Recurrence Intervals and Earthquake Probability

If the occurrence of earthquakes were random (i.e., equally likely at any time), then the odds of an earthquake having a recurrence interval of 100 years striking would be 1% in any given year.

However, many earthquake experts believe that once a fault slips, plate motions must load strain back onto that fault before the next quake can occur.  If earthquakes occurred with perfect regularity, then in this example they would occur exactly 100 years apart (once every 100 years).  Therefore, the odds of an earthquake would be 0% for the first 99 years and 100% for the final year.