| 1. |
- Abrehdary, Majid, 1983-, et al.
(författare)
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Moho density contrast in Antarctica determined by satellite gravity and seismic models
- 2021
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 225:3, s. 1952-1962
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Tidskriftsartikel (refereegranskat)abstract
- As recovering the crust-mantle/Moho density contrast (MDC) significantly depends on the properties of the Earth's crust and upper mantle, varying from place to place, it is an oversimplification to define a constant standard value for it. It is especially challenging in Antarctica, where almost all the bedrock is covered with a thick layer of ice, and seismic data cannot provide a sufficient spatial resolution for geological and geophysical applications. As an alternative, we determine the MDC in Antarctica and its surrounding seas with a resolution of 1 degrees x 1 degrees by the Vening Meinesz-Moritz gravimetric-isostatic technique using the XGM2019e Earth Gravitational Model and Earth2014 topographic/bathymetric information along with CRUST1.0 and CRUST19 seismic crustal models. The numerical results show that our model, named HVMDC20, varies from 81 kg m(-3) in the Pacific Antarctic mid-oceanic ridge to 579 kg m(-3) in the Gamburtsev Mountain Range in the central continent with a general average of 403 kg m(-3). To assess our computations, we compare our estimates with those of some other gravimetric as well as seismic models (KTH11, GEMMA12C, KTH15C and CRUST1.0), illustrating that our estimates agree fairly well with KTH15C and CRUST1.0 but rather poor with the other models. In addition, we compare the geological signatures with HVMDC20, showing how the main geological structures contribute to the MDC. Finally, we study the remaining glacial isostatic adjustment effect on gravity to figure out how much it affects the MDC recovery, yielding a correlation of the optimum spectral window (7 <= n <= 12) between XGM2019e and W12a GIA models of the order of similar to 0.6 contributing within a negligible +/- 14 kg m(-3) to the MDC.
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| 2. |
- Abrehdary, Majid, et al.
(författare)
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Remaining non-isostatic effects in isostatic-gravimetric Moho determination-is it needed?
- 2023
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 234:3, s. 2066-2074
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Tidskriftsartikel (refereegranskat)abstract
- For long time the study of the Moho discontinuity (or Moho) has been a crucial topic in inferring the dynamics of the Earth's interior, and with profitable result it is mapped by seismic data, but due to the heterogeneous distribution of such data the quality varies over the world. Nevertheless, with the advent of satellite gravity missions, it is today possible to recover the Moho constituents (i.e. Moho depth; MD and Moho density contrast; MDC) via gravity observations based on isostatic models. Prior to using gravity observations for this application it must be stripped due to the gravitational contributions of known anomalous crustal density structures, mainly density variations of oceans, glacial ice sheets and sediment basins (i.e. stripping gravity corrections). In addition, the gravity signals related mainly with masses below the crust must also be removed. The main purpose of this study is to estimate the significance of removing also remaining non-isostatic effects (RNIEs) on gravity, that is, gravity effects that remain after the stripping corrections. This is carried out by using CRUST19 seismic crustal model and employing Vening Meinesz-Moritz (VMM) gravimetric-isostatic model in recovering the Moho constituents on a global scale to a resolution of 1 degrees x 1 degrees. To reach this goal, we present a new model, named MHUU22, formed by the SGGUGM2 gravitational field, Earth2014 topography, CRUST1.0 and CRUST19 seismic crustal models. Particularly, this study has its main emphasis on the RNIEs on gravity and Moho constituents to find out if we can modify the stripping gravity corrections by a specific correction of the RNIEs. The numerical results illustrate that the RMS differences between MHUU22 MD and the seismic model CRUST1.0 and least-squares combined model MOHV21 are reduced by 33 and 41 per cent by applying the NIEs, and the RMS differences between MHUU22 MDC and the seismic model CRUST1.0 and least-squares combined model MDC21 are reduced by 41 and 23 per cent when the above strategy for removing the RNIEs is applied. Hence, our study demonstrates that the specific correction for the RNIEs on gravity disturbance is significant, resulting in remarkable improvements in MHUU22, which more clearly visualize several crustal structures.
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| 3. |
- Beckel, Ruth A., et al.
(författare)
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Comparing the performance of stacking-based methods for microearthquake location : a case study from the Burträsk fault, northern Sweden
- 2022
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 228:3, s. 1918-1934
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Tidskriftsartikel (refereegranskat)abstract
- Traditional earthquake location relying on first arrival picking is challenging for microseismic events with low signal-to-noise ratio. Over the past years, alternative procedures have been explored based on the idea of migrating the energy of an earthquake back into its source position by stacking along theoretical traveltime curves. To avoid destructive interference of signals with opposite polarity, it is common to transform the input signals into positive time-series. Stacking-based source location has been successfully applied at various scales, but existing studies differ considerably in the choice of characteristic function, the amount of pre-processing and the phases used in the analysis. We use a data set of 62 natural microearthquakes recorded on a 2-D seismic array of 145 vertical geophones across the glacially triggered Burtrask fault to compare the performance of five commonly used characteristic functions: the noise filtered seismograms and the semblance, the envelope, the short-term average/long-term average ratio and the kurtosis gradient of the seismograms. We obtain the best results for a combined P- and S-wave location using a polarity-sensitive characteristic function, that is the filtered seismograms or the semblance. In contrast, the absolute functions often fail to align the signals properly, yielding biased location estimates. Moreover, we observe that the success of the procedure is very sensitive to noise suppression and signal shaping prior to stacking. Our study demonstrates the usefulness of including lower quality S-wave data to improve the location estimates. Furthermore, our results illustrate the benefits of retaining the phase information for location accuracy and noise suppression. To ensure optimal location results, we recommend carefully pre-processing the data and test different characteristic functions for each new data set. Despite the suboptimal array geometry, we obtain good locations for most events within similar to 30-40 km of the survey and the locations are consistent with an image of the fault trace from an earlier reflection seismic survey.
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| 4. |
- Beckel, Ruth A., et al.
(författare)
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Impact of source and receiver distributions on imaging of dipping reflectors with reflection seismic interferometry
- 2022
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 230:3, s. 2098-2116
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Tidskriftsartikel (refereegranskat)abstract
- Passive reflection seismic interferometry (PRSI) facilitates imaging the subsurface structure using passive sources but according to the general theory, the target needs to be illuminated equally from all directions to obtain a kinematically correct result. In practice, this requirement is almost impossible to meet which can introduce artefacts into the PRSI results. Our study was motivated by an unsuccessful attempt to image a couple of known dipping reflectors by applying PRSI to a data set of local microearthquakes clustering around the glacially triggered Burträsk fault. Dipping reflectors are a special case since they introduce a directionality into the seismic-interferometry problem that makes the results especially sensitive to the source azimuths. To investigate which source distributions are favourable in such a case and to study the range of artefacts occurring, we analyse a number of acoustic and elastic synthetic data sets calculated using a simple model of a dipping fault. Our results show that the main contribution for imaging such a fault with PRSI comes from sources in the hangingwall whereas contributions from the footwall are often weak and kinematically incorrect. The type and position of the occurring artefacts depend upon both the source azimuth and the type of modelling. In the acoustic case, the main artefact is a gently dipping reflection caused by insufficient cancellation of the direct reflection at the fault. In the elastic case, the artefacts are dominated by a set of both gently and steeply dipping reflections related to P–S converted waves. These artefacts are present even for ideal illumination due to the use of source records containing both P- and S-wave contributions. During interpretation, it is essential to be able to distinguish between physically meaningful reflections and artefacts. We found that both acoustic and elastic artefacts stack best at lower than expected normal moveout velocities. If data quality is insufficient for velocity analysis, our results can serve as a reference point for the interpretation of dipping features in PRSI images.
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| 5. |
- Benediktsdottir, Asdis, et al.
(författare)
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Volcanic tremor of the 2010 Eyjafjallajökull eruption
- 2022
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 228:2, s. 1015-1037
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Tidskriftsartikel (refereegranskat)abstract
- Volcanic eruptions in Iceland generally start with an increase in tremor levels. These signals do not have clear onset, like many earthquakes. As the character of the tremor signal is variable from one volcano to another, locating the source of the tremor signal may require different techniques for different volcanoes. Continuous volcanic tremor varied considerably during the course of the Eyjafjallajökull summit eruption, 14 April to 22 May 2010, and was clearly associated with changes in eruptive style. The tremor frequencies ranged between 0.5 and 10 Hz, with increased vigour during an effusive and explosive phase, in comparison with purely explosive phases. Higher-frequency tremor bursts early in the eruption were caused by processes at the eruption site. Location of the tremor using a method based on differential phase information extracted from interstation correlograms showed the tremor to be stable near the eruption vent, through time, for signals between 0.5 and 2 Hz. Analyses of power variations of the vertical component of the tremor with distance from the eruption site are consistent with tremor waveform content being dominated by surface waves in the 0.5–2 Hz frequency range. The tremor source depth was argued to be shallow, less than about 1 km. The attenuation quality factor (Q) was found to be on the order of Q = 10–20 for paths in the area around Eyjafjallajökull and Q = 20–50 for paths outside the volcano. The pattern of radiated wave energy from the tremor source varied with time, defining ten different epochs during the eruption. Thus the tremor-source radiation did not remain isotropic, which needs to be considered when locating tremor based on amplitude, that is azimuthally variable source radiation.
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| 6. |
- Buntin, Laura M., et al.
(författare)
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Improved accuracy of plane-wave electromagnetic modelling by application of the total and scattered field decomposition and perfectly matched layers
- 2023
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 235:2, s. 1201-1217
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Tidskriftsartikel (refereegranskat)abstract
- In 2-D magnetotelluric modelling, the standard application of Dirichlet boundary conditions (BC) may severely diminish the solution accuracy, because the unknown scattered part of the electromagnetic field is erroneously reflected at the domain boundary. Therefore, we adapt the total and scattered field decomposition (TSFD) to geophysical modelling, enabling the application of fully absorbing boundary methods, here perfectly matched layers (PML), to the scattered field. Our novel TSFD divides the modelling domain into two regions. In the total-field region containing the area of interest, the solution is computed for the total field. In the scattered-field region containing the boundaries, the solution is obtained for the scattered field, which is fully absorbed by PML at the boundaries. The plane-wave source is excited at the TSFD interface between both regions. Thus, boundary reflections are eradicated leading to superior solution accuracy, and boundaries can be placed closer to the receivers, shrinking the computational problem. Especially for challenging models with strong lateral changes, the solution accuracy of the TSFD is superior to that of the standard Dirichlet approach. Owing to the linearity of Maxwell's equations, the inaccuracy introduced to the electric and magnetic fields by using Dirichlet BC can be expected to partly cancel out in the magnetotelluric transfer functions, for example the impedance tensor. In this work, we quantify this cancellation effect. The inaccuracy is less than typical measurement errors in the vast majority of apparent resistivity and phase data, even, when the primary fields are strongly inaccurate.
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| 7. |
- Eggertsson, Gunnar, et al.
(författare)
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Earthquake or blast? : Classification of local-distance seismic events in Sweden using fully connected neural networks
- 2024
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 236:3, s. 1728-1742
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Tidskriftsartikel (refereegranskat)abstract
- Distinguishing between different types of seismic events is a task typically performed manually by expert analysts and can thus be both time and resource expensive. Analysts at the Swedish National Seismic Network (SNSN) use four different event types in the routine analysis: natural (tectonic) earthquakes, blasts (e.g. from mines, quarries and construction) and two different types of mining-induced events associated with large, underground mines. In order to aid manual event classification and to classify automatic event definitions, we have used fully connected neural networks to implement classification models which distinguish between the four event types. For each event, we bandpass filter the waveform data in 20 narrow-frequency bands before dividing each component into four non-overlapping time windows, corresponding to the P phase, P coda, S phase and S coda. In each window, we compute the root-mean-square amplitude and the resulting array of amplitudes is then used as the neural network inputs. We compare results achieved using a station-specific approach, where individual models are trained for each seismic station, to a regional approach where a single model is trained for the whole study area. An extension of the models, which distinguishes spurious phase associations from real seismic events in automatic event definitions, has also been implemented. When applying our models to evaluation data distinguishing between earthquakes and blasts, we achieve an accuracy of about 98 per cent for automatic events and 99 per cent for manually analysed events. In areas located close to large underground mines, where all four event types are observed, the corresponding accuracy is about 90 and 96 per cent, respectively. The accuracy when distinguishing spurious events from real seismic events is about 95 per cent. We find that the majority of erroneous classifications can be traced back to uncertainties in automatic phase picks and location estimates. The models are already in use at the SNSN, both for preliminary type predictions of automatic events and for reviewing manually analysed events.
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| 8. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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Lithospheric stress, strain and displacement changes from GRACE-FO time-variable gravity : case study for Sar-e-Pol Zahab Earthquake 2018
- 2020
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 223:1, s. 379-397
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Tidskriftsartikel (refereegranskat)abstract
- Temporal variations in the Earth’s gravity field can be used for monitoring of lithospheric deformations. The network of continuously operating gravity stations is required for this purpose but a global coverage by such network is currently extremely sparse. Temporal variations in long-wavelength part of the Earth’s gravity field have been, however, observed by two satellite missions, namely the Gravity Recovery And Climate Experiment (GRACE) and the GRACE Follow-On (GRACE-FO). These satellite gravity observations can be used to study long-wavelength deformations of the lithosphere. Consequently, considering the lithosphere as a spherical elastic shell and solving the partial differential equation of elasticity for it, the stress, strain and displacement inside the lithosphere can be estimated. The lower boundary of this shell is assumed to be stressed by mantle convection, which has a direct relation to the Earth’s gravity field according to Runcorn’s theory. Changes in gravity field lead to changes in the sublithospheric stress and the stress propagated throughout the lithosphere. In this study, we develop mathematical models in spherical coordinates for describing the stress propagation from the sublithosphere through the lithosphere. We then organize a system of observation equations for finding a special solution to the boundary-value problem of elasticity in the way that provides a stable solution. In contrast, models presented in previously published studies are ill-posed. Furthermore, we use constants of the solution determined from the boundary stresses to determine the strain and displacements leading to these stresses, while in previous studies only the stress has been considered according to rheological properties of the lithosphere. We demonstrate a practical applicability of this theoretical model to estimate the stress–strain redistribution caused by the Sar-e-Pol Zahab 2018 earthquake in Iran by using the GRACE-FO monthly solutions.
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| 9. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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The temporal viscoelastic model of flexural isostasy for estimating the elastic thickness of the lithosphere
- 2021
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Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 227:3, s. 1700-1714
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Tidskriftsartikel (refereegranskat)abstract
- The (effective) elastic thickness of the lithosphere defines the strength of the lithosphere with respect to a load on it. Since the lithosphere is buoyant on a viscous mantle, its behaviour with respect to a load is not fully elastic, but rather viscoelastic. Fennoscandia is a well-known area in the world where the lithosphere has not yet reached its isostatic equilibrium due to the ongoing uplift after the last glacial period at the end of the Pleistocene. To accommodate for this changing property of the lithosphere in time, we present the flexural model of isostasy that accommodates temporal variations of the lithospheric flexure. We then define a theoretical model for computing the elastic thickness of the lithosphere based on combining the flexural and gravimetric models of isostasy. We demonstrate that differences between the elastic and viscoelastic models are not that significant in Fennoscandia. This finding is explained by a relatively young age of the glacial load when compared to the Maxwell relaxation time. The approximation of an elastic shell is then permissible in order to determine the lithospheric structure and its properties. In this way, the elastic thickness can be estimated based on combining gravimetric and flexural models of isostasy. This approach takes into consideration the topographic and ocean-floor (bathymetric) relief as well as the lithospheric structural composition and the post-glacial rebound. In addition, rheological properties of the lithosphere are taken into consideration by means of involving the Young modulus and the Poisson ratio in the model, both parameters determined from seismic velocities. The results reveal that despite changes in the Moho geometry attributed to the glacial isostatic adjustment in Fennoscandia are typically less than 1 km, the corresponding changes in the lithospheric elastic thickness could reach or even exceed ±50 km. The sensitivity analysis confirms that even small changes in input parameters could significantly modify the result (i.e. the elastic thickness estimates). The reason is that the elastic thickness estimation is an inverse problem. Consequently, small changes in input parameters can lead to large changes in the elastic thickness estimates. These findings indicate that a robust estimation of the elastic thickness by our method is possible if comprehensive information about structural and rheological properties of the lithosphere as input parameters are known with a relatively high accuracy. Otherwise, even small uncertainties in these parameters could result in large errors in the elastic thickness estimates.
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| 10. |
- Johansson, Sara, et al.
(författare)
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Spectral induced polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties
- 2020
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 220:2, s. 928-950
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Tidskriftsartikel (refereegranskat)abstract
- With advances in data acquisition and processing methods, spectral inversion of time domain (TD) induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole–Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain (FD) systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of this study is, thus, to perform a direct comparison of inverted spectral parameters from TD IP field data with FD IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50-m-long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with FD spectral IP in laboratory. The results showed that the inverted Cole–Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous–Palaeogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous–Palaeogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Palaeogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of cylindrical bryozoan fragments could be identified in the inverted Cole–Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole–Cole parameters from the field scale TD IP tomography were comparable to the magnitude and shape of FD IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.
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