Abstract: Four-component borehole strain gauges detect deformation in four orientations in the borehole to monitor long-term variation of earth stresses, with large frequency coverage including signals from earthquakes, solid earth tides, and secular changes. However, the records also contain disturbances from factors other than tectonic stresses. Therefore, it is an important task to investigate the causes of these disturbances. Guzan station, which is in Sichuan Province, is located on the bottom of a steep valley of Dadu River. This station has recorded not only obvious annual changes of amplitude up to 10^-6, but also irregular fluctuations following the rise of river water level in the wet season every year. In this study, stresses generated by the change of river water level are calculated using numerical method of elastic mechanics. Preliminary calculation results indicate that water level fluctuation of several meters in Dadu River produces stress variation of kPa magnitude. The fact that borehole strain meter can be used to record changes in river water level is significant. On the one hand, the fact shows that the borehole strain gauges indeed detect the very small change in stress variation, which indicates the high sensitivity of the instrument. On the other hand, the fact shows that the location of the strain meter station should be away from steep valleys as far as possible, in order to reduce the interference and highlight the true tectonic stress changes. Observations of improved quality can be applied in the study on relations between earth stress and seismicity.

Key words: borehole strainmeter, noise in strain observation, finite element method, numerical simulation