Personally I really don't think that 500 kg is enough to cause a 2.3 seismic event.
However just the sheer knowledge getting out is definitely going to be catastrophic for Russia if they were to have used a nuclear device and even a small nuclear device.
I'm not saying they used one of the big ones I'm saying they just used a small tactical one and most likely a remote operated vehicle to place it in a position where it would damage both these pipelines at once.
And it's four pipelines so. There's big huge problems going down and of course the initial claims are going to be understated and claim conventional munitions.
However the unfortunate fact of such a large Richter scale event? And Richter scale events are clearly measurable. And exponential.
320 kg of explosive will equal 1.5 Richter scale measurement.
That means if we extrapolate that data 10 * 10 * 10 * 10 * 10 * 10 * 10.... You end up with a massive amount of energy release.
And it really doesn't matter how much was used it was so excessive that it puts out its own message.