Abstract:

Yunnan is the front area where the material of Qinghai-Tibet Plateau moves eastward and rotates southeastward. This area is subject to active crustal movement and frequent destructive earthquakes. The activity intensity of the Honghe fault zone is the strongest in the middle part of Dali and its adjacent areas. The activity sequence of the whole fault zone since the Cenozoic has also changed gradually from the north and south ends to the Dali area in the middle section. The existing research data show that, based on the current seismic gravity monitoring network covering the whole province and surrounding areas, the mobile gravity monitoring network in Yunnan and surrounding areas can identify the gravity changes related to earthquakes of magnitude 5 and above. In this paper, the relative gravity data from 2015 to 2021 in Yunnan Province is adjusted and calculated, and combined with the regional geological tectonic environment and earthquake case studies, a systematic analysis is done on the temporal and spatial distribution characteristics of the changes in the gravity field and the horizontal crustal movement of the Yangbi earthquake and its surrounding areas to obtain the relationship between the regional gravity field change and the Yangbi M_S6.4 earthquake, the following results are obtained: 1)By analyzing the gravity observation data of seven periods since 2018, it is found that the gravity change is weak at the Yangbi survey point in the epicenter from 2018 to 2021, and the gravity change around the epicenter is relatively intense. the gravity change at the Yangbi measuring point at the epicenter is within 10×10^-8m·s^-2, and the gravity change at the Xiaguan measuring point at a distance of about 39.2km in a straight line is around 10×10^-8m·s^-2. Considering the observation accuracy of the instrument, the epicenter area is basically in a constant state. The variation of Yongsheng survey point in the north, Tengchong survey point in the west and Jingdong survey point in the south of the epicenter are larger, with the maximum amount exceeding 60×10^-8m·s^-2. 2)With the epicenter as the center, the short-term, one-year, two-year, and four-year gravitational field changes in the nearby area basically show the characteristics of four quadrants. For the short-term and multi-year gravitational changes before the earthquake, the high gradient zone of the gravity field is concentrated in Yangbi-Xiaguan area, and the zero value line of gravity change is always in this area or adjacent area. Along the north section of the Xiaojiang fault zone in the Lijiang-Xiaguan area and the east and west sides of the seismogenic fault zone in the strip-shaped area of Nanjian-Weishan Fault, the gravity transitions are drastic, and the movement of materials inside the crust is obvious. 3)The multi-year gravity changes show that the gravity change on the west side of the epicenter is negative, and that of the east side is positive. The earthquake is a normal faulting event with a strike-slip component, which is consistent with the focal mechanism solution calculated using the global network data. The change characteristics of the gravity field indicate that the preparation of earthquakes in the earthquake area is closely related to the movement of faults and material transport in the crust. 4)By analyzing the gravity change anomaly index in the one year before the earthquake, the magnitude of the anomalous gravity change is 75×10^-8m·s^-2. According to the gravity anomaly index formula, the magnitude of the earthquake is M_S6.2, which is 0.2 less than the actual magnitude of M_S6.5; According to the actual magnitude M_S6.4, the time-varying distance of the earthquake is 127.4km, and the distance between the two maximum positive change measuring points of the short-term gravity field before the earthquake and the epicenter(actual time-varying distance)is about 130km. The corresponding gravity change anomaly range value(twice the time-varying distance)is 260km, which is consistent with the actual value and falls in the range of 220~350km corresponding to M_S6.0 and M_S7.0 earthquakes. Considering the co-seismic and post-earthquake energy release of the aftershocks of 28 M_S3.0 ~4.0, 11 M_S4.0 ~5.0, 3 M_S5.0 ~5.6 earthquakes and combined with the calculation results of two gravity variation anomaly index parameters of magnitude and time-varying distance, it can be judged that the energy driving the activity of the underground materials in Yangbi area is completely released. The M_S6.4 earthquake is the main shock of the earthquake. The research results in this paperhelp us havea more objective understanding of the seismic characteristics in Yunnan area, the temporal and spatial dynamic evolution characteristics of the gravity field before the earthquake, and the performance of precursors, which provide a useful reference for earthquake analysis and prediction in this area.

Key words: Yangbi M_S6.4 earthquake, gravity change, four-quadrant, gravity gradient zone, anomaly index of gravity change

摘要：

文中对云南省区域范围内的2015—2021年的相对重力数据进行了平差计算, 结合区域地质构造环境和区域范围内已发生的地震震例, 系统分析了区域重力场变化和地壳水平运动的时空分布特征, 获得了区域重力场变化与漾濞 M_S6.4 地震之间的关系。结果表明: 1)2018—2021年震中位置的重力变化较弱, 以震中为中心, 附近区域在短期及1a期、 2a期和4a期基本呈四象限特征且重力变化较为剧烈; 2)震前短期及多年期重力变化显示, 重力场高梯度带集中在漾濞—下关区域, 且零值线交替出现在该区域; 3)震前多年期重力变化显示, 发震断裂带南涧-巍山断层的西侧重力呈负变化、 东侧重力呈正变化, 该地震为包含走滑分量的正断型地震, 与全球台网数据的计算结果一致; 4)对临震1a期重力变化异常指标进行了分析, 发现重力转折变化异常的量级为75×10^-8m·s^-2, 重力变化异常时变距约为130km。漾濞地震的实际震级 M_S6.4 稍大于重力异常指标公式的计算结果 M_S6.2, 且震后漾濞地区相继发生了42次M_S>3.0地震, 可判断出漾濞地区地下活动能量已完全释放, M_S6.4 地震为此处地震序列的主震, 主震后短期内无更大地震发生。

关键词: 漾濞M_S6.4地震, 重力变化, 四象限, 重力梯度带, 重力变化异常指标