Geology Reference
In-Depth Information
rupture events. Contrasting models for controls
on stress release propose that: (i) faulting occurs
whenever a certain stress threshold is attained,
such that strong patches, or asperities, on the
fault control the rupture pattern; or (ii) earth-
quakes release stress until a minimum threshold
stress is achieved, in which case barriers, or
patches of the fault plane that remain unrup-
tured, may control the pattern of faulting. It is
still not clear whether either model adequately
describes the behavior of many faults.
It has been proposed that some faults are
typified by characteristic earthquakes in that
they display a similar rupture length, magnitude
of displacement, and distribution of offset along
the rupture in successive earthquakes. Rupture
patterns along such faults could be controlled
by long-lived asperities. If characteristic earth-
quakes exist, knowledge of their displacement
history provides strong predictive capabilities.
Given the societal importance of earthquake
prediction, we need to improve our understand-
ing of recurrence intervals, fault strength,
characteristic behavior of faults, and controls on
fault displacement and timing. Through field
observations, patterns of displacements along
faults and styles of fault growth are becoming
better known.
In a given structural and geological setting,
a predictable relationship commonly exists
between the maximum amount of displacement
on a fault and its length. Whereas a roughly
triangular pattern of length versus displace-
ment is predicted for faults that grow laterally
without restriction, many faults display a more
bow-shaped displacement gradient. Such a
pattern can result from impediments to tip
propagation that restrict fault lengthening,
despite continued slip on the fault. In such
cases, faults extend to their full length early in
their history, rather than continually lengthen-
ing. Segmentation of faulted range fronts and
the temporal persistence of segment bounda-
ries provide support for the concept of
long-lived barriers to fault propagation. On the
other hand, arrays of faults commonly interact
such that slip deficits between major faults
are filled by displacement on minor faults.
Furthermore, compensation among multiple
faults produces smooth variations in cumula-
tive displacement across the array.
Our ability to refine and choose among the
contrasting models for controls on earthquake
cycles, accumulation of displacement, charac-
teristic earthquakes, and fault-zone segmenta-
tion depends on developing more complete
data for both past offsets and present rupture
patterns. Part of these data will come from stud-
ies of the tectonic geomorphology of fault
zones. Predictable patterns of fault rupture and
geomorphic expressions of faulting are associ-
ated with strike-slip, normal, and thrust faults
and can be interpreted in terms of imposed
stresses. Nature, however, is complicated, and,
in nearly every “compressional” or “extensional”
setting, an overlapping array of growing folds
and active normal, thrust, and strike-slip faults
will be found. It is the job of the field geologist
to sort out the relationships of these structures
to local stress fields and to define how these
stresses interact with inhomogeneities in the
underlying rocks to produce the observed
deformation.
Geomorphological studies can successfully
document deformation following faulting when
surface features have been offset. But what
about subtle folding of the surface during fault-
ing or far-field deformation? Such deformation
is often undetectable in geomorphological stud-
ies. Similarly, the build-up of strain prior to
faulting and the proportion of recovery of inter-
seismic strain during faulting usually cannot be
resolved through examination solely of coseis-
mic displacements. Instead, precise surveying
techniques are required to define both near-
field and far-field deformation prior to, during,
and after earthquakes. Such geodetic measure-
ments are the subject of the following chapter.
