Description: In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across the continental margin using ocean-bottom seismometers (OBS) and hydrophones (OBH) as well as land recorders. Two-dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles. The subducting oceanic crust of the Juan de Fuca plate can be traced from the trench to its position some 10 km landward of the coastline. At the coastline, the Moho has a depth of 30 km. The dip of the plate changes from 1.5° westward of the trench to about 6.5° below the accretionary complex and to about 16° further eastward below the coast. The backstop forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America about 50 Ma ago, is well defined by the observations. It is located about 60 km to the east of the deformation front and has a seaward dip of 40°. At its seaward edge, the base of the Siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths. The upper oceanic crust is thinner to the east of this contact than to the west. At depths greater than 18 km, the top of the oceanic crust is the origin of pre-critical reflections observable in several land recordings and in the data of one ocean bottom instrument. These reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition.

Description: The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000-km-long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y. In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore-onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

Description: Seismic investigations across the convergent Sunda margin off Indonesia provide a detailed image of the crustal architecture of the Sunda plate boundary. The combined analysis and interpretation of wide-angle and reflection seismic data along two coincident profiles across the subduction zone are complemented by additional lines within the forearc domain, which yield some three-dimensional (3-D) constraints on the velocity-depth structure across the margin. A detailed cross section of the subduction zone is presented, which is confirmed by supplementary gravity modeling. The Sunda convergence zone is a prime example of an accretionary margin, where sediment accretion has led to the formation of a massive accretionary prism, with a total width of 〉110 km between the trench and the forearc basin. It is composed of a frontal wedge which documents ongoing accretion and a fossil part behind the present backstop structure which constitutes the outer high. Moderate seismic velocities derived from wide-angle modeling indicate a sedimentary composition of the outer high. The subducting oceanic slab is traced to a depth of almost 30 km underneath the accretionary prism. The adjacent forearc domain is characterized by a pronounced morphological basin which is underlain by a layer of increased seismic velocities and a shallow upper plate Moho at 16 km depth. We speculate that remnant fragments of oceanic crust might be involved in the formation of this oceanic-type crust found at the leading edge of the upper plate beneath the forearc basin.

Description: Marine seismic wide-angle data acquisition and earthquake seismology observations are at the verge of a quantum leap in data quality and density. Advances in micro-electronic technology facilitates the construction of instrumcnts that enable large data volumes to be collected and that are small and cheap enough so that large numbers can be built and operated economically. The main improvements are a dramatic decrease of power consumption ( 〈 250 m W) and increase in clock stability ( 〈 0.05 ppm}. Several scenarios for future experiments arc discussed in this contrihution

Description: A seismic wide-angle section offshore Costa Rica is presented across the boundary between oceanic crust generated at the East Pacific rise (EPR) and at the Galápagos spreading center (GSC) as indicated by magnetic anomalies. This suture, where the Farallon plate broke up ∼23 Ma ago, is marked by pronounced velocity variations throughout the crust including a low-velocity body in the lower crust. This body is well constrained by refracted waves above the inversion zone and by strong PmP reflections from its lower boundary. The distinctness of this body and the local gravity field point to an igneous intrusion rather than serpentinized rock. Typical oceanic crust is found adjacent to the suture zone.

Description: The Cascadia convergent margin is a first-order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion. The near-trench part of the accretionary prism offshore Washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding Quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges. Multichannel seismic reflection data acquired on the ORWELL project allows us to study the structure and dynamics of the atypical frontal accretion processes. We performed a kinematical and mechanical analysis of the frontal accretion structures, and developed a dynamic Coulomb-wedge model for the landward-verging backthrust formation. Backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous Cascadia basin sediment succession entering the subduction zone, and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation. These complex deformation processes cause the passive “upward” delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle-zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits. The deformation mechanism at the tectonic front of the Cascadia margin is an immediate response to the strongly increased late Pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought.

Description: We report measurements made with an ocean bottom array which was operated for 10 days on the Mid-Atlantic Ridge just south of the 5°S transform fault/fracture zone. A total of 148 locatable earthquakes with magnitudes ∼0.5–2.8 were recorded; seismic activity appears to be concentrated within the western half of the median valley. The median valley seismic zone is bounded in along-axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south. A few scattered events occurred within the inside corner high, on the transform fault, and in the western sidewall close to the segment center. Earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle, although a few crustal earthquakes also occurred. The presence of earthquakes in the mantle indicates that it is not strongly serpentinized. We infer the median valley seismic activity to primarily arise from normal faulting.

Description: The paper presents an overview of recent seafloor long-term single-frame multiparameter platform developed in the framework of the European Commission and Italian projects starting from the GEOSTAR prototype. The main features of the different systems are described as well as the sea missions that led to their validation. The ORION seafloor observatory network recently developed, based on the GEOSTAR-type platforms and engaged in a deep-sea mission at 3300 m w.d. in the Mediterranean Sea, is also described

Description: Introduction Since the discovery of ‘bright spots’ associated with hydrocarbon deposits, ever increasing interest in determining lithological subsurface parameters has been a driving force for technological development in the hydrocarbon exploration industry. Quantification of lithological parameters is of utmost importance for reservoir prediction and monitoring. Amongst various attempts to determine these, attribute analysis of pwave data and the direct observation of shear wave data are the most visible and successful methods applied. The direct observation of shear waves in the marine environment has been attempted by several means, mainly using ocean bottom cables (OBC) that have three-component geophones (3C) and a hydrophone in addition (thus 4C in total). Some manufacturers offer two component geophones with only one horizontal component. These cables are laid out on the seafloor, sometimes even buried using specialized tools like ROVs (remotely operated vehicles). Data transfer is through the cables as in streamers or land operations, recording is made on a boat or platform where the cable terminates. Geophones are housed in tubes with a self-levelling gimballed mounting system, damped by a viscous fluid. This technique is regarded as proven technology and has been widely accepted. Especially in production areas with many man-made obstacles, this technique also offers a safe operation, and is especially suitable for monitoring purposes (4D–4C seismic). Any desired geometry and density of receivers can be laid out. Direct shear wave observations have been made by several academic institutions, both for active seismic exploration as well as for passive seismological monitoring of earthquakes. These institutions have built ocean bottom seismometers (OBS), which are also four component, two sensor instruments. Unlike OBC, they are autonomously lowered to the seafloor, record within specified time windows, and are later brought back to the surface. Amongst the various instruments designed over the past decades is the OBS range built at GEOMAR, which – due to its modular design – has been used for a wide range of applications.