Borehole Stress/Strain Observation And Earthquake Research

High resolusion borehole stress/strainmeters provides an ideal type of instruments for observing modern tectonic deformation in the earth's crust. This report involves our research works in the fields of coseismic earthquake stress triggering, fault activities, free oscillation of the earth, precursory changes prior to earthquakes and earthquake forecast. However, all the researches are related to the observations using borehole stress/strainmeters.Coseismic stress triggering is becoming a frontier subject of earthquake research. Strain steps recorded by borehole strainmeters are of special values in studies of coseismic stress triggered fault activities. According to the linear dislocation theory, we can apply the strain step data to inverse the coseismicly triggered fault slips based on a well understanding of the local active fault distributions. Taken into account the complexity of geological structure, result of such inversion may reflect a comprehensive influence of triggered activities of all local active faults. An attempt inversion has been carried out for possible triggered activities of some major avtive faults over Beijing area by the Ms8.1 Kunlun earthquake.Modern fault activities can also be inversed with the same method based on the linear dislocation theory and applying the continuos recordings of borehole strainmeters. An attempt inversion has been carried out for possible activities of some major avtive faults over Beijing area during the period from 2003 to 2005.When the great Sumatra-Andaman earthquake earthquake happened on 26 December 2004, two different types of borehole strainmeters, namely TJ-dilatometer and RYR-shear-strainmeter, recorded Earth's free occilation ecxited by this seismic event. TJ-dilatometer is suitable for recording spheroidal modes of Earth's free occilation, while RYR-shear-strainmeters particularly fits observing toroidal modes. The strainmeters also observed the coupling of spheroidal and toroidal modes as well as some split modes. Q values for modes of 0T3~0T22 are for the first time calculated with direct stran observations of toroidal modes. Since the RYR-shear-strainmeter records shear strain changes in two perpendicular directions, orientations of toroidal maximum shear strain changes are calculated and found to be stablly normal to the symmetrical axis of Earth's free occilation. It agrees with theoretical anticipation.Following a study on stress changes prior to the 1976 Tangshan earthquake, we discovered a major fault related to this disarstrous seismic event. The newly revealed great fault is at least 90km and has a coseismic vertical dislocation of 3m at the surface. What is more, it possesses the characteristics of a normal listric fault. The great fault can be divided into two parts. The southwestern part is proved by field investigation, leveling data, aerial photography and geophysical exploration, while...