Monitoring Seismic Velocity Variation Of Uppermost Subsurface Beneath Xiaojiang Fault Zone Using Direct Wave Cross Correlation Technique With Active Source

It is essential to monitor subsurface medium properties for understanding earthquake evolution process, and theoretically velocity variation can be an indicator of the variation of medium properties. Active source have become an important tool to monitor underground medium for its high repeatability. We designed a set of medium-scale seismic velocity change observation system with high precision to continuously monitor subsurface velocity variation in the seismically very active Xiaojiang fault zone.Aiming at increasing precision of monitoring velocity change in the medium, we acquired results of velocity change with high precision, analyzed factors that influence the precision of seismic velocity change measurement, and discussed the reason for the velocity change.Through analyzing and processing of the continuous observation data, we got these following conclusions:(1) Throughout the experiment, a fractional velocity perturbation (δv/v) of10-3to10-2was estimated.(2) The velocity change increased with depth, but had the same tendency at different depths.(3) The measured velocity variation appeared to well correlate with barometric pressure, which had a stress sensitivity of10-6/Pa at the experimental site.(4) We observed a positive stress sensitivity to the barometric pressure before the rain, while after the rainfall the estimated velocity changes showed a clear negative-correlation with barometric pressure.This study provides a basis for the further research on monitoring temporal variation of seismic velocity using direct wave cross correlation technique with active source.