Author: Allan Wayne Sauter
Publisher:
ISBN:
Category : Ocean bottom
Languages : en
Pages : 234

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Book Description

Author: Junzo Kasahara
Publisher:
ISBN:
Category : Marine geophysics
Languages : en
Pages : 108

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Book Description
An ocean bottom seismograph was deployed on the seaward side of the Kuril Trench off Hokkaido, Japan, in 5460 m of water, in August 1975 during the joint Soviet-American Tsunami Expedition. During the seven-day record, S-P times were distributed in three groups: 19-24 sec, corresponding to aftershocks of the 10 June and 13 June 1975 earthquakes southeast of Nemuro, Japan and to earthquakes east of Sanriku, Japan; approximately 30 sec, from south of Erimo Peninsula, Hokkaido; and approximately 100 sec, from the Izu-Bonin Islands. Seven earthquakes, with hypocenters well determined by the land seismic net, are studied in detail. A shallow focus earthquake yields typical oceanic mantle velocities shallower than 50 km in the slab which dips under the Japanese archipelago. However, deeper focus earthquakes reveal anomalously high velocities averaged over the upper 230 km, in agreement with the models of Utsu and Oliver and Isacks. Two deep earthquakes, whose paths lie in the Pacific Ocean asthenosphere, suggest a velocity 3% lower than that predicted by Jeffreys-Bullen, in agreement with the above models. Spectral analysis of S arrivals suggests Q sub s values of 1000-1500 for nearby earthquakes and 4000-6000 for longer-distance earthquakes, implying an unusual attenuation mechanism for long travel paths, which enhances the high frequencies.

Author: Robert Anthony Sohn
Publisher:
ISBN:
Category : Plate tectonics
Languages : en
Pages : 182

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Book Description

Author: Charles Edward Golden
Publisher:
ISBN:
Category :
Languages : en
Pages : 210

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Book Description

Author:
Publisher:
ISBN:
Category : Aeronautics
Languages : en
Pages : 964

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Book Description

Author: Joseph D. Phillips
Publisher:
ISBN:
Category : Ocean bottom
Languages : en
Pages : 60

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Book Description
An analysis of current ocean bottom seismometer technology has revealed that conventional, free-fall devices which simple rest on the surficial seafloor sediments could provide portable stations with short period performance only equivalent to the higher noise land stations located on islands. However, by emplacing borehole-type seismometers beneath the surficial seafloor sediments, the broadband performance of ocean bottom stations could prove superior to the best land stations. In fact, by combining 'state of the art' broadband digital seismometers with modern deep sea drilling and ocean acoustic or satellite telemetry methods, permanent subseafloor stations with essentially real-time communication are entirely feasible. A plan for preliminary seismic research is suggested.

Author: Martin Eugene Dougherty
Publisher:
ISBN:
Category : Marine geophysics
Languages : en
Pages : 572

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Book Description
Seismic studies of the oceanic crust, both experimental and theoretical, often assume a flat seafloor and laterally homogeneous crust. This is done regardless of the appearance in seismic data of obvious effects due to scattering from lateral heterogeneities both on and in the seafloor. Detailed fine scale surveys of mid-ocean ridges, where the upper oceanic crust is exposed, have revealed the presence of lateral heterogeneities in the form of complicated topography, extrusive volcanic structure, and abundant fracturing and faulting. These heterogeneities have a significant affect on the propagation of seismo/acoustic energy through the crust, especially in the immediate vicinity of the seafloor. This thesis deals with the problem of scattering of seismo/acoustic energy from a number of forms of lateral heterogeneity in the upper oceanic crust. A common theme throughout this work is that the size of the heterogeneity on or in the seafloor is of the same order of magnitude as the seismo/acoustic wavelength. This is the realm of scattering theory where the wave-like characteristics of seismic energy have a particularly large influence on the outcome of interaction with structure in the media. The work presented here involves the application of the finite difference modeling technique to problems concerning laterally heterogeneous elastic media. This method is a full wave solution to the elastic wave equation and as such includes all wave interactions with the media. The finite difference formulation is used to study four distinct phenomena; scattering from discrete deterministic seafloor features; wave propagation through continuous randomly heterogeneous upper oceanic crust; scattering from more complicated topographic profiles and the limitations of the method for the rough seafloor problem; and the problem of plane acoustic wave scattering from an infinite elastic cylinder. The principal finding of this work is that lateral heterogeneities in the upper oceanic crust can have a dramatic affect on seismo/acoustic wave propagation. Scattering from rough seafloors and/or volume heterogeneities is often quite similar and causes the occurrence of signal generated 'noise' (coda), decorrelation of primary arrivals, and anomalies in arrival travel time and amplitude. Topographic and volume scatterers acting as secondary sources of seismic energy can cause a resonant coupling of body wave energy into interface (Stoneley) waves at the seafloor. This is possibly one mechanism by which natural seismic and storm generated acoustic energy can be coupled into seafloor noise. The applicability of the use of the finite difference method for non-planar water-solid interfaces is also discussed. Models were calculated which approximate sinusoidal seafloors and plane acoustic wave scattering from an infinite elastic cylinder. The discretization of a rectangular difference grid must be extremely fine to accurately accommodate a smoothly varying water-solid interface which does not align with the grid. Regardless of the discretization concerns, the rough seafloor models presented here demonstrate the arrivals expected from larger scale sinusoidal topography as well as the importance of considering quite small (

Author: Rachel A. Mills
Publisher: Geological Society of London
ISBN: 9781862390232
Category : Science
Languages : en
Pages : 316

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Book Description

Author: Katrin Hannemann
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Book Description

Author: B.R. Kerman
Publisher: Springer Science & Business Media
ISBN: 9401116261
Category : Science
Languages : en
Pages : 750

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Book Description
To place this book in perspective it is useful for the reader to be aware of the recent history of the topic of underwater sound generation at the ocean surface by natural mechanisms. A meeting in Lerici, Italy in 1987 was convened within the NATO Advanced Research Workshop series, to bring together underwater acousticians and ocean hydrodynamicists to examine various mechanisms which generate sound naturally at the ocean surface. A record of that meeting was published in the NATO scientific publication series in 1988 under the title 'Sea Surface Sound'. That meeting was successful in inspiring and co ordinating both participants and non-attending colleagues to examine some key issues which were raised during the course of presentations and discussions. The understanding among those present was that another meeting should be convened 3 years hence to report and review progress in the subject. Accordingly the second conference was convened in Cambridge in 1990, whose proceedings are presented here. This volume represents a very gratifying increase in only a 3 year interval in our understanding of a number of physical processes which generate sound at the peripheries of oceans. In fact it represents both the acceleration of singular effort as well as the development of interdisciplinary sophistication and co-operation. The enthusiasm, goodwill, and intense scientific curiosity which characterized the Lerici meeting carried through to Cambridge. The collegial atmosphere established by the participants was perfectly timed to foster another major advance in studies of ocean surface sound.