Assuming each year has same chance,

Say, there will be earhquake in 50 years, probability of happening in first 5 years is 1/10

We are looking for probability of happening AND within 5 year range, so,

15%.10%= 15/1000

Or 1,5%

Edit: I also assumed there will be max 1 earthquake. Since big earhquakes prevent more big earthquakes