[Island Chris]

Filling a reservoir can trigger earthquakes. I am not bothering to Google it, but there was a rather large one some decades ago in California, that was linked to (I think) filling Oroville reservoir. It is more the pore fluid pressure in faults increasing than the mass of the water that is known to weaken faults. The question is whether filling a reservoir at the surface can increase pore pressure in a fault at the depth the earthquake initiates. Similarly, one would expect there to be a limit to the horizontal distance where reservoir level would affect seismicity.
Chris