You might be thinking along the wrong lines here.

The goal is to see if shockwaves from the underground nuclear tests prevented fault zones around the world from collecting enough strain energy to have a really high magnitude earthquake. There might not be any observable correlation between the time of individual tests and subsequent earthquakes.

The effect that should be easily observable if it does exist would an abrupt reduction in the number of really high magnitude earthquakes such as ones in the 7.5 and higher range between around 1945 when the nuclear tests got started and perhaps 1995 (or whenever) when testing around the world was finally stopped.

The theory is that shockwaves from the underground tests caused fault zones around the world where strain for a really powerful earthquake was building to become unstable. So, they fractured at some point before they were able to really build up a lot of strain.

Now that there are no more nuclear tests, the only shock waves like that are created by very high magnitude earthquakes such as ones in the 8 and higher range. Those earthquakes are not that common. And even when they do occur they might affect only certain fault zones that are specific distances away from them.