Investigation of regional and temporal variations in Earth’s gravitational field that are detected by the Gravity Recovery and Climate Experiment (GRACE) twin-satellites may be useful in earthquake epicentre determinations. This study focuses on monthly spherical harmonic coefficients that were extracted from GRACE observations, which were corrected for hydrological effects to determine earthquake epicentres. For the first time, we use the concept of deformation of Earth’s gravity field to estimate invariant components of strain tensors. Four different earthquakes (Iran, China, Turkey, Nepal) were analysed that occurred between 2003 and 2015 and under different hydrological regimes. Wavelet analysis was explored as a means of refining and reconstructing tectonic signals forming the disturbance gravitational potential tensor in the GRACE gravity field models. Dilatation and maximum shear were extracted from these tensors and used to map earthquake epicentre locations. Both components reached their maxima during months of the earthquakes (respectively, 11.78 and 4.93, Bam earthquake; 61.36 and 169.10, Sichuan-Gansu border earthquake; 2415.80 and 627.93, Elazig earthquake; 98.71 and 157.37, Banepa earthquake). For the aforementioned earthquakes, we estimated their respective epicentres in the ranges: φ = 29°–29.5° λ = 58.5°–59°; φ = 32.5°–33° λ = 105.5°–106°; φ = 38.5°–39° λ = 39.5°–40°; and φ = 27.5°–28° λ = 85°–85.5°. Overall, these results agree well with values from other sources. The advance that is provided by our method compared to other research is the ability of determining earthquake epicentres with magnitudes ≤7.5 based upon GRACE observations. However, the approach is of limited use for very deep earthquakes.
In this study, in order to eliminate the stripping error from GRACE gravity observations and localization of the signals of this satellite, wavelet analysis instead of Gaussian isotropic filtering approach is used. For this purpose, the scaling functions and wavelets, generated by a so-called cubic polynomial (CuP), are considered and gravity variations for two major earthquakes-Sumatra earthquake and Maule earthquake-are obtained. For the case of Sumatra earthquake-Indonesia , various scale function of CuP are tested and the most precise results of gravity variations with maximum 30 Micro-Gal is acquired in scale 4 which was similar to other studies. This result didn't change with increasing scale function even higher than 4. Moreover, for the case of Maule earthquake-Chile, the gravity variations before and after of using wavelet is attained which shows many disparity. The maximum gravity variations before applying wavelet is acquired 20 Micro-Gal and after applying that, the maximum of these variations is computed 10 Micro-Gal which was similar to other researches.
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