Geology topics

Bambang W. Suwargadi

Publications and source records attributed to Bambang W. Suwargadi.

3 recordsLinked to original sources

USGS2023

Investigating spatio-temporal variability of initial 230Th/232Th in intertidal corals

One of the key factors in obtaining precise and accurate 230 Th ages of corals, especially for corals with ages less than a few thousand years, is the correction for non-radiogenic 230 Th based on an initial 230 Th/ 232 Th value ( 230 Th/ 232 Th 0 ). Studies that consider coral 230 Th/ 232 Th 0 values in intertidal environments are limited, and it is in these environments that corals have Th concentrations 100–1000 times greater than open-ocean corals. Here we present 66 reconstructed U–Th isochrons of modern and Holocene Porites lutea and lobata corals from throughout the Sumatran forearc islands (SFI). Mean square of weighted deviates ( MSWD ) is used to evaluate the variabilities of 28 location-specific 230 Th/ 232 Th0 values before the regional mean. Results show no significant spatial difference between the means of 3.1 ± 3.2 × 10 −6 for the northern SFI and 4.7 ± 1.3 × 10 −6 for the southern SFI. Using the weighted mean of 4.3 ± 2.5 × 10 −6 for all islands has the advantage that no data need be subjectively excluded in the calculation of reliable 230 Th ages, where a local initial value is unknown. There were no clear centennial to millennial time-scale changes in 230 Th/ 232 Th 0 in the past 2500 years. This study, therefore, suggests a reliable value of initial 230 Th/ 232 Th ratio in intertidal environments.

Sumatran forearc islands↗

USGS2015

Coral 13C/12C records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios ( ) in the skeletons of massive Porites corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral response to uplift (and subsidence) produced by the March 2005 M w 8.6 Nias–Simeulue earthquake, and uplift further south around Sipora Island during a M&sim;8.4 megathrust earthquake in February 1797. At Nias, the average step-change in coral was 0.6±0.1&permil;/m for coseismic displacements of +1.8 m and −0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll is 2.3&permil;/m , nearly four times greater than for the Nias Porites . Considering the natural variability in coral skeletal , we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding M w &sim;7.2 in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral , light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia.

Earth and Planetary Science Letters↗

USGS2015

Time-varying interseismic strain rates and similar seismic ruptures on the Nias-Simeulue patch of the Sunda megathrust

Fossil coral microatolls from fringing reefs above the great ( M W 8.6) megathrust rupture of 2005 record uplift during the historically reported great earthquake of 1861. Such evidence spans nearly the entire 400-km strike length of the 2005 rupture, which was previously shown to be bounded by two persistent barriers to seismic rupture. Moreover, at sites where we have constrained the 1861 uplift amplitude, it is comparable to uplift in 2005. Thus the 1861 and 2005 ruptures appear to be similar in both extent and magnitude. At one site an uplift around AD 1422 also appears to mimic the amount of uplift in 2005. The high degree of similarity among certain ruptures of this Nias–Simeulue section of the Sunda megathrust contrasts with the substantial disparities amongst ruptures along other sections of the Sumatran portion of the Sunda megathrust. At a site on the northwestern tip of Nias, reefs also rose during an earthquake in AD 1843, known historically for its damaging tsunami along the eastern coast of the island. The coral microatolls also record interseismic vertical deformation, at annual to decadal resolution, spanning decades to more than a century before each earthquake. The corals demonstrate significant changes over time in the rates of interseismic deformation. On southern Simeulue, interseismic subsidence rates were low between 1740 and 1820 but abruptly increased by a factor of 4–10, two to four decades before the 1861 rupture. This may indicate that full coupling or deep locking of the megathrust began only a few decades before the great earthquake. In the Banyak Islands, near the pivot line separating coseismic uplift from subsidence in 2005, ongoing interseismic subsidence switched to steady uplift from 1966 until 1981, suggesting a 15-year-long slow slip event, with slip velocities at more than 120% of the plate convergence rate

Niase-Simeulue region↗