姑咱台钻孔应变观测值年变化的数值模拟解释

INTERPRETATION OF BOREHOLE STRAIN ANNUAL CHANGE AT GUZAN STATION BY NUMERICAL SIMULATION

中国是最早开始以地震预测为目标的钻孔应变观测的国家之一。YRY-4四分量应变仪等中国自行研制的应变仪已达到世界先进水平,其分辨率可达10-10量级,且可以稳定工作。为了捕捉构造应力变化的信号以及探讨它们与地震活动的联系,首先需要识别并排除降雨、气压变化、地下水位变化、河流涨落、抽注水等非构造因素引起的地应变。在某些台站(如大渡河谷旁的姑咱台),YRY-4型应变仪观测到年周期变化的应变信号。截至目前,没有相关文献解释产生这种信号的物理机制。文中试图解释该信号的成因。考虑地形因素,建立了三维有限元热-弹性耦合模型,模拟了地表温度年变化引起的热应变,说明了春季和秋季达到波峰和波谷的年周期变化的信号是地表温度年变化引起的热应变信号。建议高分辨应变仪(例如YRY-4型应变仪)地应变台站选址时应尽量选择开阔的区域而避免地形起伏较大的区域。

China is one of the earliest countries to start borehole strain observation aiming to predicting earthquake. YRY-4 and other borehole strain instruments developed by China have reached the world advanced level,with a resolution of 10-10 and can work stably. In order to capture the tectonic stress signals and analyze the relationship with seismicity,it is firstly necessary to identify and exclude the non-structure stress signals（ caused e. g. by rainfall,pressure change,groundwater level fluctuation,river fluctuation etc）. Annual strain change signals have been observed by YRY- 4 borehole strain instrument at some stations（ e.g. Guzan station in Dadu River valley）. The signal is on the wave crest in spring and trough in autumn. Up to now, no relative papers have interpreted the physical mechanism causing such strain signals. In this paper,we manage to explain the physical mechanism by using numerical simulation. Considering the terrain factor of Dadu River valley,we set up a 3D finite-lement thermoelastic coupling model to compute the heat stress caused by annual change of surface temperature. The computing results are in good accordance with observation values,which illustrates that the annual change of strain signals is caused by annual change of surface temperature.We suggest that high- resolution borehole strain instrument（ e. g. YRY- 4 strain instrument） should avoid installing in a terribly undulate area,but choose a flat one. Although the strain signal with an annual change is a noise signal,to a certain extent,the signal illustrates that the current borehole strain instrument has a high resolution. This gives us more confidence to use this instrument to measure the structural strain.