Albert Barnes Commentary 1 Samuel 14:15

"And there was a trembling in the camp, in the field, and among all the people; the garrison, and the spoilers, they also trembled; and the earth quaked: so there was an exceeding great trembling." — 1 Samuel 14:15 (ASV)

The earth quaked – This naturally increased the panic to the utmost. Compare 1 Samuel 7:10; Joshua 10:11; Psalms 114:4.