Definitely published before 1980. City on west coast experiences massive earthquake. A survivor, who I think lives in the mountains above the city, (LA, San Fran?) comes out of his house when the quake has ended and expects to see that the Pacific Ocean has dropped/disappeared, but all is well. He walks back to his house and instead looks east, and everything is gone.
I believe you are thinking of the short story "A Slight Miscalculation" by Ben Bova.
A description of the story is at:
"This is a story of a mathematician who found a way to predict earthquakes. He finds out that there will be a major earthquake in California (where he lives). After checking this prediction using CalTech's computer he notices that his own results is slightly different from the computer's. While he tries to figure out the reason of this discrepancy, other people are leaving the California in a great hurry. Being obsessed with the problem, he does not notice the panic, ignoring the fact that he might be in danger himself. Finally the earthquake begins exactly at the predicted time and date. After it is over, mathematician notices that the computer's version was correct : the rest of USA lays beneath the sea... "
You can find it in an anthology at:
The ending lines of the story read:
Beyond the parking lot, where the eucalyptus trees used to be, was the edge of a cliff, where still-steaming rocks and raw earth tumbled down to a foaming sea. Nathan staggered to the cliff's edge and looked out across the water, eastward. Somehow he knew that the nearest land was Europe. "Son of a bitch," he said with unaccustomed vehemence. "The computer was right after all."