Not my field, but...
You have a homogeneous Poisson process.
(*) The number of earthquakes within any time period of length T decades has Poisson distribution with parameter 3T.
(**) The interval (in decades) between any consecutive earthquakes has exponential distribution with parameter 3.
1) Use (**).
2) According to (*), the number N of quakes within a half-decade has Poisson distribution with parameter 1.5. P(N >= 4) = 1 - P(N = 0) - P(N = 1) - P(N = 2) - P(N = 3).