INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.4. Paleogeodetic and paleoseismic sites
3.4.10. Bai site
however, is pocked by the hollows created by boring clams. Its upper surface is most likely eroded by a few centimeters, and HLS cannot be constrained well.
We also collected the entirety of a small head, which we nicknamed “the cabbage,” because of its resemblance in both size and shape to the vegetable (Fig. 3.42a).
The annual growth bands of this head are also exceptionally clear and unaffected by erosion. The head contains a record of HLS during the period 1992–1997, which allows a comparison with the youngest portion of the record of the larger head.
3.4.10.2. HLS History
Ba97-B2 slab
The shape of the top of “the cabbage” testifies to a high emergence rate in the mid-1990s (Fig. 3.42c). The elevations of HLS on opposite sides of the head are nearly identical.
Ba97-B1 slab
The HLS history of the larger of the two heads is divisible into three periods, pre- 1962, 1962–1973, and post-1973.
Pre-1962
The oldest part of the head consists of near-hemispherical bands added between the early 1930s and 1962. When the head was only a few years old, it appears to have been overturned. Ring counting indicates that this occurred in about 1935. The coincidence of overturning in the same year as the large earthquake may indicate that the
tiny head was overturned by shaking or tsunami. This is by no means certain, however.
And even if the turnover is seismogenic, it is of no great significance.
The eroded upper surface of the central hemisphere is diachronous. Bands from about 1954 to 1962 outcrop on its upper surface. Although this may reflect an emergence of about 5 cm over about 8 years, another plausible interpretation is that these bands have suffered partial erosion from the top of the head. If this is the case, then HLS is not well constrained by the tops of these annual bands. If the tops of these bands are close to HLS during those years, HLS during the late 1950s was rapidly dropping.
1962 event
The top of the 1962/63 growth surface is about 6 cm lower than the highest preserved remnant of the earlier 1962 band. (Fig. 3.42a). This is a minimum value for emergence in late 1962. If the 1962 and older bands used to extend over the top of the inner head, then the emergence might be as large as about 15 cm.
Upward growth began in 1962 and continued until the end of 1968, at which time a slight HLS clip occurred (Fig. 3.42c). Upward growth immediately resumed and continued until the end of 1973. It is important to note that growth during this period either did not extend radially inward or has been eroded from the inward-facing surface.
Stunting of inward growth is common on the raised rims of submerging microatolls, so we suspect that in this case inward growth was minor or non-existent. We suspect that inward growth from raised rims is hindered by extreme temperatures in trapped shallow pools that form atop the low inner flat during lowest tides. Thus, the HLS clip that
occurred on this raised rim in late 1968 may reflect much localized death due to micro- environmental conditions rather than impingement of a truly regional HLS.
If the clip in 1968 did result from a regional HLS drop, the submergence that began in 1962 was at least 5 cm. If the 1968 clip is a local clip, then the submergence of the 1960s was at least 10 cm.
1973 to 1997
The upper surface of the outer raised rim exhibits remarkable preservation.
Annual fluctuations in HLS are well preserved. The annual bands display a nearly monotonic decrease in HLS during this period. The average rate of emergence for the period is about 4.7 mm/yr (Fig. 2.13c). Superimposed upon this are higher-frequency, lower-amplitude fluctuations that we interpret as non-tectonic.
This rate of emergence is about 1 mm/yr higher than the rate observed at the nearby Penang site for the same period. It is, in fact, the highest observed rate for the last three decades in this region.
3.4.10.3. Summary
In 1935, when the head was only a few years old, it flipped over, possibly due to the shaking or the tsunami of the 1935 earthquake. The head then resumed growth, forming a hemisphere that first reached HLS sometime in the period 1954-1962. If the head reached HLS early in this period, emergence at a high rate is recorded by the down- stepping of successive annual bands, and about 5 cm of emergence occurred in late 1962.
If, instead, these bands have suffered partial erosion from the top of the head, then emergence of about 15 cm occurred in late 1962.
Unfettered upward growth dominates the period 1963–1973. A minor HLS clip in late 1968 could represent merely a local effect associated with stunting of inward- growing corallites. If so, the submergence of the 1960s could have occurred as one event, with a magnitude of at least 10 cm and perhaps about 13 cm. If the 1968 unconformity reflects a regional drop in HLS, then the submergence of the 1960s occurred in at least two events or episodes, nearly equal in size.
By 1974, the microatoll reached HLS once again. Numerous HLS clips in the succeeding two decades reveal a rather continuous emergence at the exceptionally high average rate of about 4.7 mm/yr.