Abstract
AbstractWe report the results of continuous monitoring—using a seismometer array—of the travel time of seismic waves generated by an ACROSS artificial seismic source. The seismometer array, which was deployed in a surface vault located 2.4 km from the source, recorded both directP- andS-waves and refractedP- andS-waves that traveled along a velocity boundary between the granite basement and overlying sedimentary rocks. We analyzed temporal variation in differential travel time and apparent velocity for these phases for a period of 1 month and found significant temporal variation in the differential travel time. Most of the variation can be attributed to changes in environmental conditions, such as atmospheric temperature and rainfall. Variation is even observed in the seismogram that is located 50 m from the vibration source, although much smaller variation is observed in the vibration of the foundation to which the source is attached. The spectral study revealed that the effects of temperature and rainfall depend strongly on the frequency range used by ACROSS and that a large variation occurs in the 15- to 20-Hz range, especially between 17 and 20 Hz. The environmental effect on the temporal variation is comparable to the record of refractedSwaves and that of a distance of 50 m, whereas a larger variation was observed in the directSwave. This result shows that the signal is affected by the environmental change near the vibration source. The environmental effect can be drastically reduced when the signal from the 15- to 20-Hz range is eliminated in the analysis.
Highlights
Various methods have been tested for their sensitivity and reliability to detect changes in the propagation properties of seismic waves related to changes in the travel medium
The correlation coefficient in this period is −0.72, the correlation coefficient for the whole period is −0.17. These results demonstrate that atmospheric temperature affects travel time variation
As uncertainty is within 0.5 ms for the observation period, no marked variation can be correlated with other observation data. 5.4 Frequency dependence of the temporal variation in differential travel time We investigated the frequency dependence of the temporal variation in differential travel time to take advantage of the frequency-discreteness of the Accurately Controlled Routinely Operated Signal System (ACROSS) signal
Summary
Various methods have been tested for their sensitivity and reliability to detect changes in the propagation properties of seismic waves related to changes in the travel medium (see Crampin et al, 1990; Li et al, 1998; Niu et al, 2003). Saiga et al (2003) reported temporal variation in the time delay of split shear waves resulting from static stress changes associated with a moderate magnitude earthquake These observations demonstrate the sensitivity of the propagation properties of seismic waves to changes in applied stress; these researchers were not always able to analyze events of similar source parameters. As our aim was to detect temporal variations in the propagation properties of seismic waves, it was important to minimize artificial variations resulting from the observation system We deployed both seismometers and the data-logging system in the vaults, where temperature remains constant to within 0.1◦C for the entire year. We use cosine-taper windows in the following analysis as an optimum window function
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