Return Period

1. Return Period (Tr)

: A recurrence interval that is an estimate of the likelihood of events like an earthquake or wind storm of a certain intensity or size/ unit: year

2. Exceedance Probability (Pe)
: A probability that a certain incident occurs during specific period. Exceedance probability for 1 year is Pe1 = 1/Tr1

3. Non-Exceedance Probability (Pn)
Pn1 = 1 – Pe1(for 1 year), PnN=(Pn1)^N

4. Exmaples
1) The percentage of exceedance probability for 20years if exceedance probability for 1year is 1%
Sol) Pe1 = 0.01, Pn1 = 1 – Pe1 = 0.99
Pn20 = (Pn1)^20 = 0.99^20 = 0.8179
Pe20 = 1 – Pn20 = 1-0.8179 = 0.1821
Therefore, percentage of exceedance probability for 20years is 18.21%

2) The percentage of exceedance probability and the return period for 1year if exceedance probability for 50year is 10%
Sol) Pe50 = 0.1, Pn50 = 1 – Pe50 = 0.9
Pn1 = (Pn50)^(1/50) = 0.9^(1/50) = 0.9979
Pe1 = 1 – Pn1 = 1-0.9979 = 0.0021
Therefore, percentage of exceedance probability for 1years is 0.21% and 1 year return period is about 476years (Tr = 1/Pe1 = 1/0.0021)