Final answer:
The pronoun to replace 'An earthquake' is 'It', maintaining the original sentence meaning without repetition.
Step-by-step explanation:
To replace the underlined noun 'An earthquake' with a pronoun, you can use 'It'. So, the sentence would now read 'It can happen when two plates slide past each other.' This pronoun has effectively taken the place of the noun, ensuring the sentence maintains its original meaning while avoiding repetition.