Description: At convergent margins, the structure of the subducting oceanic plate is one of the key factors controlling the morphology of the upper plate. We use high-resolution seafloor mapping and multichannel seismic reflection data along the accretionary Sumatra trench system to investigate the morphotectonic response of the upper plate to the subduction of lower plate fabric. Upper plate segmentation is reflected in varying modes of mass transfer. The deformation front in the southern Enggano segment is characterized by neotectonic formation of a broad and shallow fold-and-thrust belt consistent with the resumption of frontal sediment accretion in the wake of oceanic relief subduction. Conversely, surface erosion increasingly shapes the morphology of the lower slope and accretionary prism towards the north where significant oceanic relief is subducted. Subduction of the Investigator Fracture Zone and the fossil Wharton spreading centre in the Siberut segment exemplifies this. Such features also correlate with an irregularly trending deformation front suggesting active frontal erosion of the upper plate. Lower plate fabric extensively modulates upper plate morphology and the large-scale morphotectonic segmentation of the Sumatra trench system is linked to the subduction of reactivated fracture zones and aseismic ridges of the Wharton Basin. In general, increasing intensity of mass-wasting processes, from south to north, correlates with the extent of oversteepening of the lower slope (lower slope angle of 3.8 degrees in the south compared with 7.6 degrees in the north), probably in response to alternating phases of frontal accretion and sediment underthrusting. Accretionary mechanics thus pose a second-order factor in shaping upper plate morphology near the trench.

Description: Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. Morphological structures in multibeam bathymetric data across the forearc relate with the extent of the seismogenic zone. Its updip limit corresponds to the slope break, most distinct off Java and Lesser Sunda islands, where we find coincident narrow, uniform, continuous outer arc ridges. Their landward termination and a shallow upper plate mantle mark the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. The downdip limit of the seismogenic zone coincides with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault.

Description: A bottom simulating reflector (BSR), which marks the base of the gas hydrate stability zone, has been detected for the first time in seismic data of the Black Sea. The survey area is in the northwestern Black Sea at 44°–45°N and 31.5°–32.5°E. In this paper, seismic wide-angle ocean bottom hydrophone (OBH) and ocean bottom seismometer (OBS) data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment. An image of the subsurface is computed from wide-angle data by using Kirchhoff depth migration. The image shows the BSR at 205–270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the BSR. The top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals. An analysis of traveltimes and reflection amplitudes leads to 1-D P-wave velocity–depth and density–depth models. An average S-wave velocity of 160 m s−1 between the seafloor and the BSR is determined from the traveltime of the P to S converted wave. The normal incidence PP reflection coefficient at the BSR is −0.11, where the P-wave velocity decreases from 1840 to 1475 m s−1. Velocities and density are used to compute the porosity and the system bulk modulus as a function of depth. The Gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation, which depend on porosity and on the bulk moduli of the dry and saturated sediment. A gas hydrate saturation–depth profile is obtained, which shows that there is 38 ± 10 per cent hydrate in the pore space at the BSR depth, where the porosity is 57 per cent (OBS 24). This value is derived for the case that the gas hydrate does not cement the sediment grains, a model that is supported by the low S-wave velocities. There is 0.9 or 0.1 per cent free gas in the sediment below the BSR, depending on the model for the gas distribution in the sediment. The free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity, which can be identified in the subsurface image.

Description: The continental margin of Nicaragua and Costa Rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab, a decreasing magma production and a shift in the volcanic front. To investigate this transition, a joint on- and offshore local earthquake tomography was performed. Low P-wave velocities and high Vp/Vs ratios, indicative for hydration, were found in the upper-mantle and lowermost crust beneath the Sandino Basin. The mantle wedge hydration can be estimated to 2.5 wt. per cent beneath south Nicaragua. In contrast, the mantle wedge beneath north Costa Rica is weakly or not hydrated. The hydration leads to a local gap in the seismicity in Nicaragua. The lateral transition between the hydrated and non-hydrated areas occurs within a distance of about 10 km. This transition coincides with a change in the crustal thickness in the order of 5–10 km, thickening to the south, and in the tectonic regimes. The change in the tectonic regimes towards a stronger extension along the margin of Nicaragua could be the key for understanding the observations: the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low Vp, high Vp/Vs and low seismicity.

Description: Great subduction earthquakes exhibit segmentation both within the rupture of individual events and in the long term history of the margin. The 2004 December 26 Aceh-Andaman and 2005 March 28 Nias event in northern Sumatra are two of the largest earthquakes in recent years, with both co- and post-seismic displacements constrained in unprecedented detail. Using aftershock locations from a temporary seismic array in the boundary region between both events and waveform modelling of large aftershocks, we demonstrate that the vast majority of aftershocks in the study region occur on the plate interface within a narrow band ( 20 km) seaward of the outer arc high. Comparing the seismicity distribution to the co- and post-seismic displacements, we infer that the seismic band marks the transition between the seismogenic zone and stable sliding. The location of the band and therefore the transition appears to be correlated with the ∼500 m bathymetry contour. This close correspondence is disrupted at the boundary between the two great earthquakes, where the transition to seismogenic behaviour occurs further landward by ∼25 km. To the west of Simeulue, where seafloor bathymetry throughout the forearc is deeper than 500 m, the seismic band terminates abruptly and the focus of aftershock activity is found near the trench. The seismic efficiency of afterslip varies dramatically along strike: the segment below the Banyak islands, in the gap between the two main asperities of the Nias earthquake, accommodates a much larger proportion of afterslip seismically.

Description: The structure and seismicity of the subduction zone of centralCosta Rica have been investigated with local earthquake tomography down to ca. 50 km depth. Seismic traveltime data sets of three on- and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and Vp/Vs ratio using about 2000 highquality events. The seismicity and slab geometry as well as Vp and Vp/Vs show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the Vp and Vp/Vs tomograms: a high-velocity zone with a perturbation of 4–10 per cent representing the subducting slab, a low-velocity zone (10–20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30◦ to 45◦ is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.

Description: On the Pacific margin off central Costa Rica, an anomalous lens-shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front. This feature was previously recognized in reflection seismic data when it was termed 'megalens'. Its origin and seismic velocity structure, however, could not unambiguously be derived from earlier studies. Therefore during RV SONNE cruise SO163, seismic wide-angle data were acquired in 2002 using closely spaced ocean bottom hydrophones and seismometers along two parallel strike and two parallel dip lines above the 'megalens', intersecting on the middle slope. The P-wave velocities and structure of the subducting oceanic Cocos Plate and overriding Caribbean Plate were determined by modelling the wide-angle seismic data in combination with the analysis of coincident reflection seismic data and the use of synthetic seismograms. The margin wedge is defined by high seismic velocities (4.3-6.1 km s(-1)) identified within a wedge-shaped body covered by a slope sediment drape. It is divided into two layers with different velocity gradients. The lower margin wedge is clearly constrained by decreasing velocities trenchward and terminates beneath the middle slope at the location of the 'megalens'. Seismic velocities of the 'megalens' are lower (3.8-4.3 km s(-1)) relative to the margin wedge. We propose that the 'megalens' represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate. Upward-migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing, thus causing material transfer from the upper plate to the lower plate. Results from amplitude modelling support that the 'megalens' observed off central Costa Rica is bound by a low-velocity zone documenting fluid drainage from the plate boundary to the upper plate.

Description: The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw 〉 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.

Description: The Galápagos volcanic province (GVP) includes several aseismic ridges resulting from the interaction between the Galápagos hotspot (GHS) and the Cocos–Nazca spreading centre (CNSC). The most prominent are the Cocos, Carnegie and Malpelo ridges. In this work, we investigate the seismic structure of the Carnegie ridge along two profiles acquired during the South American Lithospheric Transects Across Volcanic Ridges (SALIERI) 2001 experiment. Maximum crustal thickness is ∼19 km in the central Carnegie profile, located at ∼85°W over a 19–20 Myr old oceanic crust, and only ∼13 km in the eastern Carnegie profile, located at ∼82°W over a 11–12 Myr old oceanic crust. The crustal velocity models are subsequently compared with those obtained in a previous work along three other profiles over the Cocos and Malpelo ridges, two of which are located at the conjugate positions of the Carnegie ones. Oceanic layer 2 thickness is quite uniform along the five profiles regardless of the total crustal thickness variations, hence crustal thickening is mainly accommodated by layer 3. Lower crustal velocities are systematically lower where the crust is thicker, thus contrary to what would be expected from melting of a hotter than normal mantle. The velocity-derived crustal density models account for the gravity and depth anomalies considering uniform and normal mantle densities (3300 kg m−3), which confirms that velocity models are consistent with gravity and topography data, and indicates that the ridges are isostatically compensated at the base of the crust. Finally, a two-dimensional (2-D) steady-state mantle melting model is developed and used to illustrate that the crust of the ridges does not seem to be the product of anomalous mantle temperatures, even if hydrous melting coupled with vigorous subsolidus upwelling is considered in the model. In contrast, we show that upwelling of a normal temperature but fertile mantle source that may result from recycling of oceanic crust prior to melting, accounts more easily for the estimated seismic structure as well as for isotopic, trace element and major element patterns of the GVP basalts.

Description: The Central Costa Rican Pacific margin is characterized by a high-seismicity rate, coincident with the subduction of rough-relief ocean floor and has generated earthquakes with magnitude up to seven in the past. We inverted selected P-wave traveltimes from earthquakes recorded by a combined on- and offshore seismological array deployed during 6 months in the area, simultaneously determining hypocentres and the 3-D tomographic velocity structure on the shallow part of the subduction zone (〈70 km). The results reflect the complexity associated to subduction of ocean-floor morphology and the transition from normal to thickened subducting oceanic crust. The subducting slab is imaged as a high-velocity perturbation with a band of low velocities (LVB) on top encompassing the intraslab seismicity deeper than ∼30 km. The LVB is locally thickened by the presence of at least two subducted seamounts beneath the margin wedge. There is a general eastward widening of the LVB over a relatively short distance, closely coinciding with the onset of an inverted forearc basin onshore and the appearance of an aseismic low-velocity anomaly beneath the inner forearc. The latter coincides spatially with an area of the subaerial forearc where differential uplift of blocks has been described, suggesting tectonic underplating of eroded material against the base of the upper plate crust. Alternatively, the low velocities could be induced by an accumulation of upward migrating fluids. Other observed velocity perturbations are attributed to several processes taking place at different depths, such as slab hydration through outer rise faulting, tectonic erosion and slab dehydration.