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

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