Geology Reference
In-Depth Information
crustal rigidity on patterns of strain around
faults is theoretically clear (Fig. 5.4), our knowl-
edge of these differences is relatively limited.
Because rigidity is only one of several factors
that affect patterns of strain, it is challenging to
tease out this single variable from any given
data set. For example, viscoelastic responses to
previous earthquakes can cause deformation for
hundreds of years, such that patterns of strain
can integrate post-seismic effects from earlier
earthquakes on multiple faults (Dixon
et al
.,
2003). Most studies simply assume uniform
rigidities for country rocks, or, where differences
are defined, they are commonly based on
inference, rather than observation. As more
dense geodetic data sets become available,
improved techniques to extract reliable visco-
elastic properties should emerge.
typically spaced several kilometers apart,
although they could be up to 50 km apart.
A surveyed baseline would be determined
between two benchmarks, such that their
positions and separation distance were well
known. Subsequently, measurements of the
angles to other monuments from both bench-
marks served to define the positions of these
other points. These triangulation angles were
measured with theodolites, which typically per-
mit angular measurements with an accuracy of
≤
1 arcsecond (<1/3600 of a degree). This uncer-
tainty results in errors in the length calculations
of about 3-6 in 10
6
, equivalent to 3-6 mm/km.
When considered in two dimensions, the length
changes between each pair of benchmarks
defines the distortion of the original geometry of
the array and can be interpreted to result from
differential tectonic motion within the array.
More commonly today, distances within a fairly
complex regional array are measured with some
sort of laser-ranging electronic distance meter
(EDM), which has a precision of about 1 ppm
(1 mm/km). The laser beam is refracted as a func-
tion of the temperature of the air through which
it passes. Consequently, measured distances also
depend on air temperature, which, of course,
varies during a survey. The resultant uncertain-
ties dictate that, when several trilateration points
are surveyed from any single site at any given
time, the ratios of one distance to another are
more accurate than the distances themselves.
Most trilateration surveys are used to
investigate horizontal, rather than vertical, fault
movements. Survey results can be used to assess
both regional patterns of differential motion,
as well as the magnitude and nature of slow,
aseismic displacements on individual faults.
As with alignment arrays, abrupt changes in
displacements across an aseismic fault suggest
that the fault is creeping (Fig. 5.3), whereas a
smooth, undisrupted trend in the amount of
displacement as a fault is approached or crossed
suggests that considerable elastic energy is
being stored in the volume of rock adjacent to
the fault. In the context of large regional strains,
identification of zones across which strain is
most rapidly occurring and the differentiation
between creeping and non-creeping faults using
Alignment arrays
Horizontal motions along strike-slip faults can
sometimes be measured with alignment arrays.
These arrays can consist of simple lines of nails
that are hammered into the ground along a
linear trend, typically less than 100 m long, that
is oriented perpendicular to the trace of the
fault. Because the initial orientation of the nails
is well known, any subsequent displacement
along the linear trend can be readily detected.
Alignment arrays can reveal whether aseismic
segments of a fault are creeping or whether
they are locked. If they are creeping, alignment
arrays can indicate whether a discrete offset
occurs along the trace of the fault, and can
document the breadth of the region within
which significant shear is occurring (Fig. 5.3D).
Sometimes man-made structures provide linear
features that are serendipitously arranged
across a fault. The offsets of fence lines, foun-
dations, curbs, pipelines, railroad tracks,
painted lines on highways, and even tire tracks,
can provide useful indicators of both seismic
and aseismic displacement.
Trilateration arrays
Traditional trilateration techniques utilized an
array of triangulation monuments that were
sited across a fault or region of interest and were
