[tmm]

> And landmarks aren't static things, either? I'm curious how landmarks are done to preserve this, long term?

At least in the US, the people in charge of this (the National Geodetic Survey) take movement into account. This[1] is the datasheet for the reference monument nearest my house. It has data for 3D velocity of the reference point.

    VX =  -0.0150 m/yr      northward =   0.0040 m/yr
    VY =  -0.0008 m/yr      eastward  =  -0.0148 m/yr
    VZ =   0.0030 m/yr      upward    =  -0.0002 m/yr

I'm not sure what the difference is between VX/VY/VZ and northward/eastward/upward, but those numbers are bigger than I thought they'd be. 15 mm/yr seems like a lot!

The official coordinates get updated from time to time, and in-between, perhaps you're supposed to adjust your measurements using that movement data (IANAS)? If there is a significant measured change in location due to an earthquake, I'm sure that data would be included in the monument datasheet or a new set of coordinates would be published.

FWIW, I went looking to see when the last earthquake was here in Maryland and it turns out there was one today[2].

[1] https://www.ngs.noaa.gov/cgi-cors/CorsSidebarSelect.prl?site... [2] https://www.abc27.com/news/top-stories/2-3-magnitude-earthqu...