Rock-Mass Characteristics - Geotechnical Engineering Investigation - page 450

modeling of the rock-mass structure and the definition of the strength parameters, which is a
very difficult undertaking from the practical viewpoint.

6.5 Faults

6.5.1 General

Faults are fractures in the Earth's crust along which displacement has occurred. Their
description and identification are discussed in this chapter. Fault activity, displacement,
and their significance to earthquake engineering is presented in Section 11.3.1. Faults are
shown on geologic and tectonic maps as lineaments.
Natural causes are tectonic activity and the compressive forces in the Earth's crust.
Strains increase with time until the crust ruptures and adjacent blocks slip and translate
vertically, horizontally, and diagonally.
Unnatural causes are the extraction of fluids from beneath the surface as described in
Section 10.2.2.

6.5.2 Terminology

Fault Systems
Master faults refer to major faults, generally active in recent geologic time, that extend for
tens to many hundreds of kilometers in length. They are usually strike-slip in type, and
are the origin of much of the Earth's earthquake activity. Examples are the San Andreas
fault in California, the Anatolian system in Turkey, the Philippine fault, the Alpine and
Wellington faults of New Zealand, and the Atacama fault of northern Chile.
Minor faults are fractures connected or adjacent to the major fault, referred to as sec-
ondary, branch, or subsidiary faults, forming a fault system.
Major faults refer to faults, other than master faults, that are significant in extent.

Fault Nomenclature
Fault nomenclature is illustrated in Figure 6.50.

Net slip is the relative displacement along the fault surface of adjacent blocks.

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Strike slip is the component of net slip parallel to the strike of the fault in hori-
zontally or diagonally displaced blocks.

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Dip slip is the component of the net slip parallel to the dip of the fault in verti-
cally or diagonally displaced blocks.

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Hanging wall is the block above the fault.

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Footwall is the block below the fault.

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Heave is the horizontal displacement.

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Throw is the vertical displacement.

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Fault scarp is the cliff or escarpment exposed by vertical displacement.

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Fault Types
Fault types are defined by their direction of movement as illustrated in Figure 6.51.