Description: High-resolution seismic experiments, employing arrays of closely spaced, four-component ocean-bottom seismic recorders, were conducted at a site off western Svalbard and a site on the northern margin of the Storegga slide, off Norway to investigate how well seismic data can be used to determine the concentration of methane hydrate beneath the seabed. Data from P-waves and from S-waves generated by P–S conversion on reflection were inverted for P- and S-wave velocity (Vp and Vs), using 3D travel-time tomography, 2D ray-tracing inversion and 1D waveform inversion. At the NW Svalbard site, positive Vp anomalies above a sea-bottom-simulating reflector (BSR) indicate the presence of gas hydrate. A zone containing free gas up to 150-m thick, lying immediately beneath the BSR, is indicated by a large reduction in Vp without significant reduction in Vs. At the Storegga site, the lateral and vertical variation in Vp and Vs and the variation in amplitude and polarity of reflectors indicate a heterogeneous distribution of hydrate that is related to a stratigraphically mediated distribution of free gas beneath the BSR. Derivation of hydrate content from Vp and Vs was evaluated, using different models for how hydrate affects the seismic properties of the sediment host and different approaches for estimating the background-velocity of the sediment host. The error in the average Vp of an interval of 20-m thickness is about 2.5%, at 95% confidence, and yields a resolution of hydrate concentration of about 3%, if hydrate forms a connected framework, or about 7%, if it is both pore-filling and framework-forming. At NW Svalbard, in a zone about 90-m thick above the BSR, a Biot-theory-based method predicts hydrate concentrations of up to 11% of pore space, and an effective-medium-based method predicts concentrations of up to 6%, if hydrate forms a connected framework, or 12%, if hydrate is both pore-filling and framework-forming. At Storegga, hydrate concentrations of up to 10% or 20% were predicted, depending on the hydrate model, in a zone about 120-m thick above a BSR. With seismic techniques alone, we can only estimate with any confidence the average hydrate content of broad intervals containing more than one layer, not only because of the uncertainty in the layer-by-layer variation in lithology, but also because of the negative correlation in the errors of estimation of velocity between adjacent layers. In this investigation, an interval of about 20-m thickness (equivalent to between 2 and 5 layers in the model used for waveform inversion) was the smallest within which one could sensibly estimate the hydrate content. If lithological layering much thinner than 20-m thickness controls hydrate content, then hydrate concentrations within layers could significantly exceed or fall below the average values derived from seismic data.

Description: A joint interpretation of swath bathymetric, seismic refraction, wide-angle reflection, and multichannel seismic data was used to derive a detailed tomographic image of the Nazca-South America subduction zone system offshore southern Arauco peninsula, Chile at similar to 38 degrees S. Here, the trench basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ) is obliquely subducting underneath the South American plate. The velocity model derived from the tomographic inversion consists of a similar to 7-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities >8.4 km s(-1). In the trench-outer rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst-and-graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending-related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10-15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 6-8 km into the upper mantle, where mantle temperatures are estimated to be 400-430 degrees C. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a similar to 20 km wide frontal prism below the continental slope with Vp 〈= 3.5 km s(-1), (2) a similar to 50 km area with Vp = 4.5-5.5 km s(-1), interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp >= 6.0 km s(-1). Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.

Description: A seismic wide‐angle and refraction experiment was conducted offshore of Nicaragua in the Middle American Trench to investigate the impact of bending‐related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. On the basis of the reflectivity pattern of multichannel seismic reflection (MCS) data it has been suggested that bending‐related faulting facilitates hydration and serpentinization of the incoming oceanic lithosphere. Seismic wide‐angle and refraction data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent normal faults on the seaward trench slope. A tomographic joint inversion of seismic refraction and wide‐angle reflection data yield anomalously low seismic P wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2–0.5 km s−1 compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6–7.8 km s−1 and hence 5–7% lower than the typical velocity of mantle peridotite. These systematic changes in P wave velocity from the outer rise toward the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. If hydration is indeed affecting the seismic properties of the mantle, serpentinization might be reaching 12–17% in the uppermost 3–4 km of the mantle, depending on the unknown degree of fracturing and its impact on the elastic properties of the subducting lithosphere.

Description: The Dalrymple Trough marks part of the transform plate boundary between India and Arabia in the northern Arabian Sea. Oblique extension is presently active across this portion of the boundary at a rate of a few millimetres per year, and seismic reflection profiles across the trough confirm that it is an extensional structure. We present new swath bathymetric and wide-angle seismic data from the trough. The bathymetric data show that the trough is bounded by a single, steep, 3-km-high scarp to the southeast and a series of smaller, en-echelon scarps to the northwest. Wide-angle seismic data show that a typical oceanic crustal velocity structure is present to the northwest, with a crustal thickness of ~ 6 km. There is an abrupt change in crustal thickness and velocity structure at the northwestern edge of the trough, and the trough itself is underlain by 12-km-thick crust interpreted as thinned continental crust. Therefore we infer that Dalrymple Trough is an unusual obliquely extending plate boundary at which continental crust and oceanic crust are juxtaposed. The extensional deformation is focused on a single major fault in the continental lithosphere, but distributed over a region ~ 60 km wide in the oceanic lithosphere

Description: We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean bottom seismometers on the outer rise offshore southern Chile on young oceanic plate of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from December 2004–January 2005 and consisted of 17 instruments and 12 instruments, respectively. Approximately 10 locatable events per day were recorded by each of the arrays. The catalogue, which is complete for magnitudes above 1.2–1.5, is characterized by a high b value, i.e., a high ratio of small to large events, and the data set is remarkable in that a large proportion of the events form clusters whose members show a high degree of waveform similarity. The largest cluster thus identified consisted of 27 similar events (average inter-event correlation coefficient 〉 0.8 for a 9.5 s window), and waveform similarity persists far into the coda. Inter-event spacing is irregular, but very short waiting times of a few minutes are far more common than expected from a Poisson distribution. Seismicity with these features (high b value, large number of similar events with short waiting times) is typical of swarm activity, which, based on empirical evidence and theoretical considerations, is generally thought to be driven by fluid pressure variations. Because no pronounced outer rise bulge exists on the very young plate in the study region, it is unlikely that melt is accessible from decompression melting or opening of cracks. A fluid source related to processes at the nearby ridge is conceivable for the younger segment but less likely for the older one. We infer that the fluid source could be seawater, which enters through fractures in the crust. Most of the similar-earthquake clusters are within the crust, but some of them locate significantly below the Moho. If our interpretation is correct, this implies that water is present within the mantle. Hydration of the mantle is also indicated by a decrease of Pn velocities below the outer rise seen on a refraction profile through one of the arrays [Contreras-Reyes, E., Grevemeyer, I., Flueh, E.R., Scherwath, M., Heesemann, M., 2007. Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise. Geochem., Geophys. Geosyst. 8, Q07003.]. The deepest events within the array on the 6 Ma old plate occur where the temperature reaches 500–600 °C, consistent with the value observed for large intraplate earthquakes within the mantle (650 °C), suggesting that the maximum temperature at which these fluid-mediated micro-earthquakes can occur is similar or identical to that of large earthquakes.