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  • Elming, Sten-åke, et al. (författare)
  • A palaeomagnetic study and age determinations of Tertiary rocks in Nicaragua, Central America
  • 2001
  • Ingår i: Geophysical Journal International. - 0956-540X. ; 147:2, s. 294-309
  • Tidskriftsartikel (refereegranskat)abstract
    • Palaeomagnetic, K-Ar and Ar40/Ar39 measurements have been made to elucidate Nicaragua's plate tectonic history. These show that the Nicaraguan Highland rotated 90° counterclockwise between 30 and 18 Myr ago, but the Pacific Coastal Plain has not rotated.A lack of data has prevented a direct comparison with the tectonic history of adjacent areas, in particular, the Chortis Block, which is the part of the Caribbean Plate that is geologically most closely related to the Nicaraguan Highland. However, our data suggest that the Caribbean Plate in this part of Central America was not consolidated until c. 15 Ma.
  • Abrehdary, Majid, et al. (författare)
  • The spherical terrain correction and its effect on the gravimetric-isostatic Moho determination
  • 2016
  • Ingår i: Geophysical Journal International. - Oxford University Press. - 0956-540X. ; 204:1, s. 262-273
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, the Moho depth is estimated based on the refined spherical Bouguer gravity disturbance and DTM2006 topographic data using the Vening Meinesz-Moritz gravimetric-isostatic hypothesis. In this context, we compute the refined spherical Bouguer gravity disturbances in a set of 1 degrees x 1 degrees blocks. The spherical terrain correction, a residual correction to each Bouguer shell, is computed using rock heights and ice sheet thicknesses from the DTM2006 and Earth2014 models. The study illustrates that the defined simple Bouguer gravity disturbance corrected for the density variations of the oceans, ice sheets and sediment basins and also the non-isostatic effects needs a significant terrain correction to become the refined Bouguer gravity disturbance, and that the isostatic gravity disturbance is significantly better defined by the latter disturbance plus a compensation attraction. Our study shows that despite the fact that the lateral variation of the crustal depth is rather smooth, the terrain affects the result most significantly in many areas. The global numerical results show that the estimated Moho depths by the simple and refined spherical Bouguer gravity disturbances and the seismic CRUST1.0 model agree to 5.6 and 2.7 km in RMS, respectively. Also, the mean value differences are 1.7 and 0.2 km, respectively. Two regional numerical studies show that the RMS differences between the Moho depths estimated based on the simple and refined spherical Bouguer gravity disturbance and that using CRUST1.0 model yield fits of 4.9 and 3.2 km in South America and yield 3.2 and 3.4 km in Fennoscandia, respectively.
  • Abreu, Rafael, et al. (författare)
  • Micropolar modelling of rotational waves in seismology
  • 2017
  • Ingår i: Geophysical Journal International. - OXFORD UNIV PRESS. - 0956-540X .- 1365-246X. ; 210:2, s. 1021-1046
  • Tidskriftsartikel (refereegranskat)abstract
    • In this contribution we study elastic wave propagation via the introduction of the micropolar theory. As a generalization of a classical linear elastic medium, a micropolar medium allows each particle to have intrinsic rotational degrees of freedom (spin). We perform numerical experiments using the Pseudospectral method. We find analytical harmonic micropolar solutions for different problem configurations, which result in waveform differences between the classical linear elastic and micropolar media. In contrast to linear elastic media, wave propagation in micropolar media is dispersive. We study how the spin waveform depends on the micropolar elastic parameters and frequency content of the simulation. The micropolar effect on numerical seismograms has a direct implication on the phase, amplitude and arrival time. For frequencies lower than the cut-off frequency, the spin waveform has the same amplitude as the macrorotation field. For frequencies higher than the cut-off frequency, the amplitude of the spin waveform decreases with increasing frequency, so that then it is no longer comparable to the amplitude of macroscopic rotations. When both frequencies are equal there is no wave propagation. This work attempts to clarify the theory of micropolar media for its applications in seismology. We argue that micropolar theory should be further investigated for its potential uses in seismology to, for example, describe energy dissipation, seismograms recorded with rotational seismometers and rupture processes.
  • Abril, Claudia, et al. (författare)
  • Relocating earthquakes with empirical traveltimes
  • 2018
  • Ingår i: Geophysical Journal International. - Oxford University Press. - 0956-540X .- 1365-246X. ; 214:3, s. 2098-2114
  • Tidskriftsartikel (refereegranskat)abstract
    • A strategy is proposed to incorporate effects of 3-D velocity variations on earthquake locationsusing empirical traveltimes (ETTs). Traveltime residuals are interpolated from those predictedusing a 1-D velocity reference, mapped on to the hypocentres of corresponding earthquakesfor each station in a network. First, station corrections are computed by averaging the residualsover a fixed scale. Then, summary-ray variograms are used to estimate uncertainty and that,in turn, is used to tune a local fit to neighbouring residuals to refine the corrections. Resulting3-D traveltime estimates are then used as a description of the forward problem in a nonlineargrid-search relocation. Data are weighted according to the estimated uncertainty. Data fromthe Icelandic Southern Iceland Lowlands (SIL) national seismic network are used to test thestrategy. ETTs are estimated forP- andS-waves at 65 stations in the SIL network, basedon four million arrival time readings of 300.000 events registered between 1990 and 2012.ETTs are strongly correlated for the two wave types. The spatial variations of the predictedcorrections are consistently comparable or somewhat less forS-waves thanP-waves. Thisfeature suggests variations of theVP/VSratio in the Icelandic crust. Error estimates are alsostrongly correlated for the two wave types and between nearby stations. Relocations aretested by comparison with explosions and small populations of well-located events withindenser subnetworks. Relocations result in modestly enhanced clustering of explosions andearthquakes and significantly improved depth estimates. Estimates of the random relocationerror are statistically better behaved than those of the SIL system. They are in general reduced,as is expected since 3-D heterogeneity is now partly taken into account.
  • Adamczyk, A., et al. (författare)
  • High-resolution near-surface velocity model building using full-waveform inversion-a case study from southwest Sweden
  • 2014
  • Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 197:3, s. 1693-1704
  • Tidskriftsartikel (refereegranskat)abstract
    • Full-waveform inversion (FWI) is an iterative optimization technique that provides high-resolution models of subsurface properties. Frequency-domain, acoustic FWI was applied to seismic data acquired over a known quick-clay landslide scar in southwest Sweden. We inverted data from three 2-D seismic profiles, 261-572 m long, two of them shot with small charges of dynamite and one with a sledgehammer. To our best knowledge this is the first published application of FWI to sledgehammer data. Both sources provided data suitable for waveform inversion, the sledgehammer data containing even wider frequency spectrum. Inversion was performed for frequency groups between 27.5 and 43.1 Hz for the explosive data and 27.5-51.0 Hz for the sledgehammer. The lowest inverted frequency was limited by the resonance frequency of the standard 28-Hz geophones used in the survey. High-velocity granitic bedrock in the area is undulated and very shallow (15-100 m below the surface), and exhibits a large P-wave velocity contrast to the overlying normally consolidated sediments. In order to mitigate the non-linearity of the inverse problem we designed a multiscale layer-stripping inversion strategy. Obtained P-wave velocity models allowed to delineate the top of the bedrock and revealed distinct layers within the overlying sediments of clays and coarse-grained materials. Models were verified in an extensive set of validating procedures and used for pre-stack depth migration, which confirmed their robustness.
  • Árnadóttir, Thóra, et al. (författare)
  • Glacial rebound and plate spreading : results from the first countrywide GPS observations in Iceland
  • 2009
  • Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 177:2, s. 691-716
  • Forskningsöversikt (övrigt vetenskapligt)abstract
    • Iceland is one of the few places on Earth where a divergent plate boundary can be observed on land. Direct observations of crustal deformation for the whole country are available for the first time from nationwide Global Positioning System (GPS) campaigns in 1993 and 2004. The plate spreading across the island is imaged by the horizontal velocity field and high uplift rates (>= 10 mm yr(-1)) are observed over a large part of central and southeastern Iceland. Several earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the measurements, causing local disturbances of the deformation field. After correcting for the largest earthquakes during the observation period, we calculate the strain rate field and find that the main feature of the field is the extension across the rift zones, subparallel to the direction of plate motion. Kinematic models of the horizontal plate spreading signal indicate a slightly elevated rate of spreading in the Northern Volcanic Zone (NVZ) (23 +/- 2 mm yr(-1)), while the rates at the other plate boundary segments agree fairly well with the predicted rate of plate spreading (similar to 20 mm yr(-1)) across Iceland. The horizontal ISNET velocities across north Iceland therefore indicate that the excessive spreading rate (>30 mm yr(-1)) observed by GPS in 1987-1992 following the 1975-1984 Krafla rifting episode was significantly slower during 1993-2004. We model the vertical velocities using glacial isostatic adjustment (GIA) due to the recent thinning of the largest glaciers in Iceland. A layered earth model with a 10-km thick elastic layer, underlain by a 30-km thick viscoelastic layer with viscosity 1 x 10(20) Pa s, over a half-space with viscosity similar to 1 x 10(19) Pa s can explain the broad area of uplift in central and southeastern Iceland. A wide area of significant residual uplift ( up to 8 mm yr(-1)) is evident in north Iceland after we subtract the rebound signal from the observed rates, whereas the Reykjanes Peninsula and the Western Volcanic Zone (WVZ) appear to be subsiding at a rate of 4-8 mm yr(-1). We observe a coherent pattern of small but significant residual horizontal motion (up to 3 mm yr(-1)) away from Vatnajokull and the smaller glaciers that is most likely caused by glacial rebound. Our study demonstrates that the velocity field over a large part of Iceland is affected by deglaciation and that this effect needs to be considered when interpreting deformation data to monitor subglacial volcanoes in Iceland.
  • Artyushkov, E V, et al. (författare)
  • The cause of loss of lithospheric rigidity in areas far from plate tectonic activity
  • 2000
  • Ingår i: Geophysical Journal International. - 0956-540X. ; 143:3, s. 752-776
  • Forskningsöversikt (övrigt vetenskapligt)abstract
    • Significant losses of lithospheric strength are generally considered to be almost entirely associated with abnormal heating or steep lithospheric bending and/or stretching near to active plate boundaries. Several areas-the western Greater Caucasus, the North Crimean basin, the Carpathian foredeep, the Peri-Caspian basin and the Trans-Caspian areas-are shown to have steep basement slopes, usually comprising a difference in height of several kilometres over lateral distances of only 20-30 km, corresponding to very low, similar to 3-5 km, effective elastic thicknesses of the lithosphere. Each of these areas is shown to have undergone rapid steepening of the basement slope, usually within 1-2 Myr but in up to 10 Myr in some areas. At such times, these localities were far from active plate boundaries and in positions where bending forces could not have been transmitted to them from far-distant plate activities. Surface and/or subsurface loading can similarly be excluded as mechanisms for such steepening, and there is no apparent outflow of crustal materials into adjacent regions. It is suggested that such rapid subsidence far from plate tectonic activity is caused by rapid increases in the local density of the lithosphere. This could occur as a result of, for example, a gabbro-eclogite transformation in the lower crust, catalysed by the infiltration of volatiles from the asthenosphere. The resultant contraction of the mafic rocks would be non-uniform in space and produce high deviatory stresses, reducing the viscosity in the lower crust to similar to 10(23) Pa s. This would result in the rapid subsidence of the top of this layer, accompanied by steep ductile bending of the overlying upper crust. Such steer downwarping of the basement would be accompanied by a similar steepening of the underlying weakened mantle. The formation of such steep slopes thus indicates a weakening of the entire lithospheric layer, most probably due to the infiltration of volatiles from the asthenosphere, and unrelated to coeval plate tectonic activity.
  • Astrom, K (författare)
  • Seismic signature of the Lake Mien impact structure, southern Sweden
  • 1998
  • Ingår i: GEOPHYSICAL JOURNAL INTERNATIONAL. - BLACKWELL SCIENCE LTD. - 0956-540X. ; 135:1, s. 215-231
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • The EUGENO-S seismic profile IV in southern Sweden crossed Lake Mien, a confirmed heavily eroded 121 Ma impact structure. The Lake Mien structure has a pre-erosion rim diameter of about 9 km. It is located within the Transscandinavian Granite Porphyry Bel
  • Ayele, A, et al. (författare)
  • Spatial and temporal variations of seismicity in the Horn of Africa from 1960 to 1993
  • 1997
  • Ingår i: GEOPHYSICAL JOURNAL INTERNATIONAL. - BLACKWELL SCIENCE LTD. - 0956-540X. ; 130:3, s. 805-810
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • Spatial and temporal variations of seismic energy release and b-value are investigated in the Horn of Africa. The results indicate that the area around the Afar Depression and southern Sudan is at a higher stress level than the southern Red Sea and Gulf o
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