Your answer is the average number of tremors, not the most likely one.

The number of tremors to occur in a week can be approximately considered to be a Poisson random variable (that's in the title of the thread!) with mean (using your computation). In other words, the probability of tremors in a week is about [tex]e^{-\lambda}\frac{\lambda^k}{k!}[/tex]. Compute that value for and conclude.