Determination and analysis of near-surface stress-strain state of 4-component borehole strainmeter

Objectives: Data on near-surface continuous crustal strain variation are produced in large quantities by the 4-component borehole strainmeter, but correct analysis is needed when using these data. Methods: In this study, based on the mechanical conversion method of 4-component borehole strainmeter, we propose a method to determine the stress-strain state by using 4-component borehole strain observations under three stress environments: tension-compression stress, tension stress, and compression stress. Results: Taking the M6.9 Menyuan earthquake as an example, we investigate the background stress-strain state near the epicenter by the proposed method. The results show that the stress-strain concentration area may have strain tension near the surface. After analyzing the dynamic change of the strain direction of the Menyuan station, we suggest that the increasing rate of the strain direction change that occurred five days before the earthquake was an anomaly before the earthquake. Following the comparison between the coseismic strain state measured by the borehole strainmeter and the modeled results according to elastic dislocation theory, the results showed that the observed results are in good agreement with the modeled results. Conclusions: The application analysis shows that the method proposed in this paper to determine the stress-strain state of borehole area is feasible and effective.