Abstract: In 2012, three earthquakes occurred in Tianjin and surrounding areas, which are Tangshan M_S4.8, Baodi M_S4.0 and Baodi M_S3.5 earthquakes. This article collected water level observation data and barometric pressure data of the Baodi well, the Wang3 well, the Gaocun well and the Zhangdaokou well in 2012, which have good responses to earth tides. We removed barometric pressure effects and trend component from water level observation data, then performed digital filtering in order to enhance tidal signal of water leve. After that, we adopted fitting model similar to the Nakai to calculate loading and unloading response ratio (LURR) of water level by taking data of two days into a group. The results show that, LURR increases anomalously before the three earthquakes, and it has a better result when digital filtering window length is 96 h or 120 h. We propose that LURR shows high value change before earthquake due to accumulation of stress, and small earthquakes have relieving effect to accumulation of stress. Combining seismicity and focal mechanism with LURR, we can have a better understanding of the stress accumulationrelease process.

Key words: Water level, Earth tides, LURR, Barometric pressure effects, Seismic activity