Yusuf Djajadihardja

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.

Sunda‐Banda arc transition: Incipient continent‐island arc collision (northwest Australia)

The eastern Sunda arc represents one of the few regions globally where the early stages of continent-arc collision can be studied. We studied along the western limit of the collision zone at the Sunda-Banda arc transition, where the Australian margin collides with the Banda island arc, causing widespread back arc thrusting. We present integrated results of a refraction/wide-angle reflection tomography, gravity modeling, and multichannel reflection seismic imaging using data acquired in 2006 southeast of Sumba Island. The composite structural model reveals the previously unresolved deep geometry of the collision zone. Changes in crustal structure encompass the 10 - 12 km thick Australian basement in the south and the 22 - 24 kmthick Sumba ridge in the north, where backthrusting of the 130 km wide accretionary prism is documented. The structural diversity along this transect could be characteristic of young collisional systems at the transition from oceanic subduction to continent-arc collision. Citation: Shulgin, A., H. Kopp, C. Mueller, E. Lueschen, L. Planert, M. Engels, E. R. Flueh, A. Krabbenhoeft, and Y. Djajadihardja (2009), Sunda-Banda arc transition: Incipient continent-island arc collision (northwest Australia), Geophys. Res. Lett., 36, L10304, doi: 10.1029/2009GL037533.

Lower plate structure and upper plate deformational segmentation at the Sunda‐Banda arc transition, Indonesia

The Sunda‐Banda arc transition at the eastern termination of the Sunda margin (Indonesia) represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation. Neighboring margin segments display a high degree of structural diversity of the incoming plate (transition from an oceanic to a continental lower plate, presence/absence of an oceanic plateau, variability of subducting seafloor morphology) as well as a wide range of corresponding fore‐arc structures, including a large sedimentary basin and an accretionary prism/outer arc high of variable size and shape. Here, we present results of a combined analysis of seismic wide‐angle refraction, multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of Lombok (116°E) and Sumba (119°E). On the incoming plate, the results reveal a 8.6–9.0 km thick oceanic crust, which is progressively faulted and altered when approaching the trench, where upper mantle velocities are reduced to ∼7.5 km/s. The outer arc high, located between the trench and the fore‐arc basin, is characterized by sedimentary‐type velocities (Vp < 5.5 km/s) down to the top of the subducting slab (∼13 km depth). The oceanic slab can be traced over 70–100 km distance beneath the fore arc. A shallow serpentinized mantle wedge at ∼16 km depth offshore Lombok is absent offshore Sumba, where our models reveal the transition to the collisional regime farther to the east and to the Sumba block in the north. Our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern Sunda margin.

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.

From Subduction to Collision: The Sunda‐Banda Arc Transition

In the aftermath of the Mw 9.3 Indian Ocean earthquake and tsunami of 26 December 2004, which killed more than 250,000 people, numerous investigations have been commissioned near the epicenter offshore northern Sumatra to evaluate future earthquake and tsunami hazards. These projects have mapped seafloor morphology and imaged deep structures and faults in order to better understand the origin of megathrust earthquakes and tsunamis in the western portion of the Sunda Arc subduction system offshore northern Sumatra [e.g., Henstock et al., 2006]. In contrast, the eastern part of the arc has received relatively little attention, even though it may be just as hazardous. Our geophysical data from the eastern Sunda Arc and the transition to the Banda Arc (Figure 1) provide evidence for recent tectonic activity and thus for a similar earthquake and tsunami risk.

Seismic Survey of the Locked and Unlocked Sumatra Subduction Zone

The Sumatra subduction zone is the most seismically active region on Earth. In the past 5 years, it has been the site of three great earthquakes, including the 26 December 2004 Sumatra-Andaman earthquake. That event produced a devastating tsunami around the Indian Ocean that claimed approximately 230,000 lives and caused terrible damage and destruction. Part of the subduction zone still is locked and is likely to break in the next decade or so. To study the seismic and tsunami risk in this locked region, a deep seismic reflection survey, the Tsunami Investigation—Deep Evaluation Seismic (TIDES) project, was carried out in May 2009 using a CGGVeritas vessel towing a 15-kilometer-long streamer, the longest ever used during a seismic survey. The survey should provide the first ever seismic images of the locked zone from the seafloor down to a depth of 50 kilometers.

Petroleum systems of the Simeulue fore-arc basin, offshore Sumatra, Indonesia

Forearc basins result from plate convergence. These basins are situated offshore between an outer-arc high and the mainland. Historically, these regions have not been considered important petroleum provinces partly because low heat flow may limit significant thermal hydrocarbon generation. The Simeulue forearc basin extends between Simeulue Island and northern Sumatra. It is a frontier shallow shelf area with few wells and no wells in the basin center; therefore, it is studied using geophysical data and geologic surface samples. Multichannel seismic data show bright spots above potential hydrocarbon reservoirs in carbonate buildups. Amplitude versus offset analyses indicate the presence of gas, and surface geochemical prospecting suggests thermal hydrocarbon generation. Heat flow in the Simeulue Basin ranges between 37 and 74 mW/m2, as deduced from one-dimensional petroleum system modeling and bottom-simulating reflector depths. Two possible source rocks (Eocene and lower–middle Miocene) were assigned for three-dimensional petroleum system modeling in the Simeulue Basin. Because of a similar pre-Miocene geologic evolution of the present-day back arc and the fore arc, it can be assumed that the back-arc source rocks also occur in the fore arc. Modeling based on two heat-flow scenarios (40 and 60 mW/m2) reveals that oil and gas generation is possible within and below the main depocenters of the central and southern Simeulue Basin. This study shows that deep burial (6 km [3.7 mi]) of source rocks can compensate for low heat flow. Therefore, forearc basins may be more prolific for hydrocarbons than previously considered, and each forearc basin should be studied carefully to evaluate its hydrocarbon potential.

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.