INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.2. Coral microatolls as paleoseismic and paleogeodetic instruments
event. In the eastern region, which experienced coseismic submergence, interseismic emergence occurred, slowly raising the islands. Furthermore, the data reveals that the interseismic emergence and submergence rates have varied both temporally and spatially.
This rich data set allows us to model the kinematics and other properties of the subduction interface in a fashion that can explain the observed vertical deformation history.
Among the most interesting phenomena we have discovered in the coral record is the occurrence of a large aseismic event or “silent earthquake” in 1962, 27 years after the 1935 event. This is especially interesting in the light of recent modern instrumental documentation of rapid aseismic events on subduction interfaces, such as in Cascadia [Dragert et al., 2001; Miller et al., 2002], South America [Lowry et al., 2001], and Japan [Heki et al., 1997; Hirose et al., 1999].
3.2. Coral microatolls as paleoseismic and paleogeodetic
are more accessible than most paleoseismic sources and the raw data they contain can be acquired relatively easy.
Microatolls grow in the intertidal zones along the beaches. The upward growth of corals heads is limited by the lowest tide level, above which exposure cause coral death [Sieh et al., 1999; Taylor et al., 1987; Zachariasen et al., 1999; Zachariasen et al., 2000].
This maximum coral growth is termed "the highest level of survival" or “HLS” [Sieh et al., 1999; Taylor et al., 1987; Zachariasen et al., 1999; Zachariasen et al., 2000].
Therefore, fluctuations in sea level or HLS are accurately imprinted on the morphology and stratigraphy of microatolls. In west Sumatra, the HLS history recovered from corals is predominantly associated with the upper crustal deformations above the subduction interface. Thus microatolls serve as paleoseismic and paleogeodetic recorders [Sieh et al., 1999; Taylor et al., 1987; Zachariasen et al., 1999; Zachariasen et al., 2000].
Coral growth typically shows annual pairs of dark and light bands. This annual banding is seasonal density variations in coral skeletals caused by variations in sea temperature, rainfall, and other factors [Scoffin and Stoddart, 1978; Taylor et al., 1987;
Zachariasen et al., 1999; Zachariasen et al., 2000]. The light bands (lower density cells) occurred during the rainy seasons (September to March), and the dark bands (higher density cells) are associated with the dry season (April – August). These annual bands are similar to tree rings in that they provide a yearly record of coral growth throughout the life of the coral, and thus provide an excellent time-series for constructing the sea- level history [Sieh et al., 1999; Taylor et al., 1987; Zachariasen et al., 1999; Zachariasen et al., 2000]. The annual bands are sometime visible to the naked eye, but are commonly
more pronounced in X-ray images of the coral thin slabs [Sieh et al., 1999; Woodroffe and McLean, 1990; Zachariasen et al., 1999; Zachariasen et al., 2000].
Other geological recorders of sea level that have been used in the past include coastal terraces [Bloom and Yonekura, 1985; Ota et al., 1990; Pirazzoli et al., 1993], eroded marine notches [e.g. Hantoro et al., 1994], and coral reefs [Chappell, 1983; Ota and Chappell, 1999; Ota et al., 1993; Ota and Omura, 1992; Pirazzoli et al., 1991].
These have helped to constrain long-term climatically induced sea-level changes, cumulative tectonic uplift, and sometimes the records of past earthquake, but not the details of deformation throughout the entire earthquake cycle. As geologic tide gauges, none of these show the promise of microatolls, which have internal age control and growth characteristics that are governed by sea level [Edwards et al., 1988; Taylor et al., 1987; Woodroffe and McLean, 1990; Zachariasen, 1998b; Zachariasen et al., 1999;
Zachariasen et al., 2000].
3.2.2. The synthetic microatoll
To help understanding the natural response of our coral instrument, our collaborator, Steve Ward in the University of California at Santa Cruz, developed a computer program that simulates the natural process of coral growth. It is able to reproduce the shape of coral microatolls in various tectonic environments by incorporating the effect of tectonic submergence and emergence into the simulation [Fig.
3.4 a, b, c, d].
Based on the fact that coral growth is limited by annual lowest tide level or HLS, and any vertical motion will superimpose on the annual HLS fluctuation, we can define HLS as a function of time as in the following equation:
HLS (t) = Long-term tectonic rate (t) + Annual HLS Fluctuation (t) + Sum (coseismic steps)
Annual HLS fluctuation is simulated by a random Gaussian variable of zero mean and with a fixed standard deviation of 2 cm. This standard deviation value is estimated from the real HLS fluctuation recorded in the natural coral microatolls.
The virtual microatoll is programmed to grow at a fixed rate of 1 cm/yr. The program simulation will track the x (year) and z (elevation/HLS) position of every living coral in the outermost layers. When the simulation steps ahead one year and selects the next HLS for that year, it then scans all living coral surface position and kills any coral growth above the selected HLS. Thus, only coral bands that are below the HLS will survive and give rise to subsequent next growth bands.
The computer simulation image in Figure 3.4a demonstrates that coral heads maturing in a submerging area develop “cup-shape” morphology, because its living perimeter grows to a level that is higher than the interior, already dead coral bands. If the submergence is sudden, then the microatoll develops lower and upper flat surfaces, separated by a steep step representing the unrestricted growth of coral in the aftermath of the submergence (Fig. 3.4b). A coral head located in an emerging environment exhibits a conical shape because of the progressive fall of the upper growth limit as the coral expands outward (Fig. 3.4c). If the emergence is sudden, then the microatoll develops a
”hat-shape” morphology (Fig. 3.4d). The hat “bowl” or elevated central head represents
the continued coral growth for the period before the emergence event. The hat “brim”
represents the continued growth for decades after the event. The step from top of the bowl to the brim is a measure of the vertical displacement during the event.