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Fault Types

Schematic illustrations of three fault types. Examples of normal faults include Unnamed small-scale fault cutting across Calf Island, and may be included in some of the short faults with unknown offset mapped in GRI GIS data. Reverse or thrust faults include unnamed, mapped faults south of Slate Island and northeast of Long Island. Strike slip faults include an unnamed fault cutting across the southern end of Calf Island.

Schematic illustrations of fault types. Movement occurs along a fault plane. Footwalls are below the fault plane and hanging walls are above. In a normal fault, crustal extension (pulling apart) moves the hanging wall down relative to the footwall. In a reverse fault, crustal compression moves the hanging wall up relative to the footwall. A thrust fault is similar to a reverse fault, but has a dip angle of less than 45°. In a strike-slip fault, the relative direction of movement of the opposing plate is lateral. When movement across the fault is to the right, it is a right-lateral fault, as illustrated above. When movement is to the left, it is a left-lateral fault. A strike-slip fault between two plate boundaries is called a transform fault. Features (e.g., streams and fences) on the surface are intended to demonstrate the response to the type of fault movement illustrated and is not meant to necessarily reflect features at Boston Harbor Islands.

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Graphic by Trista Thornberry-Ehrlich (Colorado State University).

Public domain:Full Granting Rights

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Boston Harbor Islands National Recreation Area, Code: BOHA

Boston Harbor Islands National Recreation Area, Massachusetts
Latitude: 42.2621994018555, Longitude: -70.8761825561523

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