[1] Borehole strainmeters have proved very useful in geodynamic research. Because the sensors are imbedded in rock, their in situ calibration is of crucial importance. The four-gauge borehole strainmeter (FGBS) is a Chinese invention to monitor the temporal variation in horizontal strain. The four gauges in the FGBS are arranged at 45 intervals to bring about a simple self-consistency equation, which serves as a means of checking that the measurements obtained from the FGBS are correct. The instruments currently used in China are usually placed at depths of several tens of meters to avoid disturbances at the surface, while still being sufficiently near the surface for the vertical stress to be regarded as zero-the premise on which the theoretical model of this observation is based. In this paper, an index of data credibility is established, based on the self-consistency equation, to allow evaluation of the observations. A relative in situ calibration has been developed to calculate a relative correction factor for each gauge's sensitivity, termed the gauge weight, and this has proven effective in enhancing data credibility. Parameters for deriving strain from readings are determined by a concise absolute in situ calibration with the aid of the theoretical Earth tide. Instead of averaging four groups of solutions, a simpler comprehensive algorithm is developed to transform readings into strain. Data from 24 Chinese sites of YRY-4-type FGBS are processed and evaluated to be fairly good.

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An YRY-4 type borehole strainmeter at Guza station on the southwest end of the Longmenshan fault zone recorded anomalous changes from time to time since about one year before the Wenchuan earthquake. The anomaly is characterized by steps and/or asymmetrical pulses of short-periods (minutes-hours). Among about one hundred stations over the mainland of China, only Guza, the nearest one to the seismic area, observed such changes. A self-check function of the strainmeter helps confirm the credibility of the observation. The anomalous signals do not correspond to seasonal changes. The consistency in the sense of strain among long-term, anomalous interfering and coseismic changes suggests that they all resulted from the background tectonic movement. Results of an Overrun Rate Analysis further illustrate a close temporal relevance of the anomaly to the Wenchuan earthquake: its increase before, and decrease after the biggest changes during the main shock. The abnormal changes are attributed to local small-scale fractures with an evidential clue that the only M s 4.5 foreshock near Guza station on February 27, 2008 produced the biggest anomalous change. By means of Wavelet Decomposition, it is revealed that longer period portion of the anomaly took place later before the main shock, consistent with the idea that scales of small fractures increase during earthquake preparation process. The anomalous strain changes are analogous to the stress changes observed before the 1976 Tangshan earthquake.Wenchuan earthquake, earthquake precursor, strain change, 4-component borehole strainmeter, Overrun-Rate-Analysis

At a sampling rate of 100 samples per second, the YRY-4 four-gauge borehole strainmeters (FGBS) are capable of recording transient strains caused by seismic waves such as P and S waves or strain seismograms. At such a high sampling rate, data from the YRY-4 strainmeters demonstrate fairly satisfactory self-consistency. The strain tensor seismograms demonstrate the senses of motion of P waves, that is, the type of seismic wave travels in the direction of the maximum normal strain change. The observed strain patterns of S waves significantly differ from those of P waves and should contain information about the source mechanism. Spectrum analysis shows that the strain seismograms are consistent with conventional broadband seismograms from the same site.

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