Studying layers of soil in a trench they dug near the Fremont BART station, earthquake experts have made a startling discovery: The Hayward Fault has had a big earthquake every 140 years, on average, since 1315.

And this Sunday marks year 139.

Calling the fault a "tectonic time bomb" just waiting to go off, scientists today urged Bay Area residents to put together an earthquake plan, stockpile supplies and possibly have their older, two-story homes evaluated for structural weaknesses.

The announcement came just days before the 139th anniversary of the 1868 Hayward Earthquake - which was known as the "great San Francisco earthquake" until the devastating 1906 temblor came along. It remains this nation's 12th most deadly earthquake despite the East Bay's sparse population at that time.

With the number of densely built cities now straddling the dangerous fault, analysts at the Association of Bay Area Governments anticipate there would be 1,100 road closures and 94,000 destroyed homes and apartment units from a magnitude 6.7 quake. But geologists believe that the fault is capable of producing quakes as large as a magnitude 7.3, meaning that the number of deaths, injuries and damaged homes could be bigger than the area has ever planned for.

"It wouldn't be a surprise to any seismologist if it had a big earthquake tomorrow," said Tom Brocher, coordinator of Northern California Earthquake Hazards investigations for the U.S. Geological Survey in Menlo Park. "This is a real threat."

Indeed, scientists have estimated that there is a 62 percent chance of a major earthquake striking the Bay Area by the year 2031. The Hayward Fault, they said, is the most likely culprit.

"It's not a question of if, if it's a question of when," said Jim DeMersman, executive director of the Hayward Area Historical Society and Museum. The Hayward fault has been slowly creeping along just 100 feet away from the museum - the site of today's news conference on the almost overdue fault line - cracking streets and sidewalks and leaving the city's historic city hall uninhabitable.