Civil Engineering Reference
In-Depth Information
required to fracture new intact rock. Thus faults at the plate boundaries that have generated
earthquakes in the recent past are likely to produce earthquakes in the future. This princi-
ple is the basis for the development of seismic hazard maps, such as shown in Fig. 2.10.
The theory of plate tectonics also helps explain such geologic features as the islands of
Hawaii. The islands are essentially large volcanoes that have risen from the ocean floor.
The volcanoes are believed to be the result of a thermal plume or “hot spot” within the man-
tle, which forces magma to the surface and creates the islands. The thermal plume is
believed to be relatively stationary with respect to the center of the earth, but the Pacific
plate is moving to the northwest. Thus the islands of the Hawaiian chain to the northwest
are progressively older and contain dormant volcanoes that have weathered away. Yeats et
al. (1997) use an analogy of the former locations of the Pacific plate with respect to the
plume as being much like a piece of paper passed over the flame of a stationary candle,
which shows a linear pattern of scorch marks.
2.1.1
Types of Faults
A fault is defined as a fracture or a zone of fractures in rock along which displacement has
occurred. The fault length can be defined as the total length of the fault or fault zone. The
fault length could also be associated with a specific earthquake, in which case it would be
defined as the actual rupture length along a fault or fault zone. The rupture length could be
determined as the distance of observed surface rupture.
In order to understand the terminology associated with faults, the terms “strike” and
“dip” must be defined. The “strike” of a fault plane is the azimuth of a horizontal line drawn
on the fault plane. The dip is measured in a direction perpendicular to the strike and is the
angle between the inclined fault plane and a horizontal plane. The strike and dip provide a
description of the orientation of the fault plane in space. For example, a fault plane defined
as N70W 50NE would indicate a strike of N70W (North 70° West) and a dip of 50NE (50°
to the Northeast).
Typical terms used to describe different types of faults are as follows:
●
Strike-Slip Fault: During the discussion of the transform boundary in Sec. 2.1, a
strike-slip fault was defined as a fault on which the movement is parallel to the strike of
the fault. A strike-slip fault is illustrated in Fig. 2.11.
FIGURE 2.11
Illustration of a strike-slip fault. (
From Namson and Davis
1988.
)
