[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.

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.

A study of the Kunlunshan earthquake of Ms = 8.1 based on observed coseismic strain steps from the borehole strain monitoring network over China has been carried out with some interesting results. Firstly, many recordings disagree with theoretic calculation using static dislocation model. Secondly, abnormally large strain steps are observed at quite a few stations in the tectonically active east-northern China, while in the relatively inactive east-southern China no obvious steps are recorded. It is inferred that seismic stress triggering may significantly affect remote seismic strain field. In other words, whether remote faulting be seismically triggered or not may determine the pattern of local seismic strain changes. Further comparison study results of March 11, 1999 Zhangbei earthquake and November 1, 1999 Datong earthquake show that the specific pattern of seismic zones has obvious influence on seismic strain changes in the region. This supports the idea that observed abnormal strain steps might be produced by coseismicly stress-triggered local faulting.

Borehole strain observation is playing an increasingly important role in the study on the crustal movements. It has been used by many countries such as China, USA, Japan, Peru, Australia, South Africa, Iceland and Italy, in research fields of plate tectonics, earthquake, volcanic eruption, dam safety, oil field subsidence, mining collapse and so on. Borehole strainmeter has been improved rapidly and tends to get more and more components included in one probe. Based on observations by this kind of instruments, studies on seismic strain step, slow earthquake, earthquake precursor and volcanic eruption forecasting have made remarkable achievements. In the coming years, borehole strain observation is going to become one major geodetic means, together with GPS and InSAR.

Coseismic stress-triggering is becoming a new hot spot of research. Coseismic strain steps recorded by borehole strainmeters are particularly valuable in studying coseismic stress-triggered fault slips. Based on the theory of dislocation, one can invert the triggered fault slips with such data if he/she has a well understanding about the local faults. Genetic algorithm can be applied to significantly raise the efficiency of searching a best solution among all possibilities in this kind of inversion. A testifying check of the program and analyses of each parameter's influence may further enhance the reliability of inversion results. Taking complexity of geological structure into account, the inversion results should be regarded as the predominant property or a comprehensive effect of triggered local faults' activities. As an attempt, we inverted the assumingly active faults' slips triggered by the M S =8.1 Kunlun Mountain earthquake over Beijing area.

Rotation is antisymmetric and therefore is not a coherent element of the classical elastic theory, which is characterized by symmetry. A new theory of linear elasticity is developed from the concept of asymmetric strain, which is defined as the transpose of the deformation gradient tensor to involve rotation as well as symmetric strain. The new theory basically differs from the prevailing micropolar theory or couple stress theory in that it maintains the same basis as the classical theory of linear elasticity and does not need extra concepts, such as "microrotation" and "couple stresses". The constitutive relation of the new theory, the three-parameter Hooke's law, comes from the theorem about isotropic asymmetric linear elastic materials. Concise differential equations of translational motion are derived consequently giving the same velocity formula for P-wave and a different one for S-wave. Differential equations of rotational motion are derived with the introduction of spin, which has an intrinsic connection with rotation. According to the new theory, S-wave essentially has rotation as large as deviatoric strain and should be referred to as "shear wave" in the context of asymmetric strain. There are nine partial differential equations for the deformation harmony condition in the new theory; these are given with the first spatial differentiations of asymmetric strain. Formulas for rotation energy, in addition to those for (symmetric) strain energy, are derived to form a complete set of formulas for the total mechanical energy.

In the field investigations of the great 1976 Tangshan earthquake in China, several large subsidence areas with remarkable boundary fissures were reported as phenomena of the seismic event. Recently a re-interpretation of aerial photos taken immediately after the quake revealed that the areas were controlled by a giant fault of about 40 km in length; and subsidences were brought about by the movement of the fault during the shock, not being a consequence of soil liquefaction as suggested in some of the investigations. This discovery has been confirmed by the results of fine geophysical explorations carried out by the Jidong Oilfield Company, PetroChina. The newly discovered fault, named Fuzhuang-Xihe fault, is much longer than the one prevailingly accepted as the seismic fault of the Tangshan earthquake. Moreover, the former seems to be a normal fault, while the latter is regarded as a strike-slip one.