Abstract: While a traditional seismograph records the variations of displacement with time at a point, a tensor strainmeter monitors the variations of the spacial gradient of displacement. An YRY-4 borehole strainmeter at Guza, Sichuan province, recorded three local seismic events at the sampling rate of 100 sps for the first time. At this high sampling rate, the data of the YRY-4 demonstrated a satisfying degree of self-consistency and sufficiently high credibility. The observation shows that P wave strain contains areal strain components as well as shear strain component. It gives direct evidence for the property of P wave. According to the theory of seismology, P wave is clarified to have no rotation, but it has volumetric strain and shear strains.

Key words: Seismic wave strain, YRY-4 borehole strainmeter, P wave, S wave