Abstract:

The Sichuan-Yunnan rhombic block is located at the southeastern margin of the Qinghai-Xizang Plateau. Its current activities revolve around the eastern Himalayan tectonic in a clockwise direction, playing a role in regulating the eastward extrusion of materials in the Qinghai-Xizang Plateau region, or in adjusting the strong dextral shear between the Indian plate and the Yangtze block. This block is characterized by large-magnitude and high-frequency seismic activities. Seismic activities are mainly the result of crustal deformation. The linkage effect of great seismic activities can reflect the holistic and coordinated deformation of the tectonic system. This relationship of the integrity and coordination of crustal deformation is of certain significance for analyzing and predicting the trend of future great seismic activities in the region. The co-seismic response of underground fluids is one of the most effective and direct ways to reveal the response of crustal media to the stress-strain process. The spatial clustering areas of co-seismic response anomalies have certain indicative significance for future seismic danger zones.

This article analyzes the impact characteristics of major earthquakes with magnitudes 7 or above occurring within the Qinghai-Xizang Plateau block on the groundwater levels of fluid wells in the Sichuan-Yunnan region. It studies the changes in well-aquifer hydrological parameters before and after the coseismic response of fluid well levels, analyzes the genetic mechanism of coseismic step changes in water levels, and examines the indicative significance of coseismic water level step changes for future earthquake locations. The results show that: 1)In terms of morphology, after a major earthquake, some of the water levels of underground fluid wells in the Sichuan-Yunan region experienced step-up changes, while others experienced step-down changes. Both the step-up and step-down changes last for a period of time, and the duration of the co-seismic step change does not seem to be related to the well seismic distance directly. The co-seismic response patterns of the same well after different earthquakes are not constant, which is related to the changes in the permeability of the aquifer before and after major earthquakes. In terms of spatial distribution, the water level of underground fluid wells in the Sichuan-Yunan region exhibits a relatively concentrated regional pattern of synchronous rise and fall. 2)The change in permeability of aquifer rock caused by seismic waves may be the reason for the change in the seismic response pattern of well water level caused by far-field earthquakes. The “permeability enhancement” model may be the most reasonable explanation for the coseismic rise and fall of well water levels in the Sichuan-Yunan region after a major earthquake. After a major earthquake, the phase lag of the aquifer increases, the permeability decreases, and the surrounding pore water is compressed and squeezed into the well, which causes the well water level to rise. On the contrary, the phase lag of the aquifer decreases, the permeability increases, and the water in the wellbore flows out, which causes a decrease in the well water level. 3)Based on comprehensive analysis of the coseismic changes in well water levels in the Sichuan-Yunan region and GPS results, it's believed that the coseismic response changes can reflect the regional stress in a certain. It has certain indicative significance for the determination of seismic activity in the subsequent region on a medium to long-term scale. Based on the data since 2010, after 5 major earthquakes, there were 3 times causing changes in the water level of underground fluid wells, and subsequent earthquakes with magnitude of 6 occurred every time, with a probability of 60% and an interval of 15-22 months. The occurrence occurred in areas with concentrated changes in the water level of wells. In the future prediction of earthquake locations in the Sichuan-Yunan region, attention should be paid to the concentrated areas of coseismic changes. This study enhances the predictive ability of fluid observations for earthquake locations.

Key words: mechanism of the co-seismic response of water level, post-strong earthquake effects, aquifer permeability, Sichuan-Yunan region

摘要：

文中基于2010年以来青藏高原构造块体内7级以上大地震发生后川滇地区地下流体井水位同震阶变的响应特征, 分析其成因机理, 并探讨其对未来发震地点的指示意义。结果表明, 大地震发生后, 川滇地区地下流体井水位出现同震阶变上升或下降变化, 且这种阶变具有区域性相对集中的特点; 井水位同震响应前后, 井-含水层相位差的变化与井水位同震阶变的成因机理有关; 此外, 2010年以来在5次大地震引起的地下流体井水位同震阶变中, 有3次后续发生6级地震, 对应概率为60%, 间隔时间为15~22个月, 发生地点为井水位同震阶变集中区。水位同震响应阶变在一定程度上可反映区域应力场水平, 其对后续区域地震活动在中长期尺度上的判定具有一定指示意义。

关键词: 水位同震响应机理, 强震震后效应, 含水层渗透率, 川滇地区