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BOX 2.2 Continued
with
σ
v
equal to the maximum stress, and (c) strike-slip fault regime corresponding to the vertical stress being
equal to the intermediate principal stress.
Determination of three unknown quantities (
σ
h
,
σ
H
, and their orientation) remains a formidable problem.
Most of the time, the only information available is the stress regime and the broad orientation of
σ
H
, which
can be inferred using a variety of stress indicators such as earthquake focal mechanisms, wellbore breakouts,
drilling-induced fractures, and other data (Zoback and Zoback, 1980, 1989).
Furthermore, the stress varies from point to point within the Earth, subject to the constraint of having to
satisfy the equilibrium equations, a consequence of Newton's second law. Spatial variation of the state of stress
exists at various scales, as the stress is affected by the structure of the subsurface, the geometry and mechanical
properties of different lithologies, preexisting faults and other discontinuities in the crust, and other character-
istics. Yet, when viewed at the scale of hundreds of kilometers, patterns emerge that can be seen on the stress
map for North America (Figure 4). This stress map, a compilation of all available stress information, shows the
orientation of
σ
H
and the stress regime superimposed on a topographical map of North America (Heidbach et
al., 2008).
The example above refers to the initial state of stress (i.e., to the stress prior to injection or extraction of
fluid). Large variation of the pore pressure and/or temperature could also induce significant stress changes that
have to be accounted for when assessing the potential for induced seismicity.
in crustal energy (a factor often cited in news reports following large earthquakes), and the
estimates cited in the examples from empirical observations are in general agreement with
that definition.
Most existing fractures in the Earth's crust are small and capable of generating only
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