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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We have found that the current practical Newtonian formula for the gravity tide of the Earth implies a hypothesis that gravity travels at the speed of light; furthermore, we have derived and solved the propagation equation of gravity using the observation data of Earth tides from Shiquanhe and Wushi, after correction of phase lag due to the anelasticity of the Earth, and found that the speeds of gravity are from 0.93 to 1.05 times the speed of light with a relative error of about 5%. This provides first set of strong evidences to show that the speed of gravity is the same as the speed of light.

In 2004, China's digital seismic observation network project began to deploy 40 sets YRY-4 four-component borehole strainmeters in order to monitor earthquake preparation process. The paper describes observed solid tidal strain discreteness and tidal factor anisotropy, analyzes the reliability of observational data and discusses the cause for this phenomenon. After getting rid of interferences, the network, in two years practice, has observed several pre-seismic strain anomalies at stations close to epicenters especially in the Wenchuan M S 8.0 megaquake. It shows that this borehole strainmeter network is capable of monitoring seismogenic process.

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