A Discussion on the measurement and interpretation of changes of strain in the Earth - Quartz fibre accelerometers

Abstract

New vertical and horizontal accelerometers based on torsion fibres of fused quartz and differential capacitance position sensors are discussed. The instruments have low drift, flat response from d.c. to nearly 1 Hz, and gain sufficient to measure tidal and seismic accelerations. They are small, weigh less than 9 kg, and have strict control of internal temperature and pressure. Tidal, free oscillation, and seismic data from the accelerometers are presented. Records of earthquakes of surface wave magnitude 6.5 to 7.5 yield free oscillation spectra with greater than 20 dB signal-to-noise ratio. Records of distant teleseisms provide a detection threshold of surface wave magnitude 2.5 at a distance of 30°. A record of the San Fernando earthquake of February 1971 gives an upper bound of 1mm for a vertical displacement step 190 km from the source.