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Cruise P453 & P458 with R/V POSEIDON aim to conduct 3D wide-angle ocean bottom recording (OBR) da... more Cruise P453 & P458 with R/V POSEIDON aim to conduct 3D wide-angle ocean bottom recording (OBR) data during the acquisition of 3D multichannel seismic (MCS) reflection data (D. Sawyer, Rice University & T.J. Reston, Birmingham) and to analyze and interpret a highresolution densely sampled 2D OBR wide-angle profile. The primary goal of the wide-angle data will be to provide an accurate and detailed 3D P-wave velocity model for the 3D reflection data. Particularly the densely sampled 2D profile will help to determine the degree of thinning within the crust and the degree of serpentinisation of the uppermost mantle. The collection of 3D MCS data gives the unique opportunity to obtain densely sampled water column reflection data and to analyze spatial and temporal (4D) variations of the internal wave field which will yield new understanding and insights into water mass mixing processes offshore west Iberia. Contemporaneously hydrographic data will be collected to calibrate and analyze the oceanic thermohaline structures originating from the interaction between Atlantic waters and the Mediterranean Sea Outflow. Map showing survey area (red box) of the COMM3D project offshore the Iberian Margin.
The nature of the Ionian Sea crust has been the subject of scientific debate for more than 30 yea... more The nature of the Ionian Sea crust has been the subject of scientific debate for more than 30 years, mainly because seismic imaging of the deep crust and upper mantle of the Ionian Abyssal Plain (IAP) has not been conclusive to date. The IAP is sandwiched between the Calabrian and Hellenic subduction zones in the central Mediterranean. A NNE-SSW-oriented 131 km long seismic refraction and wide-angle reflection profile, consisting of eight ocean bottom seismometers and hydrophones, was acquired in 2014. The profile was designed to univocally confirm the proposed oceanic nature of the IAP crust as a remnant of the Tethys and to confute its interpretation as a strongly thinned part of the African continental crust. A P-wave velocity model developed from travel-time forward modelling is refined by gravimetric data and synthetic modelling of the seismic data. A roughly 6-7 km thick crust with velocities ranging from 5.1 to 7.2 km s −1 , top to bottom, can be traced throughout the IAP. In the vicinity of the Medina seamounts at the southern IAP boundary, the crust thickens to about 9 km and seismic velocities decrease to 6.8 km s −1 at the crust-mantle boundary. The seismic velocity distribution and depth of the crustmantle boundary in the IAP document its oceanic nature and support the interpretation of the IAP as a remnant of the Tethys lithosphere with the Malta Escarpment as a transform margin and a Tethys opening in the NNW-SSE direction.
High-resolution 3D seismic data in combination with deep-towed sidescan sonar data and porewater ... more High-resolution 3D seismic data in combination with deep-towed sidescan sonar data and porewater analysis give insights into the seafloor expression and the plumbing system of the actively gas emitting Kerch seep area, which is located in the northeastern Black Sea in around 900 m water depth, i.e. well within the gas hydrate stability zone (GHSZ). Our analysis shows that the Kerch seep consists of three closely spaced but individual seeps above a paleo-channel-levee system of the Don Kuban deep-sea fan. We show that mounded seep morphology results from sediment up-doming due to gas overpressure. Each of the seeps hosts its own gas pocket underneath the domes which are fed with methane of predominantly microbial origin along narrow pipes through the GHSZ. Methane transport occurs dominantly in the form of gas bubbles decoupled from fluid advection. Elevated sediment temperatures of up to 0.3 °C above background values are most likely the result of gas hydrate formation within the uppermost 10 meters of the sediment column. Compared to other seeps occurring within the GHSZ in the Black Sea overall only scarce gas indications are present in geoacoustic and geophysical data. Transport-reaction modeling suggests that the Kerch seep is a young seep far from steady state and probably not more than 500 years old.
Subduction-related structure at the southwest end (Albatross segment) of the Mw 9.2, 1964 megathrust rupture area offshore Kodiak Island, Alaska
Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occur... more Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture processes and their mechanisms, we relate seafloor morphology from multibeam and regional bathymetric compilations with sub-seafloor images and seismic P-wave velocity structures. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage (1994) wide angle dataset reveals the P-wave velocity distribution as well as the deep structure of the subducting plate to the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by a thick sediment cover that reaches ~3 km depth at the trench axis. Bathymetry shows two major contrasting upper plate morphologies: the shallow dipping lower slope consists of trench-parallel ridges that form the accreted prism whereas the steep rough middle and upper slopes are composed of competent older rock. Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a splay fault zone marking the boundary between the margin framework and the frontal prism. It corresponds to the most prominent lateral increase in seismic P-wave velocities, ~25 km landward of the trench axis. Major thrusts in several MCS-lines are correlated with bathymetric data, showing their > 100 km lateral extent, which might also be tsunamigenic paths of earthquake rupture from the seismogenic zone to the seafloor. The splay fault zone has been recognized as a potential tsunamigenic structure in the 1938 and 1946 earthquake rupture areas.
Influence on subduction boundary segmentation at the southwest end (Albatross segment) of the Mw 9.2, 1964 Kodiak Island, and the northeast end (Semidi segment) of the Mw 8.3 megathrust rupture areas offshore Alaska
Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occur... more Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture propagation, with a focus on barriers that limit rupture, i.e. the southwest end of the Albatross segment at the boundary with the 1938 Semidi Mw 8.3 earthquake segment, we relate multibeam seafloor morphology with sub-seafloor images and seismic P-wave velocity structure. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration and/or MCS traveltime tomography. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage wide-angle dataset reveals the P-wave velocity and the deep structure of the subducting plate to depths of the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by thick sediment cover that reaches ~3 km in the Albatross and ~1.5-2 km thickness in the Semidi segment at the trench axis. In both segments, bathymetry shows two major contrasting upper plate morphologies: trench-parallel ridges form the accreted prism of the shallow dipping lower slope whereas the steep rough middle and upper slopes are composed of competent older rock. Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a splay fault zone (SFZ) marking the boundary between the margin framework and the frontal prism. This SFZ corresponds to the most prominent lateral increase in seismic P-wave velocities, ~25 km landward of the trench axis. Major differences in the Albatross/Semidi segments are 1.) origin of subducting sediment (Surveyor vs. Kodiak fan) 2.) geometry of the subducting plate (gentle vs. steep dip) beneath the lower and middle slopes.
Subduction-Related Structure in the Mw 9.2, 1964 Megathrust Rupture Area Offshore Kodiak Island, Alaska
Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occur... more Some of the largest earthquakes worldwide, including the 1964 9.2 Mw megathrust earthquake, occurred in Alaskan subduction zones. To better understand rupture processes and their mechanisms, we relate seafloor morphology from multibeam and regional bathymetric compilations with sub-seafloor images and seismic P-wave velocity structures. We re-processed legacy multichannel seismic (MCS) data including shot- and intra-shotgather interpolation, multiple removal and Kirchhoff depth migration. These images even reveal the shallow structure of the subducting oceanic crust. Traveltime tomography of a coincident vintage (1994) wide angle dataset reveals the P-wave velocity distribution as well as the deep structure of the subducting plate to the ocean crust Moho. The subducting oceanic crust morphology is rough and partly hidden by a thick sediment cover that reaches ~3 km depth at the trench axis. Bathymetry shows two major contrasting upper plate morphologies: the shallow dipping lower slope consists of trench-parallel ridges that form the accreted prism whereas the steep rough middle and upper slopes are composed of competent older rock. Thrust faults are distributed across the entire slope, some of which connect with the subducted plate interface. A subtle change in seafloor gradient from the lower to the middle slope coincides with a thrust fault zone marking the boundary between the margin framework and the frontal prism. It corresponds to the most prominent lateral increase in seismic P-wave velocities, ~25 km landward of the trench axis. Major thrusts in several MCS-lines are correlated with bathymetric data, showing their > 100 km lateral extent, which might also be tsunamigenic paths of earthquake rupture from the seismogenic zone to the seafloor.
Seismic images of mixed water masses
Natural Hydrofractures, Gas Seeps, and Hydrates Offshore Hawke’s Bay: A High-Resolution 3D Seismic Investigation
Geophysical Observations Indicating Mechanisms that Caused the 1946 Unimak Tsunamis
AGUFM, Dec 1, 2016
Hydrophone component of Ocean bottom recording sgy-files of seismic refraction and wide angle data from profile 1172 and 2222 of expedition MGL1307 and POSEIDON cruise POS453 with links to data files
Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has b... more Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent‐ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide‐angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0–7.0 km s−1 velocity contours, corresponding to peridotite serpentinization of 60–30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8–3.5 km, with an average velocity gradient of 1.0 s−1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.
Co-existence of gas hydrates and free gas in the Black Sea: High-flux versus low-flux areas
Distribution of free gas and 3D mirror image structures beneath Sevastopol mud volcano, Black sea, from 3D high resolution wide-angle seismic data
The goal of this study is to image the sub-seafloor structure beneath the Sevastopol mud volcano ... more The goal of this study is to image the sub-seafloor structure beneath the Sevastopol mud volcano (SMV), Sorokin Trough, SE of the Crimean peninsula, Black Sea. The focus lies on structures of/within the feeder channel, the distribution of gas and gas hydrates, and their relation to fluid migration zones in sediments. This study concentrates on a 3D high resolution seismic grid (7 km x 2.5 km) recorded with 13 ocean bottom stations (OBS). The 3D nature of the experiment results from the geometry of 68 densely spaced (25/50 m) profiles, as well as the cubical configuration of the densely spaced receivers on the seafloor (~300 m station spacing). The seismic profiles are typically longer than 6 km which results in large offsets for the reflections of the OBS. This enables the study of the seismic velocities of the sub-seafloor sediments and additionally large offset incident analysis. The 3D Kirchhoff mirror image [1] time migration, applied to all OBS sections including all shots from all profiles, leads to a spatial image of the sub-seafloor. Here, the migration was applied with the velocity distribution of 1.49 km/s in the water column, 1.5 km/s below the seafloor (bsf) increasing to 2 km/s for the deeper sediments at ~2 s bsf. Acoustic blanking occurs beneath the south-easterly located OBS and is associated with the feeder channel of the mud volcano. There, gas from depth can vertically migrate to the seafloor and on its way to the surface horizontally distribute patchily within sediment layers. High amplitude reflections are not observed as continuous reflections, but in a patchy distribution. They are associated with accumulations of gas. Also structures exist within the feeder channel of the SMV. 3D mirror imaging proves to be a good tool to seismically image structures compared with 2D streamer seismics, especially steep dipping reflectors and structures which are otherwise obscured by signal scattering, i.e structures associated with fluid migration paths.
Internal wave energy estimated from seismic reflection data
The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the s... more The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus control...
Journal of Geophysical Research: Solid Earth, 2020
Large amounts of methane, a potent greenhouse gas, are stored in hydrates beneath the seafloor. S... more Large amounts of methane, a potent greenhouse gas, are stored in hydrates beneath the seafloor. Sea level changes can trigger massive methane release into the ocean. It is not clear, however, whether surficial seafloor processes can cause comparable discharge. Previously, fluid migration was difficult to study due to a lack of spatially dense seismic and thermal observations. Here we examine a gas hydrate site at Four‐Way‐Closure Ridge off SW Taiwan using a high‐resolution 3‐D seismic cube, together with bottom‐simulating reflections (BSRs) mapped in the cube, a thermal probe data set, and 3‐D thermal modeling results. We document, on a scale of tens of meters, the interaction between surficial sedimentary processes, fluid flow, and a dynamic gas hydrate system. Fluid migrates upward through dipping permeable strata in the limb, the slope basin, and along thrust faults and ridge‐top normal faults. The seismic data also reveal several double BSRs that underlie seabed sedimentary slid...
Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterized ... more Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterized by a complex pattern of faulting, thin continental fault blocks and the serpentinization, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Traveltime tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here, we demonstrate the application of 2-D time-domain acoustic full-waveform inversion (FWI) to deep-water seismic data collected at the Deep Galicia Margin, in order to attain a high-resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modeled waveforms, checkerboard tests, testing of parameter and inversion strategy and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector, there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinization. This study supports the hypothesis that normal faulting acts to hydrate the upper-mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinization. Our results confirm the feasibility of applying the FWI method to sparse, deep-water crustal data sets.
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Papers by Dirk Klaeschen