Characteristics of the Shikano–Yoshioka fault revealed by gravity anomaly
Shinya MiyataAkihiro SawadaNayuta MatsumotoYoshihiro HiramatsuNaoya SakamotoTatsuya NoguchiShigekazu Kusumoto
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Anomaly (physics)
Normal fault
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The dips of boundaries in faults and caldera walls play an important role in understanding their formation mechanisms. The fault dip is a particularly important parameter in numerical simulations for hazard map creation as the fault dip affects estimations of the area of disaster occurrence. In this study, I introduce a technique for estimating the fault dip using the eigenvector of the observed or calculated gravity gradient tensor on a profile and investigating its properties through numerical simulations. From numerical simulations, it was found that the maximum eigenvector of the tensor points to the high-density causative body, and the dip of the maximum eigenvector closely follows the dip of the normal fault. It was also found that the minimum eigenvector of the tensor points to the low-density causative body and that the dip of the minimum eigenvector closely follows the dip of the reverse fault. It was shown that the eigenvector of the gravity gradient tensor for estimating fault dips is determined by fault type. As an application of this technique, I estimated the dip of the Kurehayama Fault located in Toyama, Japan, and obtained a result that corresponded to conventional fault dip estimations by geology and geomorphology. Because the gravity gradient tensor is required for this analysis, I present a technique that estimates the gravity gradient tensor from the gravity anomaly on a profile.
Magnetic dip
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The Stratified Gravity Image (SGI) is based on the theory of gravity and data of seismology. The density structure of each layer in the internal media of the earth can be obtained by means of the transformation relation of velocity-density correlation using the velocity structure of the seismic tomography results. The images of gravity anomaly of each layer of various depths and the superposition of different layers can be obtained by calculating the gravity effect of the density model. In this paper, the Chinese continent is chosen as an example of application of SGI. The images of gravity anomaly of each layer and total gravity anomaly of seven layers in China and adjacent areas are obtained. By comparing the total gravity anomaly with the Bouguer gravity anomaly, the result shows that the features of both gravity fields are quite conform, i.e., the result of SGI reflects fairly the distribution of the internal media of the earth. This method provides a new idea and possible way to solve the difficulty problem in stratified analysis of interior structure of the earth by gravity method and data.
Free-air gravity anomaly
Anomaly (physics)
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Three dimensional gravity analysis by using FFT. FFT wo mochiita juryoku no 3jigen kaiseki ni tsuite
The three dimensional gravity analysis by means of fast Fourier transform is developed in this paper. The analysis proposed here enables more rapid and accurate calculation of potential data of three dimensional structure, and the frequency domain analysis can be so effectively conducted that the sidelobe effects can be completely reduced. This report describes the following items; Presentation of gravity distribution by DFT (Dispersed Fourier Transformation). Gravity condensing surface and subsurface structure. Measured Bougner anomaly and subsurface structure. Computer GRAV2LA (Separation of data. Calculation of 2-layer depth structure showing gravity anomaly at level surface. Calculation of level surface gravity anomaly caused by 2-layer structure, Calculation of station gravity anomaly from gravity anomaly data of level surface. Analysis of 2-layer structure which causes the station gravity anomaly. Combination of potential fields among the arbitrary surfaces). Example and time of computation. 3 refs., 9 figs.
Anomaly (physics)
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An efficient method for direct transformation of gravity anomalies into contour lines of depth is proposed. The method is applicable for surface three‐dimensional bodies whose shape may be approximated by a cone at the first step. Hypothetical and field data evaluations presented here prove the efficiency of the method.
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The 3D spectral analysis of the gravitational potential field for a given mass distribution is studied. The derived quantities, the gravitational force field and the gravity gradient tensor are computed in frequency space. As an example, the fields are theoretically and numerically evaluated for a right rectangular prism. The spectral approach finds several geophysical applications, as, e.g., in inversion processes. Gravity inversion for deep seated masses, as for instance at Moho level, are treated with an iterative inversion process, in which the downward continuation is alternated with the classical calculation of the gravity field. The theory is applied to the inversion of the gravity data in the SE-Alps, regarding only the long-period field, generated by Moho undulations. The results are used for the evaluation of the equipotential lines, the gravity field, and the gravity gradient tensor in a vertical section of the Alpine crustal root.
Geopotential
Geopotential height
Potential field
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Line integrals (LIs) are an efficient tool in calculating the gravity anomaly caused by an irregular 2D mass body because the 2D surface integral is reduced to a 1D LI. Historically, LIs have been derived for 2D mass bodies of depth-dependent density contrast. I derive LIs for 2D mass bodies with density contrast dependent on (1) horizontal and (2) horizontal and vertical directions. Assuming the density contrast depends only on horizontal position, two types of representative LIs are derived: LIs with logarithmic kernel and density-integrated LIs for any integrable density-contrast function. A general density-contrast model that depends on horizontal and vertical directions is developed to include three components: a function of horizontal position, a function of vertical position, and a sum of crossterms of horizontal and vertical positions. Based on the general density-contrast model defined and proper selection of 2D vector gravity potentials, general LIs are derived to calculate the gravity anomaly. The newly developed LI method is then compared with two cases from the literature in calculating gravity anomaly, and agreement is obtained. However, the new LI method allows for more general 2D density-contrast variations and can be used to calculate the gravity anomaly of a 2D mass body. Such a mass body can have any cross-sectional profile that can be approximated by a polygonal cross section with any density-contrast function that can be approximated by a rich set of basis functions.
Density contrast
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Anomaly (physics)
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Free-air gravity anomaly
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This paper presents two improved techniques to determine boundaries and depth from observed gravity or magnetic anomalies. The first technique is based on analysis of the largest curvature of the total horizontal gradient of the total magnetic field to determine boundaries. The second technique is based on analysis signal of the total gradient of the total magnetic field to estimate depth. The technique is just only to calculate the total gradient magnitude of gravity or magnetic anomalies, rather than two derivatives of the total gradient magnitude. It is a particularly useful transformation for reducing the effects of noise and increasing the coherency of solutions from model-independent functions. The techniques is shown to work successfully in models and yield excellent results in delineating magnetic contact edges and reasonable performance in producing depth estimates. A practical surveyed data of the South China Sea show good correlation with known structural features.
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