Hayward Fault Zone - Tectonics of The Hayward Fault Zone

Tectonics of The Hayward Fault Zone

Main articles: Seafloor spreading, Subduction, Plate tectonics, Geologic fault, Earthquake

The Pacific Plate is a major section of the Earth's crust, gradually expanding by the eruption of magma along the East Pacific Rise to the southeast. It is also being subducted far to the northwest into the Aleutian Trench and also in a northerly direction under the North American Plate at the Cascadia Fault, well north of San Francisco at the terminus of the San Andreas. In California, the plate is sliding northwestward along a transform boundary, the San Andreas Fault, toward the subduction zone. At the same time, the North American Plate is moving southwestward, but relatively southeast along the fault. The westward component of the North American Plate's motion results in some compressive force along the San Andreas and its associated faults, thus helping lift the Pacific Coast Ranges and other parallel inland ranges to the west of the Central Valley, in this region most notably the Diablo Range. The Hayward Fault shares the same relative motions of the San Andreas. As with portions of other faults, a large extent of the Hayward Fault trace is formed from a narrow complex zone of deformation which can span hundreds of feet in width.

As the transform boundary defined by the San Andreas Fault is not perfectly straight, and the motion of the North American plate is not entirely parallel to the plate boundary. Movement along the boundary creates stresses in the crust on either side of the boundary, resulting in additional faulting on both sides of the San Andreas Fault. The Hayward Fault is one of the larger faults created this way, along with the Calaveras Fault to the east and the San Gregorio Fault, west of the San Andreas.