I am really interested to learn how they calculate the probability of 1/3. I think there is a legitimate question here about the methods used. In technical term

I am really interested to learn how they calculate the probability of 1/3. I think there is a legitimate question here about the methods used. In technical terms: what is the sample space and what set are you looking for within that sample space? How strong is the justification to treat the sequence of earthquakes as a sequence of independent identically distributed random numbers? Leaves me scratching my head. I looked at Professor Goldfinger's papers and couldn't find a reasonable answer. Perhaps it is more fundamental knowledge (e.g. something one would find in geology textbooks)?