Scaling of Seismic Ground Motions Generated by Human Crowds

Human activities generate seismic ground motions that dominate the high-frequency (>1 Hz) seismic spectra in urban environments. Although vehicular and train traffic have been widely studied, the seismic signature of large pedestrian crowds remains less well quantified. Here, I analyze seismic recordings from dense urban networks in Japan and combine them with observations from outdoor gatherings in the United States and Israel to characterize crowd-generated seismic signals. Through clustering analysis of ground motions recorded by the dense Metropolitan Seismic Observation Network in the Tokyo metropolitan area, I identify distinct temporal and spatial patterns associated with vehicular traffic. I then examine 385 outdoor gatherings in the United States and 300 schools in Tokyo, together with observations from the 2021 Meron pilgrimage in Israel, a 100,000-person event that resulted in a deadly crowd crush. Crowd-generated seismic energy is strongest in the 1–2 Hz band, consistent with human walking frequencies. Distance-corrected seismic amplification factors scale approximately linearly with the logarithm of crowd size across five orders of magnitude but form two distinct populations: energetic outdoor protests produce larger distance-corrected amplification factors and stronger scaling than routine student activity in Tokyo schools, suggesting that crowd-generated seismic excitation depends on collective crowd behavior. This relationship provides a framework for using seismic observations to detect and characterize large human gatherings in urban environments.