What’s the methodology> Are there instruments to measure that? Can it be inferred from seismic readings? Or is this just somebody’s educated guess?

Is there an agency tasked to perform those calculations and ‘officially’ release the results? Accuracy, precision, uncertainty, error bars?

I’m not interested in challenging the numbers, there’s no doubt this is a huge event. I’m just curious how we know and if it can be applied to other phenomena.