[RegulatorCountry]

"Our impressions support the idea that it was destroyed not by an earthquake but by a tsunami."

The two tend to go hand in hand, so I don't know how they're making the distinction, unless the skeleton is only buried in sand, and even then, earthquake cannot be completely ruled out.

[Fatuncle]

"Our impressions support the idea that it was destroyed not by an earthquake but by a tsunami."

The distinction would be that a quake would be a local effect, distorting the earth, shaking and toppling the lighthouse, whereas a tsunami would knock it over with a wall of water which originated elsewhere, possibly from far enough away that the lighthouse never felt the quake.