[Servant of the 9]

only relative readings can be made from a bench mark. One would have to drill down below where no movement is possible and install a concrete pier or what ever but then you would need to take reading at a constant temperature of the pier. Then take an average reading from sea level after facturing in the rise of the oceans from silt from the land the world over.

Since the subsidence postulated is caused by movement in shale formations 20,000 feet down, it is gonna take a lot of concrete.
GPS can provide accuracy to inches when thousands of readings taken over time.

It's gonna have to be a 'top down' solution.

SO9

[Judge Roy]

Thousands of readings over time and only good for inches. Well why not lay pipe a 3/4 inch pipe line to a distant point to the north and one to a ocean point to the south 90 degree up with a clear plastic tube and then install an inverted yard stick on the plastic tubes and then fill with water and by checking the ocean reading you will be able to tell within an 1/8 of an inch of any movement within minutes