Fault lines: Types and characteristics

• A fault is a fracture or zone of fractures between two blocks of rock allowing blocks to move relative to each other.
• Movement can be rapid, causing an earthquake, or slow, known as creep.
• Faults vary in length from millimeters to thousands of kilometers.
• Most faults experience repeated displacements over geologic time.
• During an earthquake, rock on one side of the fault slips suddenly relative to the other side.
• Fault surfaces can be horizontal, vertical, or at an angle.
• Earth scientists classify faults by the angle of the fault to the surface (dip) and the direction of slip.
• Faults are classified based on the angle of the fault with respect to the surface and the direction of slip along the fault.
• Faults moving along the dip plane are dip-slip faults, classified as normal or reverse (thrust) depending on motion.
• Horizontal-moving faults are strike-slip faults, classified as right-lateral or left-lateral.
• Faults showing both dip-slip and strike-slip motion are oblique-slip faults.

Normal Faults

• Dip-slip fault where the block above the fault moves downward relative to the block below.
• Occurs in response to extension.
• Often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

Reverse (Thrust) Fault

• Dip-slip fault where the upper block moves up and over the lower block.
• Common in areas of compression, like subduction zones.
• A reverse fault is often described as a thrust fault when the dip angle is shallow.

Strike-Slip Fault

• Fault where two blocks slide past one another horizontally.
• The San Andreas Fault is an example of a right lateral fault.
• Left-Lateral Strike-Slip Fault: Displacement of the far block is to the left when viewed from either side.
• Right-Lateral Strike-Slip Fault: Displacement of the far block is to the right when viewed from either side.