Fault Creep

March 14, 2010

: Thomas McGuire
Summary Author: Thomas McGuire

The San Andreas Fault is generally considered a boundary between the Pacific Plate and the North American Plate. Maps often show plate boundaries as distinct, narrow lines; however, nature is seldom so simple. Geologists have discovered dozens of less well-known faults that parallel the San Andreas Fault and absorb some of the offset of the plates. One of the faults that parallel the San Andreas in the San Francisco Bay area of California is the Hayward Fault. It can be traced by a variety of fault related features above the eastern shore of San Francisco Bay. Earthquakes are often caused by sudden movement along geologic faults. But movement along fault surfaces can also occur slowly, which is called fault creep.

Memorial Stadium on the University of California Berkeley campus was completed in 1923. This structure is bisected by the Hayward Fault, which is slowly wrenching the stadium apart at a rate of 1/20 in (1.2 mm) per year. Over the years several large cracks have opened in the walls of the stadium including the opening shown here that’s bridged by a metal plate. Offset is also apparent in the floor below the seats as shown by the white arrow. A multi-million dollar renovation project has begun that will repair and reinforce this structure, help to prevent further seismic damage and help protect people in and near the stadium in the very real possibility of a major earthquake. Photo taken on February 11, 2010.