The most common and most likely answer to this crossword clue is the 7 plus 8 letter word NATURAL DISASTER
I've seen this crossword clue in The Guardian
N
A
T
U
R
A
L
D
I
S
A
S
T
E
R
Buddy explains!
The straight part
We know that "an earthquake" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an earthquake = natural disaster.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.