Studia Geophysica et Geodaetica最新文献

Most methods for processing seismological data require a suitable velocity model characteristic for the given region being defined. This is also the case of the Reykjanes Peninsula in SW Iceland, where the REYKJANET seismic network was built to monitor local seismicity in the rift zone. At present, four previously published 1D velocity models (SIL, BRA, TRY and VOG) can potentially be used, prompting us to determine which one is the best. In order to address this issue, we arranged a contest in which all four 1D models and one additional 3D model (T3D) were entered. Uniform methodology for classifying the models was applied and included an analysis of: (i) post-1ocalization travel-time residuals, (ii) residuals of the P-wave first-motion incidence angle and (iii) model-predicted and measured Rayleigh-wave dispersion. We discovered that no single model was unequivocally the most optimal, as the differences between them proved rather minor. A common shortcoming of all the models is the bias of the P-wave first motion incidence angle residuals, which may be a general problem for methods working with P-wave amplitudes (e.g., moment tensor solutions). The VOG model was selected with a weak preference. Finally, we propose a simple method for modifying any of the 1D models by adding a station-dependent surface layer with a vertical velocity gradient. This way, a pseudo-3D model is generated which is fully competitive with a true 3D model while retaining the simplicity of 1D ray tracing. The efficiency of this correction was demonstrated using the VOG model. The corrected VOG model provides post-1ocalization residuals comparable with the true 3D model T3D, has zero bias in predicting the P-wave first-motion incidence angles, and agrees acceptably in predicting the Rayleigh-wave phase-velocity known from other sources. While calculations with a 3D model can be clumsy, the proposed pseudo-3D model is defined by few parameters and is very easy to use. Its applicability is limited to earthquake sources deeper than the deepest lower limit of the topmost layer below the stations.

Staggered-grid finite-difference (SGFD) approaches are universally applied to discretize different seismic-wave equations during wavefield extrapolation. However, the traditional SGFDs may encounter numerical dispersion error and instability owing to the limited approximation accuracy. To increase the simulated accuracy, we develop an optimized SGFD with high-order accuracy based on the orthogonal-octahedral operator for 3D scalar-wave modeling. Compared with the standard orthogonal-octahedral approach, the modified approach has smaller computing cost because we reduce the SGFD stencil. In addition, the corresponding time-space domain dispersion relation is beneficial to generate the least-square-based optimized high-order SGFD coefficients. Dispersion and stability comparsions show that the developed algorithm has better performance than the classical methods. Several simulated experiments verify that the proposed scheme can significantly suppress numerical dispersion in time and space domain and effectively improve the simulated accuracy and efficiency. In conclusion, the developed scheme can provide a reliable wavefield extrapolation tool for seismic imaging and inversion.

Spectral inversion, based on the odd-even decomposition principle of reflectivity, used the relationship between seismic data and wavelet amplitude spectrum to establish the inversion equation and achieve resolution-enhancement processing. Compared with deconvolution based on the L₂ norm, the odd and even components of reflectivity using spectral inversion can weaken the tuning effect, identify thin layers, and obtain data with higher resolution. However, most post-stack seismic data are non-stationary, i.e., attenuation of amplitude, phase, and frequency with time exists. We derived a resolution-enhancement algorithm of non-stationary seismic data with quality factor Q based on the short-time Fourier transform. Due to the instability of the spectral inversion algorithm, the lateral continuity of the obtained result is poor. Therefore, we proposed a multichannel spectral inversion algorithm with lateral constraints. The algorithm inherits the high-resolution characteristics of spectral inversion and effectively enhances lateral continuity. Applications to model and field data sets show that the proposed L₂ norm-based non-stationary multichannel spectral inversion method can be effectively applied to the resolution-improvement processing of non-stationary seismic data.

The earthquake of 9 November 1880 was one of the most important moments in the seismic history of Zagreb (Croatia). It is the strongest earthquake to have occurred in the greater Zagreb area, and as such it defines the seismic hazard in northwestern Croatia, the most populated part of the country. The main objective of this study was to reanalyze the location and magnitude of the earthquake, the input parameters which are crucial for a better assessment of seismic hazard, as there were macroseismic indications that the previous assessments should be revised. In addition, the strongest aftershock occurred two days after the main event, so it can be assumed that the observed intensities were caused by the cumulative effect of these two events. Therefore, a new isoseismal map was created, synthetic macroseismic modelling was performed and additional geophysical and microtremor measurements were taken. Based on all the information collected, the attempt to separate the effects of the strongest aftershock from the effects of the mainshock (to avoid a cumulative effect), a new assessment of the location of the epicentre of the main 1880 earthquake in Zagreb and its magnitude was made. When it comes to historical earthquakes, from a seismological point of view, even small improvements in the definition of the main seismological parameters of an earthquake significant for a given area are very important - for a better understanding of the geodynamics of the area, the earthquake mechanism and the spatial distribution of damage after the earthquake, as well as for the consistent assessment of seismic hazard and thus risk.

The oblique collision zone of Arabia-Eurasia is a seismically active region with complex crustal deformation patterns. While GPS measurements provide valuable data, their sparse distribution limits our understanding of the full extent of deformation. This study addresses this limitation by using a robust interpolation method for GPS velocity data in the collision zone. We utilized biharmonic splines to interpolate horizontal components of sparse GPS velocity data independently and in a coupled manner by altering Poisson ratio. This method is an effective means of interpolating sparse vector data in cases where deformation mechanics can be explained by elasticity principles. The interpolation process included fitting trends to the input data, calculating residuals, and analyzing them. The prediction process consisted of trend and spline fitting stages. We interpolate horizontal GPS velocities onto a standard geographic grid with a 30-minute interval, excluding data points with significant deviation. The data was partitioned into training and testing subsets, with the training set used for calibration and the testing set for evaluation of the interpolation method. Our analysis revealed an irregular spatial distribution of crustal movement. The northern component of the velocity field consistently points towards Eurasia and is greater than the eastern component. The amplitude of the northern component decreases from south to north and from west to east, indicating variations in deformation intensity. The eastern component exhibits a change in direction, moving westward in the western half of Iran and eastward in the eastern half, with a reversed trend in the north. This change in direction highlights the presence of solid blocks within the collision zone. Undeformed regions, major faults, convergence deformation, and compressing high-elevation regions are also observed in the collision zone. These findings provide a detailed picture of present-day crustal deformation in the Arabia-Eurasia collision zone, enhancing our understanding of the collision process.

Global Geopotential Models (GGMs) provide valuable information about Earth’s gravity field functionals, such as geoid heights and gravity anomalies. However, ground-based datasets are required to validate these GGMs at the regional and local scales. In this study, we validated the accuracy of GGMs by comparing them with ground-based Global Navigation Satellite System (GNSS)/levelling data for the first time in Nigeria. We employed two validation scenarios: with and without considering spectral consistency using the spectral enhancement method (SEM) to incorporate high and very high frequencies of the gravity field spectrum from the combined global gravity field model (XGM2019e_2159) and the residual terrain model (RTM) derived from the Shuttle Radar Topography Mission (SRTM) data, respectively. The results of this evaluation confirmed that the application of SEM improved the assessment of the GGM solutions in an unbiased manner. Integrating XGM2019e_2159 and SRTM data to constrain the high-frequency component of geoid heights in Gravity Field and Steady-State Ocean Circulation Explorer (GOCE)-based GGMs led to an improvement of approximately 10% in reducing the standard deviation (STD) relative to when SEM was not applied. GO_CONS_GCF_2_TIM_R6 at spherical harmonics (SH) of up to degree and order 260 demonstrated the lowest STD when compared to GO_CONS_GCF_2_DIR_R6 and GO_CONS_GCF_2_SPW_R5, with a reduction from 0.380 m without SEM application to 0.342 m with SEM implementation. In addition, four transformation models, namely, linear, four-parameter, five-parameter, and seven-parameter models, were evaluated. The objective is to mitigate the reference system offsets between the GNSS/levelling data and the GGMs and to identify the particular parametric model with the smallest STD across all GGMs. This effort reduced the GGMs misfits to GNSS/levelling to 0.30 m, representing a 15.3% decrease in STD. Notably, the XGM2019e_2159 model provides this improvement.

The formula for the area of a rhumb polygon, a polygon whose edges are rhumb lines on an ellipsoid of revolution, is derived and a method is given for computing the area accurately. This paper also points out that standard methods for computing rhumb lines give inaccurate results for nearly east- or west-going lines; this problem is remedied by the systematic use of divided differences.

Earth’s surface velocities are routinely extracted from Global Navigation Satellite System (GNSS) position time series. In addition to velocity estimates, acceleration may be a crucial parameter for modeling non-linear motion. Typically, a statistical hypothesis test is employed to evaluate the significance of the involved parameters and guide the selection of the appropriate model. In this contribution, we formulate a statistical test procedure from the generalized likelihood ratio test to analyze the significance of the acceleration in the model. The proposed procedure is compared with results obtained using the Akaike Information Criterion and Bayesian Information Criterion. Additionally, Minimal Detectable Horizontal Acceleration is provided as an indicator of the sensitivity of the acceleration detection. The GNSS time series of position estimates from the Nevada Geodetic Laboratory were used for this study. The experiments demonstrated a good agreement between the statistical test proposed and the information criteria approach. Therefore, the proposed statistical test may be another criterion to help the user in the important task of model selection.

Compared to gravity method, the gravity gradient has multi-component advantages and can emphasize short wavelength features. By providing more detailed features in the image display, it could present a more accurate determination of the spatial distribution of the underling anomaly body. In our study, based on the gravity gradient data that was synthesized from the high-precision gravity model from Gravity Recovery and Interior Laboratory mission, we analyzed the tectonic structure of the Moon’s shallow layers in Oceanus Procellarum region. Bouguer anomalies of gravity gradient are used for geological boundary recognition and three-dimensional density inversion. Theta Map method is adopted for the edge identification of geological structures. It fully utilizes the characteristics of multi-components combination of gravity gradient to sharpen the boundaries identification of abnormal bodies. During the density inversion, in order to decrease the non-unique solution problem, the depth weight constraint is added to the inversion equation. Furthermore, the method of wavelet coefficient compression and Least Squares QR-decomposition is applied to accelerate the inverse calculation of large ill-conditioned equations. According to the result illustration, we found that: 1) The combination of gravity tensors has a strong boundary recognition ability in the horizontal direction. There is close consistency with the results of density inversion. 2) Our study supports the expansion and intrusion hypothesis of lunar magma in the research zone of Oceanus Procellarum, since the distribution of density tomography results at different depths is higher than the normal assumed crust density of the Moon in the majority of areas. Moreover, the intrusion source in our research area is concentrated at a direction depth of 30–45 km below the mean lunar radius surface approximately.