Upon what two factors is the probability of earthquake occurance based

The answers are:
1. The history of earthquakes in an area
2. The rate at which strain builds up in the rocks

The recurrence rate of earthquakes in a fault can indicate that the fault in question ruptures regularly and repeatedly which could generate similar earthquakes.

The accumulation of strain and the rate at which it builds up is also a good indicator. Seismologists look into how much strain is released as of the last quake in a particular section of the fault. They also check how long has it been since an earthquake occurred along that section of the fault. At it measures how much strain has accumulated in that same section of the fault.