For decades, a sidewalk curb at Rose and Prospect streets in Hayward, Calif., tracked the slow creep of Earth along a notorious seismic fault line.

Then, last month, the sidewalk record was erased by a city crew, which replaced the curb with a wheelchair-accessible ramp.

The curb’s disappearance resulted in the loss of one of the most visible historical markers of the Hayward Fault, which runs 74 miles along San Francisco Bay, parallel to the San Andreas Fault. Over time, as the Pacific and North American tectonic plates slipped past each other along the fault, the curb slowly pulled apart.

The resulting offset was a perfect demonstration of the Hayward Fault’s steady movement of a few millimeters a year, said Luther Strayer, a structural geologist at California State University, East Bay. He brought his students on field trips to the curb every year.