Geology Reference
In-Depth Information
instrumentation. Hand-held GPS units can very
successfully define the deformation of most
geomorphic features. Alignment arrays, trilat-
eration surveys, leveling lines, and tide gauges
all offer useful information on short-term
deformation. Sometimes, better vertical resolu-
tion can be obtained through these approaches
than through GPS measurements. Often the
instigation for a more elaborate study will
originate from simple observations of the
deformation associated with faults and folds.
Moreover, the database for these older meas-
urements extends farther back in time and, thus,
provides a longer temporal basis for calculating
deformation rates.
The expanded geodetic knowledge that is
rapidly emerging prompts a new array of
important unanswered questions. We can now
determine plate motions, rotations of crustal
blocks, near- and far-field fault displacements,
and folding rates with a greater precision and
across a broader area than ever before. Armed
with the knowledge of present-day rates of
deformation, we would like to know the
persistence of these rates in the past. Were the
same faults and folds that are accom-
modating differential motion today also
primarily responsible for accumulating strain
in the past? What fraction of observed inter-
seismic strain is retained as permanent crustal
deformation and how much is simply elastic
strain that is recovered during earthquakes?
Does the shape of large-amplitude folds mimic
geodetically measured, coseismic deformation?
Were the displacement patterns and lengths
of ruptures due to major earthquakes repeated
consistently in the past, or were different
patterns associated with each earthquake? Given
a steady plate-tectonic forcing (or is it as steady
as we assume?), what causes the formation and
abandonment of faults? As the poles of plate
rotation changed position in the past, how were
the new stress orientations accommodated at
plate margins? As discussed in the next chapter,
some of the answers to these questions can be
revealed through paleoseismological studies
that seek to reconstruct deformational patterns
associated with past earthquakes.
