Here's a surprising fact. If you allow after-the-fact reprocessing of the GPS and significantly increased modeling technology, you can get millimeter accuracy from civilian GPS.

For more on this global network of re-analyzed GPS, see:

http://sideshow.jpl.nasa.gov/mbh/series.html

Here's a particular time series, near Claremont, CA, a few tens of miles east of LA proper:

http://sideshow.jpl.nasa.gov/mbh/all/plots/CLAR.jpg

These plots are lat/lon/height (in centimeter units) versus time. In the height plot, you can see a several-cm drift which has annual/seasonal features. This is due to subsidence due to pumping of ground water in the summer. You can also see some green lines which I believe are large earthquakes.

Besides airborne and (coming soon) satellite radar interferometry, reprocessed GPS is one of the main ways we have to observe seismic displacement fields.