In geology, faults are discontinuities (cracks) in the Earth's crust that are the result of differential motion within the crust. Faults are the source of many earthquakes that are caused by slippage vertically or laterally along the fault. The largest examples are at tectonic plate boundaries, but many small faults are known to exist that are far from active plate boundaries.
The two sides of a fault are called the hangingwall and footwall. By definition, the fault always dips away from the footwall. Faults can be categorized into three groups: normal faults, transform (or strike-slip) faults and reverse (or thrust) faults.
This occurs when the crust is in tension. The hangingwall moves downwards (i.e. towards the centre of the Earth) relative to the footwall. The depressed ground between two parallel normal faults is called a graben. A ridge between two parallel normal faults is called a horst.
Reverse (or thrust) fault
This occurs when the crust is in compression. The hangingwall moves upwards (i.e. away from the centre of the Earth) relative to the footwall.
Schematic illustration of normal and reverse faults. Note that the view is a cross-section through the Earth, such that the up-direction on the page is away from the centre of the Earth.
Strike-slip faults
The fault surface is vertical and the footwall moves either left or right (with respect to the plane perpendicular to the fault and to the Earth's surface). Strike-slip faults with left-lateral motion are also known as sinistral faults Those with right-lateral motion are also known as dextral faults.
Schematic illustration of the two strike-slip fault types. The view is of the Earth's surface as from space.
The Hayward fault is considered to be particularly dangerous due to the poor soil conditions in the alluvial plain that drops from the East Bay Hills to the eastern shoreline of San Francisco Bay.
In its northern extent the Hayward Fault lies directly beneath the portion of Highway 13 (the Warren Freeway) that is south of its intersection Highway 24 and north of its teminal connection with Highway 580 (the Foothill Freeway).
Further north the fault is under the centerline of the football field of Memorial Stadium at the University of California, Berkeley.
The New Madrid Seismic Zone, also known as the Reelfoot Rift or the New Madrid Fault Line, is a major seismic zone located in the Southern United States and Midwestern United States.
The New Madrid Seismic Zone is made up of reactivated faults that formed when North America began to split or rift apart during the breakup of the supercontinent Rodinia in the Neoproterozoic Era (about 750 million years ago).
The zone saw four of the largest North American earthquakes in recorded history, with magnitude estimates greater than 7.0 on the Richter scale, all occurring in a 3 month period.
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