INDONESIAN OCEAN
Chapter 3. Paleogeodetic records from microatolls above the central Sumatran subduction zone
3.4. Paleogeodetic and paleoseismic sites
3.4.12. Masin site
3.4.11.3. Summary
Microatoll H17 provides a long, continuous record of sea-level change at Lago Island over the past 130 years. Between 1863 and 1920, the site was submerging steadily at a rate of about 1.1 mm/yr. In about 1920, this trend appears to have reversed, with submergence at an average rate of about 3.3 mm/yr continuing (and possibly accelerating) up to the date of the 1935 earthquake. The 1935 event and any transients in the subsequent two decades yielded a net submergence of the site of 30 to 35 cm.
Submergence during the 1962 event amounted to about 8 cm. In the past three decades (1970 to 1997), emergence at an average rate of 2.5 mm/yr has been occurring.
The significance of this history will be considered in concert with the histories of the other sites in Chapter 4.
some about 4m in diameter, rest on the steep southeastern flank of the sandy spit, at the brink of the spit’s plunge into much deeper water. Eighteen surveyed heads, all on top of the sandy spit, have diameters between 1 m and 4 m, and their crests rise above substrate from a few tens of centimeters to a meter (Fig. 3.46b and 3.46c). The larger ones commonly have perimeters that overhang the substrate (for example, H19 in Fig. 3.46c).
The nearly flat tops of these microatolls suggest that HLS has been nearly stable over the past few decades.
When we visited the site in mid-1997, many of the living perimeters showed signs of distress. In particular, the elevation of the upper limit of the “carpet” of living polyps (i.e., the HLS) fluctuated tens of centimeters around many of the head perimeters. We ascribed this to cyanide fishing, which our crew noted had been prominent in the early 1990s. When we revisited the site in 2000, all of the heads were dead.
Nonetheless, our visit in 1997 and our survey of the perimeter crests of 18 heads in 2000 convinced us that these heads are all part of the same modern population. Crest heights of all the surveyed heads are concordant within 5 centimeters of their mean (Fig.
3.46d).
3.4.12.2. Microatoll H19
We collected a 1 m long slab from the south radius of microatoll H19 (Fig. 3.46b and 3.46d). The morphology of this head is representative of the shape of many of the larger heads at the site, although some of the larger heads have a low, central flat (for example, H14, Fig. 3.46c). The annual banding within the slab is of average quality.
Thus, we were able to assign ages without great difficulty. Our uncertainties of the
assignments are typically about a few years. We did not use U-Th analyses to check the reliability of our visual counting of the annual bands. We assumed that the outermost full band grew in 1997.
Initial HLS history
Analysis of the slab reveals a half-century of slow submergence, interrupted by slight drops in HLS in 1962 and about 1992. The slab also reveals a large submergence event in 1935 (Fig. 3.47b and 3.47c).
The oldest band present in the slab grew in about 1925. The band was growing horizontally, which suggests that the head had already reached HLS and upward growth had been restricted. In fact, the slab appears to contain an HLS impingement in 1930, nearly 30 cm below the upper surface of the microatoll.
The 1935 event
Another HLS impingement, barely recovered on the left edge of the slab, occurred in about 1935. The growth ring just above this impingement clearly wraps around the impingement.
Free upward growth of annual bands occurred from 1935 to 1943. This is consistent with submergence of about 18 cm in 1935. As elsewhere, we know only the date of inception of the submergence event or episode. We do not know the precise duration of the submergence event, but we know that it occurred more rapidly than vertical growth could keep up with it. Thus, in this case, submergence had to have been more than about 2 cm/yr for each of the years between 1935 and 1943. As with the other
sites, we suspect that most of the submergence occurred during the 1935 earthquake and during post-seismic transients within a year or two of that event.
Slow, steady submergence since 1943
Between 1943 and 1961, HLS was nearly stable. Submergence occurred at about 0.65 mm/yr. Similarly, from 1964 to 1991, submergence was very stable at about 0.95 mm/yr. Thus, if we extrapolate the post-1943 continuous submergence back to 1935 the size of the 1935 submergence event will be slightly smaller (Fig. 3.47c). Between 1991 and 1997, HLS also appears to have been stable or nearly so.
The 1962 event
The unusually clear record of HLS and its stability between 1943 and 1991 allows us to see quite clearly a slight perturbation in 1962. This begins with an emergence of about 10 cm, which appears on the outside of the 1961 ring. Within two years, by the end of 1964, the microatoll had grown back up to a new HLS, within only 1 cm of the pre-1961 level.
The 1992 event
A clear die-down of about 4 cm occurred in 1991/1992. This event is possibly related to an Mw5.2 earthquake that occurred in 1991 in this vicinity (Fig. 3.3). However, the die-down may also be induced by cyanide poisoning related to fishing practices that were common in the early 1990s but which have subsequently been made illegal.
3.4.12.3. Summary
The Masin site contains an exquisite record of slow submergence preceded by a major submergence event in 1935 and minor emergence events in 1962 and 1992. The magnitude of submergence in 1935 was about 18 cm. Emergence in 1962 was about 10 cm, but this recovered almost fully by late 1964. The 4 cm die-down of HLS in 1992 is possibly a tectonic event, but it may also be an effect of cyanide fishing, or it is a combination of two.