冰川均衡调整对东亚重力和海平面变化的影响

新的全球冰川均衡调整(GIA)模型RF3L20(β=0.4)+ICE-4G考虑了地幔黏滞度沿横向的变化,其黏滞度参数得到大地测量、历史相对海平面变化观测和地震剪切波层析模型的较好约束.本文利用该模型预测了东亚现今重力变化和海平面变化,根据当前末次冰川时空变化和黏滞度参考模型中下地幔下部黏滞度认识的差异,评估了预测的不确定性.结果表明,GIA对东亚地区重力场和海平面长期变化有显著的影响:例如,在哈尔滨、长春、泰安、蓟县、郑州、武汉等测站,GIA重力影响达几十纳伽,可用超导重力仪和未来原子重力仪观测出来;在东亚大陆GIA对GRACE监测的等效水柱长期变化的影响为3%~10%,其中青藏高原西部、华北和三峡地区的影响较大.在东海—太平洋区,GIA的相对影响高达20%~40%;GIA使东亚海域绝对海平面以0.27~0.37 mm/a的速率在长期下降,在黄海、东海卫星测高监测的绝对海平面长期变化中,GIA的相对影响分别达6.9%和7.5%;在58个验潮站,平均相对海平面长期上升速率为2.22 mm/a,GIA影响为-0.17 mm/a,其中14个测站GIA的影响达-0.3~-0.4 mm/a.本文GIA预测的结果,对在东亚地区发现弱的地球动力学过程信号、监测水质量长期变化、监测海平面长期变化和分析其机制,提供精密的改正模型.

Effects of global glacial isostatic adjustment on the secular changes of gravity and sea level in East Asia

We have developed a new glacial isostatic adjustment (GIA) model, RF3L20(β=0.4)+ICE-4G, which is well constrained by geodetic measurements, relative sea levels and shear-wave seismic tomographic data. Based on this model we will show how GIA affects the secular rate of change of gravity and sea level in East Asia. An advantage of this model is that the viscosities in the mantle are allowed to vary both in the radial and lateral directions. The laterally homogeneous reference viscosities are given by model RF3 (modified from Peltier's VM2). Lateral viscosity perturbations are converted from seismic tomographic data using the well-known scaling relationship multiplied by the factor β. The latter is used to quantify the contribution of thermal effect to lateral variation in shear wave velocity, and is chosen to be 0.4 by fitting the GIA data. In addition, the deglacial phase of the ice loads is given by the ICE-4G model. We assess the uncertainties of the GIA predictions by considering the differences between two reference viscosity models and two ice models. The GIA predictions are computed using the coupled-Laplace Finite-Element Method. The results show that GIA has obvious effects on the secular rate of change of gravity and sea level in East Asia. For example, the GIA effect on gravity can be several tens of nGals per year which can be detected by superconducting gravimeter and future atomic gravimeters at the geodetic stations Harbin, Changchun, Taian, Jixian, Zhengzhou and Wuhan. Also GIA is estimated to contribute 3%~10% on the secular trends of equivalent water heights on land in East Asia. The larger GIA effect appears in the west Tibetan Plateau, North China and Three Gorges Area. From East China Sea to the Pacific Ocean, the relative effects of GIA can be 20%~40%. GIA has caused a long-term depression of the absolute sea level in the ocean area by 0.27~0.37 mm/a. For the secular change in absolute sea level measured by satellite altimetry, the GIA effects can be 6.9% and 7.5% in Yellow Sea and East China Sea respectively. For the 58 tide gauges, the average value of the observed long term RSL rate is 2.22 mm/a while the GIA effect is -0.17 mm/a and the relative effect is 8.3%. For the 14 gauges, the GIA effects are -0.3~-0.4 mm/a. Therefore the results of this paper provide useful GIA corrections, so that the weak signals from geodynamic processes in the East Asia can be revealed.