my life trying to figure out how events like this happen.

P056 is located near the Porterville Municipal Airport.

State: CA
Country: United States
Elevation: 101.9 m
Lat/Long:  36.0274 / -119.0629

Aquifer Depletion in Central California

Land subsidence has been a significant issue in the San Joaquin Valley in California for nearly 100 years; the primary cause has been pumping water from the underlying aquifer for agricultural use. Before GPS, the subsidence was documented by the surveying technique of leveling, which was conducted manually every few years. Because the surveys were so infrequent, there was no way to identify changes during the year.

The Plate Boundary Observatory (PBO) installed GPS stations such as P056 in the San Joaquin Valley to study tectonics. Because PBO operates continuously, we could see strong seasonal variations; these variations are not tectonic and in this case were related to hydrology.

The first step is to see if the seasonal variation is scientifically interesting. If all you're seeing is the swelling and contraction of the nearby dirt, that's not interesting. If you're seeing variation because the GPS equipment is unstable, that's not interesting. If the variation appeared at only one station, we might dismiss it as bad data.

However, many GPS stations in the valley showed similar patterns and the patterns were recognizable as connected to the growing season. The measurements at P056 show subsidence when water is pumped for irrigating crops and then a slight rebound during winter when the aquifer is recharged. If more water is pumped than recharged, the ground moves down along with the seasonal variations.

Data from P056 shows again that the distinction between signal and noise depends on the perspective of the user. Scientists looking at tectonics filter out the seasonal changes to get the long-term movements. Hydrologists filter out the long-term movements to see the seasonal changes.

To learn more about aquifer depletion, see our background page and this USGS Fact Sheet.

Figure 1. Position changes for P056 in a North American fixed reference frame. (For help interpreting the graphs, see the GPS Data page.)

Figure 3. The P056 vertical component of motion with the 2007-2012 linear rate removed. This figure makes it clear that the deformation has accelerated since then.


Figure 2. The GPS station is surrounded by irrigated agricultural fields.

Spotlight Questions

  • Does plate tectonic theory predict large vertical motions? Compared to other sites, does P056 have large vertical displacements?
  • If this site continues to sink at the given rate, how much will it have moved between 2007 and 2017?
  • What would the vertical graph look like if pumping and recharge were in equilibrium? How would the vertical graph change if pumping stopped?

Last modified: 2019-12-26  16:24:53  America/Denver  


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