Chasm by Stephen Laws
Edmonville: an ordinary town full of ordinary citizens, going about their everyday business. Then the earth tremor hits, followed by a second devastating tremor, followed by a great darkness. The next morning, when the dust cloud swirls away, the few survivors discover that most of Edmonville has disappeared into an enormous crevasse. As they struggle to survive surrounded by a bottomless gulf, waiting for the rescue that never comes, one by one people start to disappear without trace.