Coral geochemical response to uplift in the aftermath of the 2005 Nias–Simeulue earthquake

On 28 March 2005, the Indonesian islands of Nias and Simeulue experienced a powerful Mw 8.6 earthquake and coseismic uplift and subsidence. In areas of coastal uplift (up to ~ 2.8 m), fringing reef coral communities were killed by exposure, while deeper corals that survived were subjected to habitats with altered runoff, sediment and nutrient regimes. Here we present time-series (2000–2009) of Mn/Ca, Y/Ca and Ba/Ca variability in massive Porites corals from Nias to assess the environmental impact of a wide range of vertical displacement (+ 2.5 m to − 0.4 m). High-resolution LA-ICP-MS measurements show that skeletal Mn/Ca increased at uplifted sites, regardless of reef type, indicating a post-earthquake increase in suspended sediment delivery. Transient and/or long-term increases in skeletal Y/Ca at all uplift sites support the idea of increased sediment delivery. Coral Mn/Ca and Ba/Ca in lagoonal environments highlight the additional influences of reef bathymetry, wind-driven sediment resuspension, and phytoplankton blooms on coral geochemistry. Together, the results show that the Nias reefs adapted to fundamentally altered hydrographic conditions. We show how centuries of repeated subsidence and uplift during great-earthquake cycles along the Sunda megathrust may have shaped the modern-day predominance of massive scleractinian corals on the West Sumatran reefs

Persistent elastic behavior above a megathrust rupture patch: Nias island, West Sumatra

We quantify fore-arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible. Elevated coral microatolls, paleoreef flats, and chenier plains show that the Sumatran outer arc island of Nias has experienced a complex pattern of relatively slow long-term uplift and subsidence during the Holocene epoch. This same island rose up to 2.9 m during the Mw 8.7 Sunda megathrust rupture in 2005. The mismatch between the 2005 and Holocene uplift patterns, along with the overall low rates of Holocene deformation, reflects the dominance of elastic strain accumulation and release along this section of the Sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long-term plate convergence. The fraction of 2005 uplift that will be retained permanently is generally <4% for sites that experienced more than 0.25 m of coseismic uplift. Average uplift rates since the mid-Holocene range from 1.5 to −0.2 mm/a and are highest on the eastern coast of Nias, where coseismic uplift was nearly zero in 2005. The pattern of long-term uplift and subsidence is consistent with slow deformation of Nias along closely spaced folds in the north and trenchward dipping back thrusts in the southeast. Low Holocene tectonic uplift rates provide for excellent geomorphic and stratigraphic preservation of the mid-Holocene relative sea level high, which was under way by ∼7.3 ka and persisted until ∼2 ka

Long-term growth trends of massive Porites corals across a latitudinal gradient in the Indo-Pacific

Previous studies have reported recent substantial declines in the growth rates of massive Porites corals under warming oceans. However, the majority of these reports are from inshore reefs, and few have explored growth responses in offshore reefs from remote locations with low levels of pollution, sedimentation or nutrient loading. Here, we examined continuous growth records of massive Porites from remote locations spanning a 25 degrees latitudinal gradient in the Indo-Pacific, including Palau, central Sulawesi, West Papua and the central Great Barrier Reef (GBR). Between 1982 and 2012, no significant changes in calcification or extension anomalies were observed at any study location, despite significant increases in sea surface temperature (SST) at all sites. Skeletal density increased linearly by similar to 0.4 % yr(-1) in Palau, but no change was found in Sulawesi, yet skeletal density showed a significant nonlinear change in West Papua and the GBR. Skeletal density displayed a significant positive linear relationship with SST at Palau and West Papua, whereas no relationship was observed in Sulawesi. In the GBR, skeletal density exhibited a nonlinear parabolic relationship with SST, with strong negative anomalies occurring following major thermal events. Unlike the ongoing declines in growth rates of inshore corals that have been widely reported, we found that calcification and extension anomalies of the majority of Porites from offshore remote locations do not appear to be exhibiting negative growth responses to warming SST. Our results suggest that reefs experiencing low levels of local stressors may show increased resilience to warming SST in an era of rapidly warming oceans

Using coral geodesy to investigate seismic cycle in the banyak islands, Indonesia

The location of the Banyak Islands above the down-dip limit of the 2005 Nias-Simeulue earthquake rupture provides a unique opportunity to study the seismic cycle. The synthesis of sea level histories based on Porites genus corals at three sites spanning a 40-km transect perpendicular to the Sunda trench allows a reconstruction of the interseismic, coseismic, and postseismic deformation that occurred before, during, and after the 2005 event. During the earthquake, the western island of Bangkaru uplifted while the eastern islands subsided, suggesting that the island group is located above the down-dip limit of slip. Early postseismic motions suggest that much of the coseismic subsidence in the Banyak Islands will be recovered quickly by postseismic uplift in the decades following the earthquake, with low rates of subsidence occurring in the latter part of the interseismic period. This deformation pattern implies that the Banyak Islands overlie the down-dip edge of a locked patch on the megathrust. This asperity probably slips only during large earthquakes whereas the adjacent down-dip region of the fault slips with initially accelerated postseismic rates, slowing and perhaps becoming completely locked late in the interseismic interval. However, the island of Bangkaru which experienced coseismic uplift has had little or no interseismic or postseismic subsidence, suggesting that an updip slip event is required for elastic recovery

Deformation and Slip Along the Sunda Megathrust in the Great 2005 Nias-Simeulue Earthquake

Seismic rupture produced spectacular tectonic deformation above a 400-kilometer strip of the Sunda megathrust, offshore northern Sumatra, in March 2005. Measurements from coral microatolls and Global Positioning System stations reveal trench-parallel belts of uplift up to 3 meters high on the outer-arc islands above the rupture and a 1-meter-deep subsidence trough farther from the trench. Surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward. A saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 Sumatra-Andaman rupture. The southeastern edge abuts a predominantly aseismic section of the megathrust near the equator