Geology Reference
In-Depth Information
afterslip, with no apparent deep slip. Instead, the
geodetic pattern could be best explained by shal-
low afterslip up-dip from the coseismic rupture
(Fig. 5.16). More study is needed to understand
what controls the location and magnitude of
afterslip. Such studies will certainly emerge from
future post-seismic geodetic studies and should
help place useful uncertainties on paleoseismic
slip reconstructions.
wavelength) or more can be readily defined.
Under favorable conditions, even displacements
of less than 10 mm with uncertainties of less than
5 mm can be delineated (Massonnet
et al
., 1994).
Radar interferograms for use in tectonic studies
are probably best suited for arid settings where
seasonal changes in vegetation and moisture
are minimal. Because of the steep look angle of
the radar pulse, InSAR's greatest sensitivity is to
vertical change, rather than horizontal change -
opposite sensitivities to those for GPS.
The unusual research attribute of radar
interferometry lies not in its resolution (which
is considerably less than that achieved through
extensive GPS measurements), but rather in the
tremendous spatial coverage it provides. One
60 ×60 km SAR image comprises more than
300 000 pixels measuring 100 ×100 m! The
distance changes recorded by these many
thousands of pixels can be displayed as a
contour map of deflections, in which each
successive
fringe
represents an additional
28 mm of displacement. In addition to seismic
displacements, volcanic inflation or subsidence
(Massonnet
et al
., 1994), large landslides
(Fruneau
et al
., 1996), and the details of glacial
flow (Dowdeswell
et al
., 1999; Mohr
et al
.,
1998) can be observed and depicted through
radar interferometry. Unlike most other geodetic
techniques, radar interferometry can be done
remotely, and it provides regionally extensive,
high-resolution maps of interference patterns
resulting from surface displacements. Although
radar interferometry will have restricted appli-
cability in highly vegetated regions, it can be
extremely useful for quantitative analysis of
deformation in remote, relatively arid areas.
An interferogram of the ground displacement
associated with the Landers earthquake
(Fig. 5.17) shows concentric, but asymmetric,
fringes that extend at least 75 km in an east-
west direction (Massonnet
et al
., 1993). Note
that the pattern appears to consist of several
groups of distorted, concentric fringes. One
explanation for the abrupt changes in displace-
ment along the trace of the Landers rupture is
that faulting occurred along a segmented fault.
In fact, the rupture appears to have linked
together individual fault segments that were not
Radar interferometry
A new geodetic technique for measuring
ground displacements over larger areas has been
developed based on radar interferometry
(Bürgmann
et al
., 2000). Using
synthetic aperture
radar
(SAR) carried aboard satellites at 785 km
altitude, radar pulses are transmitted along west-
pointing ray paths at an angle of 23
°
from the
vertical. Based on the return signal, the distance
from the satellite to the ground is calculated, and
a phase shift due to the reflection from the ground
is recorded. These measurements are made for
each pixel (picture element; with dimensions
commonly of 4 × 20 m). If the same area of the
Earth's surface is contained within two different
SAR images, if the positions of the satellites are
well known, and if the ground and atmospheric
moisture distributions are approximately the
same for both images, then the differences in path
lengths between the two images are attributable
to some combination of: (i
)
the ground topography
as seen stereoscopically from the satellite in
slightly different orbital positions; and (ii
)
changes
in the position of the ground in the time between
acquisition of the two images (Massonnet
et al
.,
1993). If two images are used to construct the
topography of a region, then a third image can
be used to determine ground deformation with
respect to that topography along the
look
direction
of the radar. Alternatively, if a detailed digital
topography already exists for the area, then the
differences between two images can define the
ground deformation. The resultant depiction
of ground displacements is termed an
SAR
interferogram
, and the overall approach is
referred to as
InSAR
. The wavelength of the radar
emitted by the satellite is 56 mm, and, using
interferometry, displacements of 28 mm (half a
