Characteristics of isostatic gravity anomaly in Sichuan-Yunnan region,China

Sichuan-Yunnan region in China,a tectonic transition belt where earthquakes occurred frequently and intensely,has a distinct variation characteristic of gradient zone of Bouguer gravity anomaly(BGA).Many deep faults and epicenters of severe earthquake scatter along the BGA gradient zones.Here we apply two forward models(Airy model and Vening Meinesz model) of isostatic gravity mechanisms(local versus regional) in this region to calculated the isostatic gravity anomaly(IGA).Afterwards,the relationship between IGA and distribution of faults as well as seismicity is also illustrated.The IGA results show that the two models are similar and most parts of the study area are in an isostatic state.Most featured faults are distributed along the steep anomaly gradient zones;earthquakes tend to occur in the non-isostatic area and steep gradient belt of IGA.The distribution of root thickness based on regional mechanism can be associated with the main trend of BGA variation.The regional mechanism is more plausible and closer to the reality because of its relatively further consideration of the horizontal forces derived from adjacent particles in the crust.Then we analyze the effect of isostasy on the tectonic movements and find that the isostatic adjustment is not the main cause of the continuous uplift process of Longmenshan Mountain fault zone,which is due to the Indian-Eurasian continental collision.