Asymmetry of coda cross-correlation function: dependence of the epicenter location

K. Emoto , M. Campillo , F. Brenguier , X. Briand , and T. Takeda

Tuesday 3 March 2015, by briandx // Publications

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Asymmetry of coda cross-correlation function: dependence of the epicenter location

K. Emoto , M. Campillo , F. Brenguier , X. Briand , and T. Takeda


Abstract

We analyze the cross-correlation function (CCF) of coda of earthquakes, which is used to retrieve the Green’s function between two stations as
well as the CCF of ambient noise. We select 74 Hi-net stations located in eastern Japan and 66 earthquakes to calculate the CCF. For each earthquake, we calculate the CCFs 10 between possible pairs for the frequency bands of 0.1-0.2Hz, 0.2-0.4Hz and 0.4-0.8Hz. Then we stack the CCFs for different earthquakes at each pair to obtain the average CCF.
Although the correlation coefficients between the average and each CCFs are lower than 0.5 for most of the earthquakes, we obtain the propagating
Rayleigh wave trace from average CCFs. We focus on the ratio of the amplitude in the positive lag time of the CCF to that in the negative lag time. CCFs for different earthquakes show different ratios which depend on the angle between the path of two stations and the epicenter. The
amplitude in the lag time corresponding to the signal traveling from the near source station to the far source station is larger than that in the opposite lag time. Therefore the energy flux is not isotropic even in the
coda and the energy from the source side is dominant. We average the ratios of pairs whose absolute values of angles are less than 45 degrees. The average ratios are 0.5 at 0.1-0.2Hz. For higher frequencies, the ratio is not clear because of the bad signal to noise ratio. According to the diffusion model, the ratio is predicted as 0.6. Therefore, the coda is represented as the diffusion state in 0.1-0.2Hz with our observation setting.

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