Lapse-time dependence of dv/v in response to tides
Takano, T., T. Nishimura, H. Nakahara, E. Fujita, and H. Ueda (2019), Sensitivity of seismic velocity changes to the tidal strain at different lapse-times: Data analyses of a small seismic array at Izu-Oshima volcano, J. Geophys. Res. Solid Earth, 124, https://doi.org/10.1029/2018JB016235. [PDF]
We investigate seismic velocity changes in response to the tidal strain at Izu-Oshima volcano, Japan, by analyzing the data of permanent seismic stations and a small seismic array to evaluate the characteristics of strain sensitivity of velocity changes. We estimate the seismic velocity changes by phase differences between cross-correlations functions of ambient noises at the frequency of 2-4 Hz stacked for time periods with different tidal strain amplitudes. The seismic velocity changes decrease and increase during dilatation and contraction periods, respectively, when analyzing the cross-correlations functions at early lapse-times ranging from 2 to 7 s. However, we find that strain sensitivity of the seismic velocity changes decreases when analyzing the cross-correlation functions at later lapse-times from 7 to 35 s. Applying an array analysis to the cross-correlation functions, we observe apparent velocities of about 1 km/s at the early lapse-times and those of higher than 1 km/s at the late lapse-times. Since the group velocity of Rayleigh waves is 1.1 km/s at Izu-Oshima volcano, the apparent velocities at the late lapse-times may indicate the scattered or reflected body waves incident from a deeper region. Decrease of strain sensitivity with the lapse times therefore results from the emergence of body waves on the late lapse-times. These results highlight the need to pay attention to wave types of cross correlation functions and their paths to interpret seismic velocity changes.