The Collaborative International Dictionary of English v.0.48:
Earthquake \Earth"quake`\, n.
A shaking, trembling, or concussion of the earth, due to
subterranean causes, often accompanied by a rumbling noise.
The wave of shock sometimes traverses half a hemisphere,
destroying cities and many thousand lives; -- called also
earthdin, earthquave, and earthshock.
Earthquake alarm, a bell signal constructed to operate on
the theory that a few seconds before the occurrence of an
earthquake the magnet temporarily loses its power.