I'm revising for a cryptography exam at the moment and I'm having some problems with a question. 

The question looks for the key length of a cipher given that the word "earthquake" appears in the plaintext.

Ciphertext:
aaatr ksams itahb netrs weurs rheti aadne ttrht eakgt snqid s

I was wondering how I use the fact that the word "earthquake" appears in the plaintext to make it easier to obtain the key length? 

At the moment, I'm just using the method of looking for repeated letters in the ciphertext to obtain a possible key length.