Abstract:

The May 21, 2021, M_S6.4 earthquake occurred in Yangbi, Yunnan, which caused 35 people died and possibly affected an area of 6 500 square kilometers. The hypocenter is located near the Weixi-Qiaohou-Weishan Fault which is the northern extension of the Red River Fault. However, the existing data show that this earthquake may not occur on the Weixi-Qiaohou-Weishan Fault. Further research is needed to explore the seismogenic fault. It is of great significance to understand the crustal structure and physical properties of the epicenter and its surrounding regions.
Affected by the Indian plate pushing on the Tibet, the southeastern margin of Tibetan plateau is characterized by intense intracontinental deformation, many deep faults and strong seismic activities. Gravity is one of the methods to study the crust structure and deep seismogenic environment. Many researches have been carried out in the study area, such as on the characteristics of gravity field, Moho depth, density structure, and isostatic anomaly. Gravity isostasy can reflect the crustal structure and its relationship with tectonic stress, which relates to seismic activities. However, the relationship between isostasy anomaly and earthquake distribution is still controversial. Some researches show that the Wenchuan earthquake locates in the extreme value region of isostatic anomaly. They believe the places where the difference between the theoretical and actual Moho depth is large are more likely to generate earthquakes. Other studies find that strong earthquakes in the western Sichuan and eastern Tibetan plateau occur mostly in the isostatic anomaly gradient regions, where the earthquake frequency is obviously higher than the extreme value regions. The crust-mantle density contrast is often taken as a constant in calculating Airy theoretical crustal thickness. The crust-mantle density contrast is inconsistent calculated with different data and methods. The gravity isostasy anomaly, which takes into account the density lateral variation, can better reflect the real structure.
In this paper, we calculate the gravity isostasy of the epicenter and surrounding regions based on the WGM2012 gravity anomaly, ETOPO1 elevation and Moho depth estimated from receiver functions in the previous study. China Earthquake Administration conducted gravity and topographic surveys on two profiles in the study area in 2011. We compared the observed Bouguer gravity and topography with the same stations extracted from WGM2012 and ETOPO1 models. The gravity anomaly standard deviation of two profiles is 4.89mGal and 8.42mGal, respectively. The standard deviation of topography is 17.86m and 28.74m, respectively. The WGM2012 and ETOPO1 data are in good correspondence with the actual measurements and can be used for the isostatic research.
We calculate the radial average logarithmic power spectrum of Bouguer gravity anomaly to estimate the reference Moho depth. The density contrast of the crust and upper mantle is obtained by apparent density estimation method in undulating interfaces. Based on the reference Moho depth and density contrast of crust and upper mantle, we get the theoretical isostatic Moho depth. We further compare the difference between the theoretical isostatic Moho depth and the actual depth obtained by the receiver function to analyze the isostatic anomalies in the whole study area, discuss the relationship between the isostatic anomalies and the distribution of moderate-strong earthquakes, and determine the isostatic state of the epicenter area of Yangbi earthquake.
Our results show that the difference between crustal mean density and uppermost mantle density varies from 0.3g/cm³ to 0.55g/cm³. The epicenter locates in the transition zone of density contrast. The change direction of transition zone is approximately perpendicular to the strike of Weixi-Qiaohou-Weishan Fault. The Sichuan Basin shows a relatively low density contrast, representing it is a stable tectonic block. The complete Bouguer gravity anomaly in the southeastern margin of Tibetan plateau has a low negative value. The crust-mantle density contrast is high(0.5~0.55g/cm³), which is speculated to be related to the extensive existence of low-velocity layers in the middle and lower crust revealed by previous tomography studies.
The theoretical isostasy Moho depth is 35~60km, reducing from northwest toward southeast. Local high value beneath the Panxi region is due to the low crust-mantle density, and the elevation is similar with adjacent areas. The theoretical Moho depth has negative correlation with crust-mantle density contrast. Wide angle reflection profile and seismic tomography studies also show there are obvious high velocity anomalies in the middle and lower crust. High velocity generally corresponds to high density, which is consistent with the low crust-mantle density contrast in Panxi region.
We obtain the gravity isostatic anomaly by subtracting the real Moho depth estimated through receiver functions H-κ stacking method from the theoretical Airy Moho depth. The Chongming M_S8.0 paleo-earthquake occurred on the 4km-long isostatic anomaly gradient belt. There are 29 earthquakes of M_S7.0 ~7.9, of which 16 earthquakes(55%)locate in the transition zones of positive and negative isostatic anomalies, and 5 earthquakes(17%)situate on the accompanied gradient zones with high value. Among the 42 earthquakes of M_S6.5 ~6.9, 21 earthquakes(50%)and 13 earthquakes(31%)are in the transition zones of positive and negative isostatic anomalies and the accompanied gradient zones with high value, respectively. 9 earthquakes(21%)are distributed in the extreme value zones of isostatic anomaly.
The epicenter of the Yangbi M_S6.4 earthquake locates on the 4km-long isostatic anomaly gradient belt. The gradient zone is basically consistent with the trend of Weixi-Qiaohou-Weishan Fault. The gradient zones of gravity isostatic anomaly are the area where anomalies change rapidly. We suggest that these regions may be in a more unstable state and liable to accumulate strain energy, thus, adjustments often begin to occur in these regions so as to achieve equilibrium in the crust.

Key words: Yangbi earthquake, crust-mantle density contrast, Airy isostasy, gravity isostatic anomaly

摘要：

2021年5月21日云南漾濞发生了6.4级地震, 造成35人伤亡, 受灾面积近6 500km²。文中基于WGM2012布格重力异常、 ETOPO1高程和接收函数估计得到的莫霍面深度资料, 考虑密度差的横向变化, 计算得到了该地震震源区及周边地区的重力均衡异常。研究结果表明: 地壳平均密度与上地幔顶部密度差的变化范围为0.3~0.55g/cm³, 漾濞地震震源区位于密度差过渡带。研究区6.5级及以上地震分布在重力均衡异常(艾里均衡理论莫霍面深度与接收函数H-κ叠加扫描法得到莫霍面的深度之差)梯度带的频率高于极值区。漾濞地震震源区处于均衡异常为4km的梯度带上, 且梯度带与维西-乔后-巍山断裂带的走向基本一致。重力均衡异常梯度带为异常变化较快的地区, 我们推测这些地区可能处于不稳定状态, 有利于应变能的积累, 为使地壳达到均衡, 多从这些地方开始调整, 从而导致地震的发生。

关键词: 漾濞地震, 壳幔密度差, 艾里均衡, 均衡异常