INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.4. Paleogeodetic and paleoseismic sites
3.4.4. Pono site
3.4.4.4. A 100-year record from large microatolls and H10
In 2000, we studied larger microatolls a few tens of meters northwest and southwest of the H1–H9 cluster (Fig. 3.20b). Heads H10 and H16, representatives of these larger heads, are depicted in Figures 3.22a and 3.22b. All of these microatolls have a shallow cup-shaped morphology indicative of submergence at rates that, on average, have been low. Abrupt steps upward, however, suggest that they have been subjected to a couple of rapid submergence events. As we shall see in the slab of one of these heads, the highest, outermost flat represents coral bands that grew after 1961. The next flat toward the center formed between 1935 and 1962. The third flat toward the interior represents a period of growth prior to the 1935 event. This flat is a bit lower on average in H16 than in H10. We tentatively interpret this to be the result of settling of H16 into
the sandy substrate during the 1935 earthquake. H16 has a still-older step down to another flat, which forms the center of the head. Judging from the radius of the head, this central flat probably grew during a period of relative stability in the late 19th century.
In July 2000, we collected a diagonal slab across head H10 (Pn00A3), (Fig. 3.22).
We chose this head, rather than the larger H16 head, because that 5 m head has been significantly degraded by bioerosion. The record from H10 is particularly valuable, because it extends farther into the past than the record of H2, and because we can compare HLS records from two radiuses
The 2 m long slab from H10 contains a continuous record of HLS variations through the 20thcentury. However, the geometry of the head is rather asymmetric (Fig.
3.25a). To determine the ages of the annual bands, we relied both on visual ring counting and U-Th age determinations. Two U-series disequilibrium analyses from the 62nd band inward on the western radius yielded dates AD 1933±2 and 1934±4 (Table 3.1). Using these dates and counting outward, we derive perimeter ages of 1995±2 and 1996±4. This is consistent with death in the regional environmental catastrophe of late 1997–early 1998. We assume, then, a date of AD 1997 for the outermost ring of the eastern wing in assigning the band dates shown in Fig. 3.25a. These show that the western radius contains growth bands from about 1926 to 1997.
To determine ages for the eastern radius, we first assumed those two small irregularities on the western radius in about 1935 and 1962 correlate with similar irregularities on the eastern radius. These assignments yield dates of 1957 and 1927 for bands that U-Th analyses indicate formed AD 1957±4 and 1927±6 (Fig. 3.25a and Table
3.1). Visual ring counting, then, shows that the eastern radius grew during the century between about AD 1886 and 1986. Both radii of H10 show a general submergence throughout the 20th century. That submergence, however, is “eventful,” with abrupt submergence events in about 1935 and 1962. Long periods of nearly stable HLS occurred through most of the remainder of the microatolls growth history (Fig. 3.25a).
Submergence of 5 to 8 cm occurred in the early 1930s. Emergence of 2 to 5 cm occurred in 1962, followed by submergence of 7 to 8 cm (Fig. 3.25 b and 3.25c).
HLS history prior to and during 1935 event
The lower flat on the eastern radius of H10 formed during growth between about 1890 and the early 1930s (Fig. 3.25a). The least-squares fit to all HLS on these bands yields an average submergence rate of about 1.1 mm/yr (Fig. 3.25c). The western radius’
oldest bands record just the last few years of this period and are consistent with HLS levels on the East. The eastern radius records a small (2 cm) emergence in the early 1930s. This does not appear on the western radius. But both radii reveal a subsequent, prominent, fast submergence that enabled free upward growth for several years. The magnitude of the free upward growth is about 7.8 cm on the eastern radius and about 4.7 cm on the western (Fig. 3.25b-c). The U-Th dates constrain the onset of this submergence to AD 1931±4/7 and 1931±6. This is strong circumstantial evidence that the submergence is associated with the large earthquake of 1935.
HLS history between the 1930s and 1961
Both radiuses of H10 show relative stability of HLS between the disruptions of 1935 and 1961. However, the rates differ by a fraction of a millimeter per year. The
average rate on the western radius is ~0.1 mm/yr, and it is emergent. The average rate on the eastern radius is ~0.5 mm/yr submergence (Figs. 3.25b and 3.25c). This small discrepancy is easily attributable to differential bioerosion, since only an additional centimeter or two of bioerosion of the western radius would be required to produce the apparent difference between these two very low rates. Nonetheless, true differences as small as these could also be attributable to slight variations in HLS from one side of the microatoll to the other [Zachariasen et al., 2000].
The 1962 event
The HLS disruption of 1962 in H10 mimics that of 1935 in that a lesser emergence event precedes the submergence event. As with the 1935 event, the precursory emergence is most prominent on the eastern side of the head. The magnitude is 4.7 cm on the eastern radius and 2.0 cm on the western radius. This difference could be a hint that micro-environmental conditions on the eastern side were more favorable to die-down than those on the western flank.
HLS history after 1962
HLS restricted the upward growth on both sides of the microatoll after 7 to 8 years of free upward growth after 1962. Only about a decade of growth at HLS occurred on the western flank before death of the perimeter in 1986. But growth continued on the eastern flank throughout the 1970s, 80s, and most of the 90s. The average rate of emergence recorded by HLS clips on the western flank is about 1.5 mm/yr (Fig. 3.25b).