Thursday, residents of South Carolina felt a strong rumble and thought it was a powerful earthquake. But seismologists said it came from the air, not the ground. COLUMBIA, S.C. — The revelation that a mysterious rumble heard across South Carolina Thursday night was a sonic boom, rather than an earthquake, has left more questions than answers. Thursday evening, residents of the South Carolina Midlands felt a strong rumble and thought it was a powerful earthquake — which is already a rare occurrence in the state. South Carolina regularly gets small earthquakes, with about 10 to 15 recorded each year by…