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.