Comparison to mobility data
To generalize the relationship between seismic noise and mobility data,
we calculated an averaged seismic noise change for a single day by first
averaging 24 hours noise changes for each channel and then averaging
them across all channels. Figure 5 shows a crossplot between the noise
data and Google mobility data. The noise reduction in the frequency band
of 1–10 Hz is compared with the workplace mobility data, while the
10–50 Hz noise level reduction is compared with transport mobility
data. The good linear correlation between mobility change and changes of
seismic noise level allows us to relate variation of seismic noise level
and mobility changes as,
\begin{equation}
M_{\text{TL}}=1.49N_{\text{TL}}\nonumber \\
\end{equation}where \(M_{\text{TL}}\) is time-lapse mobility change and\(N_{\text{TL}}\) is time-lapse noise change. This linearity implies
that the seismic noise variation (1–50 Hz) is linearly proportional to
the amount of human activities, including foot traffic and road traffic.