Fire and Firestorms
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FAULT William A. Bryant California Geological Survey, Sacramento, CA, USA
Synonyms Seismic source; Shear Definitions Fault – a tectonic fracture in the earth’s crust along which displacement (horizontal, vertical, or diagonal) of one side relative to the other has taken place. The fracture may be either a discrete plane or a zone containing multiple fracture planes. Cumulative displacement may be measurable from millimeters to kilometers. Slip – distance, measured on the fault plane, between two originally adjacent points situated on opposite sides of the fault. It would be represented by a straight line on the fault plane connecting these two points after displacement. Introduction The word fault was derived from a late eighteenth-century mining term used to describe a surface, or plane, across which coal layers were displaced (Twiss and Moores, 1992). Realization that sudden displacement along a fault produces earthquakes evolved from observations of four large events that ruptured the ground surface in the late nineteenth and early twentieth centuries: the 1888 Marlborough (New Zealand), 1891 Nobi (Japan), 1892 Tapanuli (Indonesia); and 1906 San Francisco (California) earthquakes (Yeats et al., 1997). Earthquakes are the dominant natural hazard associated with faults. Although strong ground shaking accounts for the vast majority of potential damage from earthquakes, significant damage to structures and infrastructure can result when the rupture of a fault at depth extends to
and offsets the ground surface. Surface displacement can also disrupt drainage patterns, such as by damming streams, with resultant flooding and/or catastrophic breaching. Rapid uplift of the seafloor resulting from surface fault rupture is one of the ways seismic sea waves, or tsunamis, are formed.
Types of faults Faults can be recognized by the juxtaposition of dissimilar rock types across a generally planar surface exhibiting textures and structures characteristically produced by shearing. A fault plane can be vertical, or inclined at an angle (dip) less than 90 . Dip is the acute angle between the fault plane and a horizontal surface. High-angle faults dip greater than 45 ; low-angle faults dip less than 45 . Faults are classified according to the direction the bounding crustal blocks have been displaced parallel to the fault plane (Figure 1a–e). Dip-slip displacement is vertical movement oriented parallel to the dip of the fault plane. Horizontal offset parallel to the fault plane is known as strike-slip displacement. Oblique-slip faults will have components of both dip-slip and strike-slip displacement. Dip-slip faults Relative movement of crustal blocks bounding a fault plane determines whether a dip-slip fault is described as normal or reverse. The hanging wall is the structurally upper block bounding a dipping fault; the structurally lower block is the footwall. If the hanging wall has been displaced down relative to the footwall, the fault is considered a normal fault (Figure 1a). A reverse fault has the opposite sense of dis
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