What causes the remarkable tilt anomalies at the Hancheng geodynamic observatory in Shaanxi Province?
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摘要: 巨幅地倾斜异常既可能是地震前兆异常信息，也有可能是仪器问题或环境干扰所致的异常信号. 有效地厘清其性质，对地震前兆异常的及时识别与科学判定至关重要. 陕西韩城台金属水平摆EW分量自2010年以来连续两次出现巨幅东倾和西倾异常，幅度分别约达140″和180″，该巨幅地倾斜异常产生的根源至今尚未被厘清. 鉴于此，本研究依据韩城台所在区域的水文、构造和地震活动等特征，提出并分析了地下水动力变化、韩城断裂慢滑移和地壳应力场变动等3种可能的成因机制. 结果表明，第一种成因机制难以有效地解释巨幅地倾斜异常，第二和第三种成因机制则具有一定的可能性，但证据还不够充分. 因此，更可靠的物理解释尚需更多的观测和更深入的研究. 尽管本文未能给出该巨幅异常的真正成因，但所采取的分析方法可为今后巨幅地倾斜异常性质的判定工作提供有益的参考.Abstract: The remarkable tilt anomalies could be the earthquake precursors, but may also be caused by instrumental factors and environmental disturbances. Thus, the question arises on how to distinguish the earthquake precursors from the non-tectonic factors, which is very important to effectively and reasonably detect earthquake precursors. Since 2010, two remarkable tilt anomalies have been recorded by metallic horizontal pendulums in E-W component at the Hancheng observatory in Shannxi Province, and the amount of east- and west-ward tilt approxi-mately reach up to 140″ and 180″, respectively, but these two remarkable tilt anomalies have not been reasonably and clearly interpreted till now. Here, we propose and compare three different causal mechanisms possibly responsible for these anomalous phenomena according to the regional hydrological, tectonic and seismicity characteristics, i.e. ① hydrodynamics-induced surface tilt, ② a long-term slow slip event on the northeastern segment of the Hancheng fault, and ③ variations of the regional tectonic stress field during the anomalous period. We then theoretically calculated the poroelastic deformation and the fault slip amount, and finally inversed the focal mechanism solutions of 85 earthquakes （2.0≤ML≤4.8） that occurred between 2008 and 2015 with the aim of determining the regional stress field changes （35°N—36°N, 110°E—111°E） in the crust. Our results show that the first possibility can be shown unlikely, but it is difficult to rule out the second and the third possibility according to the current eviden-ce. To further prove and confirm the causal relationship between deformation of tectonic origin and the anomalies, more comprehensive tilt and crustal deformation measurements are necessary in the Hancheng region in the future, furthermore, more intensive researches are also needed to reveal and determine the causal mechanisms of these anomalies. Unfortunately, we fail to find the real causal mechanism, but the approaches used in this study could be helpful to investigate the causal origin of remarkable anomalies recorded by tiltmeters in the near future.
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图 1 研究区域及台站概况
（a） 区域构造背景、台站分布、历史强震及2008—2016年ML≥2.0地震分布；（b） 韩城台、龙门水文站、韩城断裂NE段和黄河的地理位置；（c） 韩城台及韩城断裂剖面图；（d） NS和EW分量金属水平摆；（e） 韩城台及周边地区与黄河之间地下水动力示意图
Figure 1. Map view of the investigation region and stations containing regional tectonic setting，earthquake events，hydrology and station locations
（a） Regional tectonic setting，the distribution of ML≥2.0 events during the period from 2008 to 2016 and significant historical earthquakes；（b） Distribution of the Longmen hydrological station，Hancheng station，NE segment of Hancheng fault and Yellow River；（c） Picture showing the Hancheng station and Hancheng fault plane；（d） Picture of NS and EW components of the metallic horizontal pendulums；（e） Conceptual cartoon illustrating the groundwater hydrodynamics between Hancheng station and Yellow River
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图 2 韩城台地倾斜观测曲线
（a） NS分量变化；（b） EW分量巨幅异常变化；（c） 地倾斜的矢量变化
Figure 2. The secular ground tilt recorded by Hancheng station
（a） Tilt variations in NS component；（b） Remarkable tilt anomalies in EW component；（c） The hodograph showing the changes of tilt vector
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图 3 经验模态分解方法提取的地倾斜周年变化信号（a）和观测室气温变化（b）
Figure 3. Annual variations of tilt retrieved by EMD （a） and room temperature changes recorded at Hancheng observatory （b）
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图 4 1977年黄河特大洪峰引起的巨幅地倾斜
（a） 龙门水文站记录的黄河特大洪峰；（b） 韩城台记录到的SE向巨幅地倾斜
Figure 4. Remarkable ground tilt induced by the great flood peak of the Yellow River in 1977
（a） The 1977 great flood peak of the Yellow River recorded at Longmen hydrological station；（b） SE-ward tilt recorded at Hancheng station
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图 5 黄河水位（a）、S34井水位（b）及运城气象站（c，d）观测的降雨变化
Figure 5. Hydrology and rainfall information of Hancheng station and its adjacent regions
（a） The water level of Yellow River recorded at Longmen hydrological station since 2007；（b） Changes in groundwater level in S34 well；（c） Time series of cumulative annual and daily precipitation observed at Yuncheng meteorological station；（d） Cumulative daily and detrended time series from 2001 to 2017
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图 6 断裂慢滑移引起的EW分量的理论倾斜场
Figure 6. Theoretical ground tilt in EW component induced by slow slip event on fault
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图 7 巨幅地倾斜异常前后研究区内85次2.0≤ML≤4.8地震的震源机制解及应力场反演结果
（a） 2008年10月10日至2010年2月26日研究区内28次地震的震源机制解；（b） 2010年3月14日至2015年9月15日研究区内57次地震的震源机制解；（c） 2008年10月10日至2010年2月26日研究区内应力张量；（d） 2010年3月14日至2015年9月15日研究区内应力张量
Figure 7. Focal mechanism solutions for the 85 earthquakes with 2.0≤ML≤4.8 and stress field inversion in the studied area before and after the remarkable tilt anomalies
（a） Focal mechanism solutions for the 28 earthquakes from Octorber 10，2018 to February 26，2010；（b） Focal mechanism solutions for the 57 earthquakes from March 14，2010 to September 15，2015；（c） The principal stress axes for the period from October 10，2008 to February 26，2010；（d） The principal stress axes for the period from March 14，2010 to September 15，2015
表 1 韩城台概况
Table 1 General information of Hancheng station
台基
岩性海拔
/m观测室情况 金属水平摆仪器参数 仪器支墩
材质及尺寸覆盖层
厚度/m室内气温
年变幅/℃室内气温
日变幅/℃相对
湿度仪器
类型记录
方式折合摆长
/mm使用周期
/s奥陶系
灰岩460 0 17—18 ≤0.5 90% JB 光记录 NS分量：25.2
EW分量：25.418—19 混凝土，
高0.7 m
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