Detection of Changes of the Earth's Volume and Geometry by Using GPS and VLBI Data

Abstract

Abstract:In previous studies, the northern hemisphere of the Earth is considered to be in compression while the southern one is in expansion. In this study, based on three different methods, we calculate average vertical variations of the two hemispheres from velocity field data under the ITRF2008 (International Terrestrial Reference Frame 2008) solution. Results show that the northern hemisphere is in expansion at the rate about 1 mm/yr, while the compression rate of the southern hemisphere is one order smaller than the expansion rate of the northern one. After the post glacial rebound effect is subtracted, results show that the expansion and compression rates of the northern and southern hemispheres are 0.46 mm/yr and -0.19 mm/yr, respectively. Transformation between the velocity fields under ITRF2008 and ITRF2000 can explain why different authors have different conclusions about the expansion/compression pattern of one hemisphere or the other. Anyway, the entire Earth is expanding at a rate about 0.2 mm/yr, and this estimation coincides with results of our previous studies. The mean variation rates of the radii at different latitudes have been calculated.Key words: reference systems, tectonic deformation, space geodetic datasets, expansion, compression. Recommended by V.A. San'kov GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E S R e c e n t G e o d y n a m i c s 45 W.-B. Shen et al.: Estimation of the asymmetric vertical variation…Аннотация: В предыдущих геодезических исследованиях считалось, что Северное полушарие испытывает тенденцию сжатия, а Южное полушарие, наоборот, расширения. В данной работе аналогичные исследования были проведены на основе трех различных методов. Для расчета средних вертикальных изменений этих двух полушарий мы использовали поле скоростей в Международной земной системе координат (ITRF) в эпоху 2008. Наши результаты показали, что в Северном полушарии из-за деформации происходит расширение со средней скоростью около 1 мм/год, а Южное полушарие испытывает сжатие со скоростью на порядок меньше скорости расширения. После исключения из рассмотрения эффекта постледникового отскока было установ-лено, что темпы расширения и сжатия Северного и Южного полушарий составляют 0.46 мм/год и -0.19 мм/год, соответственно. Преобразование между полями скоростей для двух международных систем коорди-нат ITRF 2008 и ITRF 2000 может объяснить, почему разные авторы делают различные выводы о характере расширения / сжатия Северного и Южного полушарий. Тем не менее, в целом, Земля расширяется со скоро-стью около 0.2 мм/год, что совпадает с нашими предыдущими исследованиями. Были рассчитаны также средние скорости изменения радиуса Земли в зависимости от широты.Ключевые слова: земные системы координат, тектонические деформации, космические геодезические наборы данных, расширение, сжатие.

Estimation of the asymmetric vertical variation of the southern and northern hemispheres of the Earth

“…7, from which we can see that the northern hemisphere is in compression, while the southern hemisphere is in expansion. This conclusion is in accordance with results published in [Jin, Zhu, 2003;Shen, Zhang, 2008;Sun et al, 2000Sun et al, , 2006.…”

“…Exactly to say, after a trend in the velocity in Z direction of about 1.8 mm/yr is removed, the southern hemisphere is in expansion at a rate of 0.708±0.007 mm/yr, while the northern hemisphere is in compression at a rate of -0.436±0.012 mm/yr. This is in accordance with the conclusions stated in the majority of the previous studies [Jin, Zhu, 2003;Sun et al, 1997Sun et al, , 2000Sun et al, , 2006. Hence, the different expansion/compression patterns given in this study and the previous studies may be attributed to the average motion in Z direction of the Earth mantle with respect to the Earth center of mass.…”

“…Huang et al [2002] also used the velocity field under ITRF2000 to estimate volume variations of the northern and southern hemispheres and concluded that the volume variations of the northern and southern hemispheres are -1.57×10 3 km 3 /yr and 0.936×10 3 km 3 /yr, respectively, which correspond to the compression rate of the northern hemisphere at -6.2 mm/yr and the expansion rate of the southern hemisphere at 3.5 mm/yr. Furthermore, Sun et al [2006] obtained similar results by using Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) solutions provided by the International Earth Rotation Service (IERS), and they concluded that the variation rates of the volume in the northern and southern hemispheres are -2.5428×10 3 km 3 /yr and 0.6641×10 3 km 3 /yr, respectively, resulting in a total volume variation of -1.8787×10 3 km 3 /yr. Based on data from 617 space geodetic stations under ITRF2000, Shen and Zhang [2008] found that the volume variation rates of the northern and southern hemispheres are -1.3765×10 3 km 3 /yr and 1.6517×10 3 km 3 /yr, respectively.…”

“…In the previous studies related to the average vertical variation rates of the two hemispheres, the southern hemisphere is considered in expansion, while the northern one is in compression [Jin, Zhu, 2003;Shen, Zhang, 2008;Sun et al, 2000Sun et al, , 2006. Such a difference might be caused by the difference in time spans represented by the available data.…”

“…Here we note that this study only considered the translation parameters listed in Table 3, because the consideration here focuses on the possible explanation for the expanding northern hemisphere and the compressing southern hemisphere instead of the opposite expansion/compression pattern described in the previous studies [Jin, Zhu, 2003;Shen, Zhang, 2008;Sun et al, 2000Sun et al, , 2006. As a remark, the variation of scale factor D (see Table 3) may also influence the magnitude of expansion/contraction of the two hemispheres.…”