Can a 99.9% probability of an M≥ 5 earthquake within a 3-year period be verified?
Based on Probabilistic Seismic Hazard Analysis the probability of an earthquake of magnitude $m$ occurring at the source is obtained with the Truncated Exponential Distribution can be given by 

$$P(M &gt; m) = \int_{m}^{m_{max}} \frac{\beta e^{-\beta (m - m_{min})}}{\left( 1 - e^{-\beta (m_{max} - m_{min})} \right)}dm$$

where :

$m$ is the magnitude; $m_{min}$ is the minimum magnitude of earthquakes in source; $m_{max}$ is the maximum magnitude of earthquakes in source; $P(M &gt; m)$ is the probability of occurrence of an earthquake with magnitude $M$ larger than $m$; $\beta = b \ln(10)$; $b$ is the slope in the Gutenberg–Richter (G–R) magnitude–frequency relation.

Source : Development of probabilistic seismic hazard analysis for international sites, challenges and guidelines

Additional Read : Gutenberg Richter Law - why exponential distribution was used