Compression Knoll
 

Central Park has many features that define the Hayward Fault. A fault is a break in the Earth’s crust. When a fault moves rapidly it is called an earthquake. However, some faults “creep” or move slowly over time. The Hayward Fault creeps at about 3-10 millimeters per year, depending on location. The Hayward Fault is part of the San Andreas Fault System which is the boundary between the North American and Pacific Plates. It is a nearly-vertical strike-slip fault, but then dips eastward with depth.

Over time the Hayward Fault has not only moved horizontally, but is responsible for the uplift that created the East Bay hills.Central Park has a rolling topogragphy of knolls and depressions. Knolls, such as the one located here, are caused when movement of strike-slip faults push the earth into a small hill.  The pressure causes a movement upward called a positive flower structure.

How are compression knolls formed?

Strike-Slip Fault Creating Small Hill	Flower Structure

 This knoll is created by opposite motion along the Hayward Fault causing a compression ridge or knoll.

On the right was the former City Hall building, built at this location on a compression ridge created by motion from the Hayward Fault. The Loma Prieta Earthquake in 1989 created devastating and irreparable cracks throughout the concrete structure. It was considered a safety hazard and deemed uninhabitable, and was demolished in 2004.