Haryadi Permana

Limits of the seismogenic zone in the epicentral region of the 26 December 2004 great Sumatra-Andaman earthquake: Results from seismic refraction and wide-angle reflection surveys and thermal modeling

The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated around 30 km depth and ruptured 1300 km of the Indo‐Australian–Sunda plate boundary. During the Sumatra‐OBS (ocean bottom seismometer) survey, a wide‐angle seismic profile was acquired across the epicentral region. A seismic velocity model was obtained from combined travel time tomography and forward modeling. Together with reflection seismic data from the SeaCause II cruise, the deep structure of the source region of the great earthquake is revealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, and the subducting slab can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km from the trench axis, below the fore‐arc basin. A high‐velocity zone at the lower landward limit of the ray‐covered domain, at 22 km depth, marks a shallow continental Moho, 170 km from the trench. The deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault. Assuming 100°–150°C as its updip limit, the seismogenic zone is predicted to begin 5–30 km from the trench. The downdip limit of the 2004 rupture as inferred from aftershocks is within the 350°–450°C temperature range, but this limit is 210–250 km from the trench axis and is much deeper than the fore‐arc Moho. The deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate.

Contrasting Décollement and Prism Properties over the Sumatra 2004–2005 Earthquake Rupture Boundary

Quake Control Large earthquakes occur at the margins of two colliding plates, where one plate subducts beneath the other at a shallow angle. These megathrust earthquakes often cause destructive tsunamis owing to the displacement of large volumes of water at the fault along the plate boundary. Two related studies of the seismic structure of subduction zones attempt to reveal the underlying mechanisms of megathrust earthquakes (see the Perspective by Wang). Kimura et al. (p. 210) compared seismic reflection images and microearthquake locations at the Philippine Sea plate where it subducts obliquely beneath Japan. The locations of repeating microearthquakes correspond to active transfer of material from the subducting plate to the continent—a process only previously assumed from exhumed metamorphic rocks. Dean et al. (p. 207) observe an expansive structure in the sea-floor sediment near the location of the 2004 and 2005 Sumatra earthquakes in Indonesia that suggests sediment properties may influence the magnitude of megathrust ruptures and their subsequent tsunamis. A plate boundary fault reflector suggests different rupture styles in the last two major Sumatran earthquakes. Styles of subduction zone deformation and earthquake rupture dynamics are strongly linked, jointly influencing hazard potential. Seismic reflection profiles across the trench west of Sumatra, Indonesia, show differences across the boundary between the major 2004 and 2005 plate interface earthquakes, which exhibited contrasting earthquake rupture and tsunami generation. In the southern part of the 2004 rupture, we interpret a negative-polarity sedimentary reflector ~500 meters above the subducting oceanic basement as the seaward extension of the plate interface. This predécollement reflector corresponds to unusual prism structure, morphology, and seismogenic behavior that are absent along the 2005 rupture zone. Although margins like the 2004 rupture zone are globally rare, our results suggest that sediment properties influence earthquake rupture, tsunami hazard, and prism development at subducting plate boundaries.

Can turbidites be used to reconstruct a paleoearthquake record for the central Sumatran margin

Turbidite paleoseismology aims to use submarine gravity flow deposits (turbidites) as proxies for large earthquakes, a critical assumption being that large earthquakes generate turbidity currents synchronously over a wide area. We test whether all large earthquakes generate synchronous turbidites, and if not, investigate where large earthquakes fail to do this. The Sumatran margin has a well-characterized earthquake record spanning the past 200 yr, including the large-magnitude earthquakes in 2004 (Mw 9.1) and 2005 (Mw 8.7). Sediment cores collected from the central Sumatran margin in 2009 reveal that surprisingly few turbidites were emplaced in the past 100–150 yr, and those that were deposited are not widespread. Importantly, slope basin deposits preserve no evidence of turbidites that correlate with the earthquakes in 2004 and 2005, although recent flow deposits are seen in the trench. Adjacent slope basins and adjacent pairs of slope basin and trench sites commonly have different sedimentary records, and cannot be correlated. These core sites from the central Sumatran margin do not support the assumption that all large earthquakes generate the widespread synchronous turbidites necessary for reconstructing an accurate paleoearthquake record.

Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event

Current models predict that the seismogenic zone along subduction thrusts, where the largest earthquakes nucleate and propagate, does not extend to the forearc mantle below the crust of the upper plate. Stable sliding conditions have been shown to prevail there, particularly along several circum-Pacific margins that underwent great megathrust earthquakes (Mw > 8.5) during the twentieth century. Based on geophysical investigation, we show that the great 2004 Sumatra-Andaman earthquake (Mw = 9–9.3) contradicts these models: not only did it propagate downdip along the interface between the forearc mantle and the subducting plate, but it actually nucleated along this reportedly aseismic part of the interplate contact. Petrological models can therefore underestimate the downdip extent of rupture zones to be expected in megathrust earthquakes, and need to be revised to account for this observation, albeit unusual.

Downgoing plate topography stopped rupture in the A.D. 2005 Sumatra earthquake

Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmentation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the M w = 8.7, A.D. 2005 Simeulue-Nias earthquake, and the northern termination of a major 1797 earthquake that was partly filled by an M w = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.

3‐D active source tomography around Simeulue Island offshore Sumatra: Thick crustal zone responsible for earthquake segment boundary

We present a detailed 3-D P-wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P-wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures.

Exploring Structural Controls on Sumatran Earthquakes

A series of linked marine and land studies have recently targeted the Sumatra subduction zone, focusing on the 2004 and 2005 plate boundary earthquake ruptures in Indonesia. A collaborative research effort by scientists from the United Kingdom (UK Sumatra Consortium), Indonesia, United States, France, and Germany is focusing on imaging the crustal structure of the margin to examine controls on along-strike and updip earthquake rupture propagation. The fundamental science objective is to examine how margin architecture and properties control earthquake rupture location and propagation.

High-resolution multi-proxy reconstruction of environmental changes in coastal waters of the Java Sea, Indonesia, during the late Holocene

ABSTRACT To obtain insight into the natural variability of the coastal ecosystems off southern Kalimantan, late Holocene environmental conditions between ca. 2850 and 990 cal yr BP in the Java Sea were investigated. A 134-cm-long sediment core collected ∼50 km off the Pembuang River mouth was analysed for organic-walled dinoflagellate cysts, pollen/spores and biogeochemical parameters, e.g. organic carbon (Corg), total nitrogen (N) and calcium carbonate (CaCO3) as well as carbon and nitrogen stable isotope composition (δ13C, δ15N). Sediments consist of mixed terrestrial as well as marine organic matter, are characterised by low nutrient uptake and suggest generally low river discharge that is supported by very low pollen and spore concentrations (256 pollen grains cm-3 and 20 spores cm-3 at maximum, respectively). Foraminifera and coccolithophores dominated the plankton over cyst-producing dinoflagellates and diatoms. Dinoflagellate cyst assemblages are composed mainly of oxidation-resistant species of the genera Operculodinium and Spiniferites with a minor contribution of Impagidinium (mainly I. strialatum). The percentages of round brown and peridinioid cysts are low and decrease from the bottom of the core to the top. Palynological and biogeochemical data appear well correlated and synchronously reflect changes in the marine environment. It is reconstructed that after ca. 2480 cal yr BP, bottom waters became increasingly ventilated. A typical open-water dinoflagellate cyst association is gradually replaced by a more coastal association between ca. 2480 and 1530 cal yr BP that is most likely attributed to El Niño-induced seasonal differences between dry and wet periods of the year. After 1530 cal yr BP, a more pronounced influence of the Pembuang River is indicated by an increase in δ15N and decreased δ13C which is supported by the occurrence of nutrient-sensitive Lingulodinium machaerophorum and Nematosphaeropsis labyrinthus. The overall results indicate short-scale local environment fluctuations attributed to abiotic factors.