CHARACTERISTICS OF GRAVITY CHANGE BEFORE THE 2021 YUNNAN YANGBI MS6.4 EARTHQUAKE AND ITS DEEP MATERIAL MIGRATION PROCESS

doi:10.3969/j.issn.0253-4967.2025.06.20240062

Abstract

Abstract:

This study presents a comprehensive analysis of mobile gravity observation data from the western Yunnan region, focusing on the gravity variations leading up to the Yangbi M_S6.4 earthquake in Yunnan in 2021. The analysis utilizes the GLDAS global hydrological model to examine regional hydrological gravity changes based on load theory. Additionally, GNSS observations of long-term vertical displacement rates are used to assess gravity changes resulting from crustal deformation. By isolating gravity changes due to hydrological effects, crustal deformation, and other factors, the study explores potential deep-seated material migration processes in and around the epicentral area. After removing gravity changes attributed to terrestrial water loads and crustal deformation, gravity variations linked to deep structural activities were confirmed. The Bott density interface inversion method was then applied to evaluate deep material migration before the Yangbi earthquake.
The main findings are as follows: 1)Mobile gravity observations show a steady increase in regional gravity changes since 2016, culminating in 2018 in a “four-quadrant” distribution pattern around the epicenter, with maximum positive and negative gravity differences exceeding 50μGal. This pattern extends across the entire western and southwestern Yunnan region, with a spatial range of 200~300km. After 2019, cumulative gravity changes slightly decreased, with only minor local variations remaining before the earthquake. Gravity change gradients align with block boundaries, and the earthquake occurred at the center of the gravity “four-quadrant” and within a region of high gradient. 2)Gravity changes calculated from a small number of permanent GNSS observation stations range from -13 to 1μGal, indicating significant magnitude and clear seasonal variations. When annual differential GNSS data are used, the gravity effect reduces to between -8 and 1μGal. However, the limited number of GNSS stations does not fully represent the conditions across the entire survey area. Data from 165 long-term mobile GNSS observation stations revealed that long-term gravity changes range from -0.68 to 0.37μGal, indicating minimal impact. Hydrological effects, including seasonal changes, contributed between -3.3 and 0.6μGal, suggesting a minor influence. The gravity effects from vertical deformation(based on mobile GNSS data) and hydrological influences were too small(no greater than 4μGal) to account for the observed gravity changes in the mobile gravity surveys. 3)Thus, after excluding gravity changes due to terrestrial water loads and crustal deformation, gravity variations attributable to deep structural activities were confirmed. The Bott density interface inversion method was then used to evaluate regional deep material migration prior to the Yangbi earthquake.
The results show that when the reference depth for material migration is set at 10km and the density change is 1kg/m³, there is a strong correlation between the material migration-induced column height changes and the observed gravity changes. By analyzing the cumulative mass changes since 2016, it was found that material exchange within a 300km radius of the epicenter covered the earthquake zone. Specifically, from September 2016 to September 2018, the simulated net material inflow within this 300km radius reached 5.2×10¹²kg, with stress changes causing the epicentral area and surrounding regions to exhibit a “four-quadrant” pattern. From September 2016 until just before the earthquake, the simulated peak material inflow within the 300km radius centered on the epicenter reached 1.45×10¹³kg, leading to rock layer rupture and triggering the earthquake.

Key words: Yangbi M_S6.4 earthquake, gravity change, hydrological modeling

摘要： 文中对滇西地区流动重力观测资料进行精细处理, 获得了2021年云南漾濞6.4级地震前重力变化特征, 利用GLDAS全球水文模型, 基于负荷理论分析了区域水文重力变化效应, 利用GNSS观测获取的地表长期垂直位移速率评估了地壳形变引起的重力变化。进而在分析和剥离水文、地壳形变等重力变化效应的基础上, 探讨了震中及周边区域可能的深部物质迁移过程。结果表明: 1)2016年以来, 区域重力变化明显, 至2018年, 在震中周边形成“四象限”分布, 重力变化正-负差异量级最大超过50μGal, 其演化空间范围覆盖了整个滇西-滇西南地区, 演化半径达200~300km, 地震发生在“四象限”中心, 2019年后, 累积变化幅度略有减小, 至发震前仅余小幅度的局部变化; 2)基于密度界面反演方法, 评估了漾濞6.4级震前区域深部物质的迁移情况, 2016年至震前, 地震周边地区地壳内部物质交换频繁, 可能引起断层周边应力场发生反复调整, 2016年9月-2018年9月的应力变化使得震中及周边区域呈现出“四象限”特征, 以震中为中心300km范围内模拟物质流入达5.2×10¹²kg。模拟物质的质量经历了增加-减小-增加的变化过程, 临震前涌入的模拟物质质量达到峰值, 为1.45×10¹³kg, 最终引发地震。

关键词: 云南漾濞6.4级地震, 重力变化, 水文模型

LIU Zhi-hui, HAO Hong-tao, HU Min-zhang, LI Zhong-ya, WANG Jian, ZHANG Xin-lin, WANG Jia-pei. CHARACTERISTICS OF GRAVITY CHANGE BEFORE THE 2021 YUNNAN YANGBI M_S6.4 EARTHQUAKE AND ITS DEEP MATERIAL MIGRATION PROCESS[J]. SEISMOLOGY AND GEOLOGY, 2025, 47(6): 1625-1648.

刘志惠, 郝洪涛, 胡敏章, 李忠亚, 汪健, 张新林, 王嘉沛. 2021年云南漾濞6.4级地震前的重力变化特征及其深部物质迁移过程探析[J]. 地震地质, 2025, 47(6): 1625-1648.

Figures/Tables 15

Fig. 1 Seismotectonic map of western Yunnan region.

Fig. 2 Location of mobile gravity observation sites(a) and field observation sites of: Changyi(b), Pingpo(c), Huaping(d) and Shilin(e).

Table1 Results of mobile gravity leveling observation in western Yunnan region

观测年月	采用仪器	格值系数			点值平均精度 /μGal
		C169	C170
2015-09	C169、C170	1.000098	1.000040	8.4	7.0
2016-09	C169、C170	1.000006	1.000066	8.3	8.5
2017-09	C169、C170	1.000058	1.000116	9.1	8.8
2018-09	C169、C170	1.000250	0.999980	10.0	9.6
2019-09	C169、C170	1.000240	1.000020	7.9	11.5
2020-09	C169、C170	1.000435	1.000109	9.3	10
2021-04	C169、C170	1.000405	1.000127	10.5	11.5

Fig. 3 Differential evolution of the gravity field in western Yunnan region prior to the M6.4 Yangbi earthquake.

Fig. 4 Cumulative gravity change before the Yangbi MS6.4 earthquake in western Yunnan region.

Fig. 5 Temporal variation of gravity point values near the epicenter.

Fig. 6 Inter-annual variations of gravity from 1~5a intervals before the earthquake.

Fig. 7 Sequence of co-seismic displacements at continuous stations.

Table2 GNSS inter-annual vertical displacements and gravity effects at each permanent station

站点	类别	2016-09-2017-09	2017-09-2018-09	2018-09-2019-09	2019-09-2020-09	2020-09-2021-04
YNYL	形变量/mm	-4.33	9.03	32.85	25.46	66.58
	重力变化/μGal	0.85	-1.78	-6.46	-5.01	-13.1
YNYS	形变量/mm	16.32	7.49	26.87	21.37	61.75
	重力变化/μGal	-3.2	-1.47	-5.29	-4.20	-12.14
YNYA	形变量/mm	15.87	10.98	25.74	19.45	60.86
	重力变化/μGal	-3.12	-2.16	-5.06	-3.83	-11.97
YNSD	形变量/mm	41.09	11.13	34.93	22.74	62.1
	重力变化/μGal	-8.08	-2.19	-6.87	-4.47	-12.22

Fig. 8 Distribution of vertical uplift rate in western Yunnan.

Fig. 9 Vertical gravity changes from September 2016 to September 2017.

Fig. 10 Inter-annual differential hydrological effect.

Fig. 11 Schematic diagram of density interface inversion(Martins et al., 2011).

Fig. 12 Residual plots of the inversion results against the input gravity anomalies.

Fig. 13 Cumulative column height changes since 2016.

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