Regions under tectonic stress release the accumulating strain with earthquakes of different magnitudes. Large earthquakes occur less frequently than smaller ones. Nevertheless, according to their sizes, all contribute to releasing the accumulating strain within the region. It is impossible to deterministically predict the magnitudes and the frequency of occurrences of earthquakes within a region or on any fault, even when we know the rate of regional and local strain accumulation. Magnitude-rate distribution is a practical way of formulating the expected frequency of occurrences of different magnitude earthquakes needed to release the accumulating strain within a long time window, i.e. the long-term strain rate, assuming earthquakes occur randomly and independently, i.e., following a Poisson process.