Atsushi Saiga

Continuous observation of seismic wave velocity and apparent velocity using a precise seismic array and ACROSS seismic source

We 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 direct P- and S-waves and refracted P- and S-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 refracted S waves and that of a distance of 50 m, whereas a larger variation was observed in the direct S wave. 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.