[robocat]

> our super accurate GPS measurements are not 100% reliable over time

The GPS measurements are 100% reliable. Think of it like getting GPS co-ordinates while on a boat - it's the boat that moves.

A point of interest on land doesn't stay at the same latitude/longitude/altitude because land is a tectonic plate "boat" floating on the mantle.

Close to the earthquake faultplane, land crumples and shears sideways and up/down. Plus secondary effects of the shaking: landslides, settlement, liquifaction.

Finally: I think it helps to remember that earthquakes are fault _planes_. Talk of epicentres and depths and faultlines often misleads our intuitions. For one of the Christchurch earthquakes my parents were about a kilometre away from the faultline (where the faultplane met the surface) and many kilometres away from the epicentre (which is just a synthetic average point), but the faultplane actually was relatively close to them somewhere underneath their home.

Also images of cracks in roads are often extremely misleading. They tend to be spectacular subsidence and are not the faultline. The actual faultline is usually not so photographic and less likely to have great photos early on. Media choses photos for their emotional appeal - not because they are a good approximation for the truth.

[oasisbob]

> The GPS measurements are 100% reliable. Think of it like getting GPS co-ordinates while on a boat - it's the boat that moves.

I'm not sure this is a good metaphor. When doing high-accuracy GNSS (GPS) survey work, a static reference station is typically used to help correct errors from the GPS measurements themselves.

If GPS was 100% consistent and reliable, differential correction wouldn't be a common and often-required technique:

https://www.e-education.psu.edu/natureofgeoinfo/c5_p23.html

[helen___keller]

While you’re correct that GNSS is not 100% accurate (like everything in the physical world, it comes with error bars), I think the point stands that the error is unrelated to the movement of continents.

Ultimately GNSS is just measuring your position relative to some celestial objects using the time it takes for signals to propagate space (and some other info: their ephemerides, and a shared-ish clock). The fact that land exists, and where it exists, are not relevant

Your error corrections might be thrown off when the base station moves, but that’s not an issue of GNSS that’s an issue of RTK

[dnedic]

GNSS satellites talk to base stations on earth to get correction data using the measurements they obtain (including measurements obtained by tracking satellites from the ground). I believe this is what the poster above is reffering to.

RTK is a whole another beast and the meaning of an RTK base station is something else.

[oasisbob]

RTK is exactly what I was referring to.