Abstract:

As part of the frontal edge of the Tibetan plateau, the eastern Tibetan plateau is featured by large-scale active fault systems, intense tectonic movement, and has experienced many devastating earthquakes, which have attracted high attention. The Fubianhe Fault is located inside the Songpan Block of eastern Tibetan plateau, and to its east is the Longmenshan Fault, which is a strong earthquake-prone zone. However, there are less earthquakes having occurred in the area around the Fubianhe Fault, and whether the area around the Fubianhe Fault has potential of strong earthquakes needs to be analyzed based on the crustal stress pattern. In this study, we calculate the Bouguer gravity anomalies by using two profiles with hybrid gravity and GPS observations, analyze the difference between measured gravity anomalies with the EGM2008 model, as well as the crustal density structures and isostatic additional stress(IAS)around Fubianhe Fault based on the Bouguer gravity anomalies. We analyzed the uplift mechanism of eastern Tibetan plateau based on the inverted IAS. At last, we discussed the medium-strong seismic risk in the eastern Xiaojin County of Sichuan Province, based on the IAS, geological active faults, historical earthquakes, and the regional gravity changes from 2018 to 2021. The main conclusions obtained in this study are as follows:

(1)The measured free-air gravity anomalies near the Maerkang-Xiaojin range from -230mGal to 180mGal, which is less systematic than the EGM2008 model results, with the difference standard deviation being 57mGal. The measured gravity anomalies would be used to analyze the regional characteristics in the eastern Tibetan plateau, due to the poor accuracy of EGM2008 in this region. The crustal density structure and Moho depth are inversed based on the measured gravity anomalies, and the Moho depth beneath the Maerkang-Xiaojin is approximately 60km.

(2)We estimated the isostatic depth in the study region based on the Airy isostatic theory, and the Moho depth beneath the study area is approximately 60km, which is generally deeper than depth of isostatic interface. We calculate the IAS in the study region based on the Moho depth and isostatic depth, and the result shows that the maximum IAS is approximately 20MPa, and the direction of IAS is upward in the whole, which indicates that the crustal uplift in the eastern Tibetan plateau attributes to compressing uplift, which is caused by the Tibetan plateau eastward extrusion and that the Sichuan Basin is inserted downward into the Songpan Block. The gravity profile crossing through Maerkang shows that there are fewer earthquakes in the east and more earthquakes in the west of the Fubianhe Fault. The IAS in the west of Fubianhe Fault is smaller than that in the east. This phenomenon is considered to be due to the difference in stress release in the crust by the earthquakes, with the Fubianhe Fault as the boundary. The relationship between the IAS and earthquakes across the Xiaojin profile is similar with that of the Maerkang profile. In addition, the IAS profile crossing through Xiaojin shows that there is an obvious high gradient zone in the east of Xiaojin, we suggest that there is a concealed fault located in the IAS gradient zone, which needs to be further explored in combination with other observation means.

(3)The IAS change gradients appeared in the eastern Xiaojin County, which is located in the earthquake-prone arc crest zone of Xiaojin arc geological structure belt. The Jiaochang arc geological structure belt is located in the northeastern Xiaojin arc geological structure belt, and the 1933 M7.5 Diexi earthquake and 1941 M6 Heishui earthquake occurred in the arc crest zone of Jiaochang arc geological structure belt. The Jintang arc geological structure belt is located in the southwestern Xiaojin arc geological structure belt, and the 1941 M6 kangding earthquake occurred in the arc crest zone of Jintang arc geological structure belt. While, there are no medium-strong earthquakes in the arc crest zone of Xiaojin arc geological structure belt. Besides, regional gravity changes from 2018 to 2021 around the study region show obvious four quadrant spatial distribution in the gravity gradient belt area, with the gravity changes reaching approximately 90 microgals. Based on the results obtained above, we suggest that there exists medium-strong earthquake risk in the eastern Xiaojin County.

Key words: Fubianhe Fault, gravity and GPS observation, gravity anomaly, isostatic additional stress

摘要：

文中基于扶边河断裂周边的2条重力和GPS观测剖面数据计算了该地区的重力异常, 并利用该异常反演了马尔康—小金地区周边的地壳密度结构与垂向构造应力, 分析了该地区的隆升机制; 同时结合地质构造背景、 历史发震情况及多年尺度的区域流动重力变化资料分析了小金及周边地区的地震危险性。结果表明, 马尔康—小金地区周边自由空气异常较模型结果整体偏小, 差异中误差为57mGal, EGM2008模型结果在该地区的精度较低。马尔康—小金地区的莫霍面深度约为60km, 整体大于Ariy均衡面深度, 受到的地壳垂向构造应力整体为正, 认为是由于松潘块体向E挤压并受到四川盆地的阻挡, 产生向上的垂向构造应力, 且青藏高原东缘的抬升主要由挤压隆升所致。此外, 小金东部地区位于小金弧形构造带的弧顶区域, 存在明显的垂向构造应力变化梯度带, 且2018—2021年3a尺度的重力变化显示该地区存在重力变化四象限及变化梯度带, 重力变化的量级超过90μGal, 分析认为该地区存在发生强震的背景。

关键词: 扶边河断裂, 重力和GPS观测, 重力异常, 垂向构造应力, 地震危险性