I just returned from a week in Mexico. At the beginning and end of our trip we stayed at the Zocalo, the main plaza in Mexico City. The rest of the time we were in San Miguel de Allende.
Serious subsidence problems in Mexico City. There have been many studies. One showed that from 1951 to 1957 some areas subsided 1.5 m. Since 1891 there has been an overall accumulation of 7.5 m of subsidence. A lot of it is differential settling. Here is a picture of an old building next to a newer one:
The most interesting was the main cathedral, on the Zocalo. It was built in sections from 1573 to 1813. Every pillar inside has several monitoring pins in it:
I am always against overmarking of points, in this case I think it is ridiculous that they have marked them up so much in a historic church.
What is really cool is that they have a massive plumb bob hanging from the middle of the cupola in the roof:
The white panel on the floor to the left in the picture is a plot of the plumb point over the years. They would not allow flash, so my picture didn't come out very well. But, I can say that it has moved a few FEET over the years.
I could definitely feel the slope of the floor. They also had some kind of electronic plumb line nearby, but it was inside a locked case, seen in the third picture above.
Here is one article I found about it:
Interesting, but unfortunate. Thanks for sharing.
Makes the Harris-Galveston Subsidence District look like a minor problem in comparison.
Are you sure the plumb bob isn't a Foucault pendulum? See http://en.m.wikipedia.org/wiki/Foucault_pendulum it seems more likely as it is tied off.
There are similar problems in the Las Vegas Valley in Nevada. I saw some interesting InSAR imagery while visiting the LVVWD. The late Paul Wolf's (former professor at U of Wisconsin and original author of surveying and adjustment texts) son was a surveyor working there when I visited.
Subsidence due to ground water withdrawal and the complication of deep-seated tectonics was (is?) a subject of research in the Harris-Galveston Subsidence District. The late Professor Roy Dokka wrote some very interesting papers on the area. Too bad he died so young.
DMM
Yes, that is the same device in the picture. I could not get close to the white panel to see exactly what it said, but I could see that there were dates on it, several hundred years worth.
I'm sure that it's not. Not with the photo showing deviation and dates.