关键词: 云南漾濞6.4级地震, 重力变化, 水文模型

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