Geology Reference
In-Depth Information
of the terraces were used to estimate rock
uplift rates, the Holocene rates would appear
more rapid than the pre-8 ka rate. Instead, when
sea-level change is included, the Late Pleistocene
rate is nearly three times faster than the
Holocene rate.
In some tropical settings where reefs flourish,
detailed reconstructions of both coseismic and
interseismic deformation are sometimes possible
(Natawidjaja
et al.
, 2007; Sieh
et al.
, 2008;
Zachariasen
et al.
, 2000). Based on the annual
growth banding of corals (Fig. 5.13), changes in
relative sea level are recorded on an annual
basis. Recall also that, if permanent deformation
is excluded, vertical motions during inter-
seismic intervals should be balanced by abrupt
movement in the opposite direction during
earthquakes. Whereas corals grow upward and
outward during times of steady interseismic
submergence (or downward and outward during
emergence), earthquakes abruptly shift the
position of the coral head with respect to local
sea level (Fig. 6.11). If part of the coral head is
lifted coseismically above sea level (Fig. 5.13A),
those corals exposed to the air will die and
slowly erode, whereas those that remain below
the highest level of survival (HLS) will continue
to grow outward. Such coseismic emergence
will create a fringing annulus below the HLS
and an unconformity above. After an earthquake,
the height of the coral heads above the HLS
provides an excellent measure of the vertical
component of seismic deformation (Briggs
et al.
, 2006, 2008). If interseismic submergence
then ensues, annual layers will grow upward
and will unconformably overlap the eroded
coral surface, thereby preserving the record of
the minimum magnitude of coseismic emergence
(Fig. 6.11). Moreover, with new high-precision
protactinium-231 and thorium-230 dating
techniques on carbonates (Cheng
et al.
, 2000;
Edwards
et al.
, 1997), corals as young as a few
decades old can now be dated with remarkably
high precision. Such dates help to anchor
chronologies based on counting annual bands.
These dates permit synthesis of data from living
coral heads with older, dead ones in order to
produce paleoseismic records hundreds of years
long (Sieh
et al.
, 2008).
The extensive, emergent, and coral-fringed
forearc above the Sumatran subduction zone is
a rich source of high-resolution data that permits
studies that exploit dense two- and three-
dimensional spatial coverage of seismological
signals. For example, along transects oriented
perpendicular to the trench, the detailed pattern
of interseismic and coseismic deformation
allows both testing of models for slip on the
subduction interface (Fig. 6.12), as well as
deducing lithospheric properties such as rigidity
and temperature that affect flexural wave-
lengths. The coral record yields well-defined
spatial variations in the magnitude and rates of
uplift and subsidence. In order to reproduce
these data, an elastic dislocation model of the
subduction zone (Fig. 6.12A and B) can be
implemented in which slip occurs along both
the upper and lower surfaces of the subducting
slab (Sieh
et al.
, 1999). Given the duration of
interseismic interval derived from the corals and
their pattern of vertical motion, slip rates along
the subduction interfaces can be varied until the
best match is found to the observed interseismic
deformation (Fig. 6.12C). When the stored elastic
strain is abruptly released, the model should
reproduce the observed, coral-based coseismic
deformation (Fig. 6.12D). When successful, such
data-to-model couplings can provide tremendous
insight on the behavior of a subduction interface
that is hidden many kilometers beneath the
surface of the forearc.
In some tectonically active coastal settings,
constructional beach ridges provide useful
coseismic markers. For example, at Turakirae
Head in New Zealand, beach ridges are built
through the accumulation of clasts, shells, and
organic debris during large storms. The modern
beach ridge is about 2.3 m above the average
high-tide level. Nearly 6 m above the modern
ridge is a beach ridge that was uplifted in the
1855 earthquake along the nearby Wairarapa
Fault (Fig. 6.13A and Plate 1B). Still higher
above the modern shoreline are three addi-
tional beach ridges that are interpreted to
represent older coseismic offsets of 8, 6, and
2 m and that probably extend back to about
6-8 ka when global sea level stabilized. The
correlated crests of these beach ridges indicate
