Abstract:

Fixed point deformation observation, with a broad application prospect, plays an important role both in monitoring crustal deformation and capturing possible seismic precursors. Due to advantages of high precision, broad bandwidth and relative good continuity, fixed-point deformation observation, together with InSAR and GPS, has become one of the major methods in crustal deformation researches.
During the daily observation, it is usual to encounter various load disturbances, such as heavy rain, reservoir, river, building constructions etc. However, when quantitatively evaluating the effects of such load disturbance, a number of studies were conducted without taking into account the rock mass integrity in the study area, i.e. simply using general rock mechanical parameters rather than specific rock mass mechanical parameters, which therefore greatly affect the final interpretation and the elimination of disturbance, or even the identification of seismic precursory anomalies.
In this study, the strain survey data at Xiaoxinzhuang seismic station in Jixian County of Tianjin are taken as an example to illuminate that the integrity or fragmentation of rock mass in each specific site should be considered when conducting quantitative analysis, only thus can the influence of external load on deformation observation be evaluated more accurately.
Firstly, a detailed joint statistic of the rock mass around Xiaoxinzhuang and Yuqiao reservoir was carried out; then the elastic modulus of rock mass of the study area was derived from the equation between rock mass volumetric joints and elastic modulus; finally the theoretical compressive strain at Xiaoxinzhuang station produced by Yuqiao reservoir was calculated by an irregular load model, and the influence of Yuqiao reservoir on the strain of Xiaoxinzhuang station was also evaluated.
Geological surveys, combined with numerical simulation indicate that: 1)The exposed rocks are dominated by Middle Proterozoic dolomites and dolomites with minor limestone at Xiaoxinzhuang seismic station and its vicinity, and the corresponding volumetric joint counts of rock mass are more than 32, the highest values of 41 and 39 occurred at Ligezhuang and south of Yuqiao reservoir, respectively, which means the bedrocks are moderately fragmented; 2)The elastic modulus of rock mass in the study area is only 0.10~0.19 times(an averaged value of 0.14)of that of integral rock, which means that under the same stress the effect of external load disturbance will be magnified about 5~10 times(an averaged value of 7 times); 3)During the impoundment period of Yuqiao reservoir from September to November in 2019, the theoretical compressive strain at Xiaoxinzhuang station in the north-south direction induced by Yuqiao reservoir impounding is about(93.76~200.91)×10^-8, calculated by an irregular load model, accounting for about 10%~21% of the actual observed strain; 4)Therefore, it is concluded that the earlier claimed anomaly is not an earthquake precursory anomaly, but probably the effect of water level change of Yuqiao reservoir.

Key words: deformation observation, load disturbance, rock mass integrity index, elastic modulus, joint statistics, volumetric joint counts

摘要：

定点形变观测是监测地壳形变和捕捉地震前兆的重要手段, 具有广阔的应用前景。 在定点形变的日常观测中, 经常碰到各种“载荷干扰”, 然而, 在定量分析此类干扰的影响时, 不少研究没有考虑具体场地岩体的完整(破碎)程度, 简单地采用了通用的“岩石弹性模量”而非“岩体弹性模量”, 从而影响到最终结果的解释及异常或干扰的识别与排除。 文中以天津蓟县小辛庄的洞体应变为例, 阐明应当考虑具体场地岩体的完整性, 才能更准确地评估外界载荷干扰的影响。 野外调查及数值模拟结果表明: 1)小辛庄周围岩体的体积节理数普遍>32条/m³, 达到比较破碎的程度; 2)研究区岩体的弹性模量仅为完整岩石弹性模量的0.10~0.19倍, 即在相同的应力条件下, 外界载荷干扰的影响将被放大5~10倍; 3)2019年9—11月于桥水库蓄水期间, 水库水位的上升对小辛庄应变SN向所产生的压缩应变理论值为(93.76~200.91)×10^-8, 约占实际观测值的10%~21%; 4)2019年9月小辛庄洞体应变出现的破年变变化并非是地震前兆异常, 而有可能是受于桥水库的库容变化影响所致。

关键词: 形变观测, 载荷干扰, 岩体完整性指数, 弹性模量, 节理统计, 体积节理数