摘要
An ASKANIA borehole tiltmeter which consists of a vertical pendulum with two degrees of freedom was installed in Xiangshan station,Beijing,in May 1989 in order to monitor the earthquake-reiated crustal deformations.The preliminary results indicate that the resolution of the instrument is better than 0.05 msec for tidal amplitudes.The daily total tidal amplitude is about 20 msec.The spectral analysis of the first four months data provides a signal-to-noise ratio of 】 100 for the main semi-diurnal waves.The non-tidal tilts for one year data are dominated by a long term drift with the rate of 0.9 msec/day for X-component and 1.1 msec/day for Y-component.In addition,the tilt observations are affected by meteorological factors,especially by rainfall.The results of modelings of the fault-creep related tilts and responses of tilts to earthquakes suggest that the tilt measurements are very sensitive to the change in tectonic strain and stress fields near the site where the tiltmeter is installed.The
An ASKANIA borehole tiltmeter which consists of a vertical pendulum with two degrees of freedom was installed in Xiangshan station,Beijing,in May 1989 in order to monitor the earthquake-reiated crustal deformations.The preliminary results indicate that the resolution of the instrument is better than 0.05 msec for tidal amplitudes.The daily total tidal amplitude is about 20 msec.The spectral analysis of the first four months data provides a signal-to-noise ratio of > 100 for the main semi-diurnal waves.The non-tidal tilts for one year data are dominated by a long term drift with the rate of 0.9 msec/day for X-component and 1.1 msec/day for Y-component.In addition,the tilt observations are affected by meteorological factors,especially by rainfall.The results of modelings of the fault-creep related tilts and responses of tilts to earthquakes suggest that the tilt measurements are very sensitive to the change in tectonic strain and stress fields near the site where the tiltmeter is installed.The fault-creep related effects on the horizontal surface are concentrated in the area with Xt=±(L/2)(half length of the creeping-fault),X2=±W(width of the fault).On the surface of a hill the tilt effects are amplified by the elastic medium of the hill.The distance of the borehole site from a fault should be at least bigger than the crept segment size of the fault in order to reduce the creep-related tilt effects for earth tide observations,but for earthquake prediction research the borehole site might be close to the fault in order to monitor effects of fault creep.The results of modelings of the variations in strain and tilt observations in BTTZ region(Beijing-Tianjin-Tangshan-Zhangjiakou)area caused by the 1989 Datong-Yanggao earthquake with M=5.8 are given in this paper.The calculated values of co-seismic volumetric strain changes for Changping station agree well with the recorded ones.
