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Seismic Observations (seismic + observation)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

Radial profiles of seismic attenuation in the upper mantle based on physical models

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2008
Fabio Cammarano
SUMMARY Thermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100 km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100 km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ,1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. [source]

A glassy lowermost outer core

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2009
Vernon F. Cormier
SUMMARY New theories for the viscosity of metallic melts at core pressures and temperatures, together with observations of translational modes of oscillation of Earth's solid inner core, suggest a rapid increase in the dynamic viscosity near the bottom of the liquid outer core. If the viscosity of the lowermost outer core (F region) is sufficiently high, it may be in a glassy state, characterized by a frequency dependent shear modulus and increased viscoselastic attenuation. In testing this hypothesis, the amplitudes of high-frequency PKiKP waves are found to be consistent with an upper bound to shear velocity in the lowermost outer core of 0.5 km s,1 at 1 Hz. The fit of a Maxwell rheology to the frequency dependent shear modulus constrained by seismic observations at both low and high-frequency favours a model of the F region as a 400-km-thick chemical boundary layer. This layer has both a higher density and higher viscosity than the bulk of the outer core, with a peak viscosity on the order of 109 Pa s or higher near the inner core boundary. If lateral variations in the F region are confirmed to correlate with lateral variations observed in the structure of the uppermost inner core, they may be used to map differences in the solidification process of the inner core and flow in the lowermost outer core. [source]

Radial profiles of seismic attenuation in the upper mantle based on physical models

A thermochemical boundary layer at the base of Earth's outer core and independent estimate of core heat flux

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2008
David Gubbins
SUMMARY Recent seismological observations suggest the existence of a ,150-km-thick density-stratified layer with a P -wave velocity gradient that differs slightly from PREM. Such a structure can only be caused by a compositional gradient, effects of a slurry or temperature being too small and probably the wrong sign. We propose a stably stratified, variable concentration layer on the liquidus. Heat is transported by conduction down the liquidus while the light and heavy components migrate through the layer by a process akin to zone refining, similar to the one originally proposed by Braginsky. The layer remains static in a frame of reference moving upwards with the expanding inner core boundary. We determine the gradient using estimates of co, the concentration in the main body of the outer core, and cb, the concentration of the liquid at the inner core boundary. We determine the depression of the melting point and concentrations using ideal solution theory and seismologically determined density jumps at the inner core boundary. We suppose that co determines ,,mod, the jump from normal mode eigenfrequencies that have long resolution lengths straddling the entire layer, and that cb determines ,,bod, the jump determined from body waves, which have fine resolution. A simple calculation then yields the seismic, temperature, and concentration profiles within the layer. Comparison with the distance to the C-cusp of PKP and normal mode eigenfrequencies constrain the model. We explore a wide range of possible input parameters; many fail to predict sensible seismic properties and heat fluxes. A model with ,,mod= 0.8 gm cc,1, ,,bod= 0.6 gm cc,1, and layer thickness 200 km is consistent with the seismic observations and can power the geodynamo with a reasonable inner core heat flux of ,2 TW and nominal inner core age of ,1 Ga. It is quite remarkable and encouraging that a model based on direct seismic observations and simple chemistry can predict heat fluxes that are comparable with those derived from recent core thermal history calculations. The model also provides plausible explanations of the observed seismic layer and accounts for the discrepancy between estimates of the inner core density jumps derived from body waves and normal modes. [source]

Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001
W. Zhao
Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW,SSE wide-angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong-Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P- wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along-strike to the east based on prior fan profiling; (3) a thick high-velocity lower crustal layer is evident along the length of the profile (20,35 km thick, 6.5,7.3 km s,1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid-crustal low-velocity layer in the P- wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide-angle profile with other seismic results allows a cross-section of Moho depths to be constructed across Tibet. This cross-section shows that crustal thickness tends to decrease from south to north, with values of 70,80 km south of the middle of the Lhasa terrane, 60,70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low-density, hot upper mantle with respect to southern Tibet. [source]

Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea

BOREAS, Issue 3 2010
ALASTAIR G. C. GRAHAM
Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471,491. 10.1111/j.1502-3885.2010.00144.x. ISSN 0300-9483. Geological constraints on ice-sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30-m-long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British,Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice-sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine-to-marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE,NW-flowing Witch Ground ice stream. AMS 14C dating confirms ice-stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 14C ka BP. Sediments overlying the ice-compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 14C ka BP. A re-advance of British ice during the last deglaciation, dated at 13.9 14C ka BP, delivered ice-proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg-ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 14C ka BP, and may correspond to oscillating ice-sheet margins during final, dynamic ice-sheet decay. [source]