SAN FRANCISCO—For years, scientists believed the mighty San Andreas—the 800-mile-long fault running the length of California where the Pacific and North American plates meet—could only rupture in isolated sections.

But a 2014 study by federal, state and academic researchers showed that much of the fault could unzip all at once, unleashing a rare, singular catastrophe. Now, a firm has used that research to come up with a new analysis of the damage that could be caused by statewide break of the San Andreas.

The...