Geology Reference
In-Depth Information
well. Although the axis of maximum interseismic
uplift coincides with the axis of maximum
coseismic subsidence, coastal zones of strong
coseismic uplift are still experiencing inter-
seismic uplift (Fig. 5.12). Hence, there is no
“mirror image” of subsidence and uplift along
the outermost coastal areas. Finally, along
accretionary prisms, both strata and marine
terraces are commonly observed to dip toward
the arc. This dip indicates that at least some
component of non-recoverable deformation
occurs within the overriding plate.
Despite the uncertainties in the interpretation
of these tide-gauge data, the Japanese coastal
data clearly demonstrate their usefulness in
delineating vertical deformation patterns. On
highly digitate coasts with numerous embay-
ments and islands, a regional three-dimensional
pattern of emergence and submergence can
potentially be obtained. Because tide-gauge
records are available from many harbors with a
long history of use, these records provide a rich
database for studies of temporal and spatial
changes in vertical motions in coastal regions.
Fig. 5.10), but they provide remarkable
high-fidelity records at time scales ranging
from decadal to millennial (Natawidjaja
et al
.,
2007; Sieh
et al
., 2008; Zachariasen
et al
., 2000).
Remarkable insights about earthquake behav-
ior are emerging from coral studies above the
Sumatra subduction zone. Rapid convergence
(
∼
50 mm/yr) along this plate boundary has
produced the world's largest earthquake since
1964 (
M
w
=
9.1 on 26 December 2004 (Chlieh
et al
., 2007; Subarya
et al
., 2006)) and several
magnitude 7.5-8.6 temblors over the past century.
A tropical archipelago sits above the equatorial
segment of the Sumatran outer-arc high and hosts
diverse coral communities that have been
exploited for geodetic studies. For example,
studies of coral heights along the fringes of
Simeulue Island (Briggs
et al
., 2006) reveal
complementary patterns of uplift that occurred in
two closely spaced, great earthquakes just a few
months apart (Fig. 5.14A). When summed together,
the uplift data reveal a saddle of low displacement
(Fig. 5.14B), where no record of large displace-
ments in past earthquakes has been uncovered.
This saddle represents a deficit in coseismic dis-
placement that has been interpreted to result from
aseismic slip on a segment of the Sumatra meg-
athrust (Briggs
et al
., 2006). If true, little stress
would be expected to accumulate on the megath-
rust in this specific region, and it might act as a
barrier to propagation of large earthquakes.
In many tectonic settings, it is unknown
whether cumulative deformation from repetitive
coseismic displacements has resulted in the
growth of the observed topography or geological
structures. At Coalinga (Fig. 5.8), for example,
the similarity of patterns of coseismic displace-
ments and large-scale structures argues for
earthquake-driven structures and topography. In
striking contrast, coral data from Nias Island
in Sumatra (Fig. 5.14C) support the opposite
conclusion. There, the pattern of coseismic uplift
bears no spatial relationship to the long-term
uplift rate. Whereas the coseismic data reveal
strong gradients in uplift, Holocene uplift rates
deduced from uplifted corals are spatially
homogeneous and clearly uncoupled from the
coseismic patterns (Briggs
et al
., 2008). This
de-coupling argues that the coseismic deformation
Tropical corals
In some of the world's tropical oceans, Nature
has created its own geodetic recorder. Certain
corals, such as
Porites
, produce annual growth
bands (Fig. 5.13A) that are similar to tree rings,
but develop in response to seasonal variations
in the density of coral cells. Because these cor-
als cannot survive continuous exposure to air
even for relatively short durations, each year
these corals grow up to the level of the annual
lowest tide, which is also termed the “highest
level of survival” (HLS). Thus, when corals
become established in somewhat deeper water,
they initially grow upward to the sea surface
and then grow outward. The uppermost edge
of their annual rings, therefore, serves as a nat-
ural sea-level gauge that can commonly record
long-term upward or downward trends,
depending on whether the coast is subsiding or
rising, respectively (Fig. 5.13B and C). Because
corals represent a dynamic natural system, they
are imperfect recorders of subtle year-to-year
tidal variations (oceanographic corrections;
