The People’s Magnitude: Characterizing Seismic Motions Generated by Human Crowds

Human activity induces ground motions that dominate >1 Hz seismic data recorded in semi-rural and urban areas. Whereas the seismic noise generated by industrial and traffic activities is relatively well characterized, weaker pedestrians-related noise remains less well understood. Here, I examine the spatiotemporal distribution of seismic amplitudes to uncover the effects of human crowds and derive semi-empirical attenuation relationships between seismic amplitudes and crowd sizes. I utilize recordings from the dense MesoNet seismic network, which consists of about 300 accelerometers, primarily located near schools in the Tokyo metropolitan area. These data exhibit strong temporal variations: maximum daytime amplitudes are recorded during school hours, and minimum amplitudes coincide with daily breaks in activity during lunch and afternoon teatime. Outside school hours, I observe a wide-spread amplitude peak at 3 to 5 am daily, likely due to truck traffic. The peak amplitudes correlate very well with the number of students in each school. Given that ambient traffic volumes differ substantially in the area covered by the MesoNet stations, this correlation suggests the amplitudes are primarily influenced by daytime pedestrian school activity. I supplement the MesoNet dataset with seismograms of well-recorded outdoor cultural activities with participant sizes ranging from 200 to about 100,000. After correcting for attenuation due to surface-wave geometrical spreading, the amplitudes are found to scale logarithmically with the crowd’s size. This finding indicates that seismic data can be effectively used to study trends in pedestrian mobility within urban environments.