Whispering of the city: Characteristics and origin of environmental shaking in the Taipei metropolitan area

Abstract Examining continuous seismic data recorded by a dense broadband seismic network throughout Taipei shows for the first time, the nature of seismic noise in this highly populated metropolitan. Using 140 broadband stations in a 50 km x 69 km area, three different recurring, strong noise signals characterized by dominant frequencies of 2–20 Hz, 0.25-1 Hz, and < 0.2 Hz are explored. At frequencies of 2–20 Hz, the seismic noise exhibits daily and weekly variations, and a quiescence during the Chinese New Year holidays. The largest amplitude occurred at a station located only 400 m from a traffic-roundabout, one of the busiest intersections in Taipei, suggesting a possible correlation between large amplitude with traffic flow. The median daily amplitude for the < 0.2 Hz and 0.2-1.0 Hz frequency bands are mostly synchronized with high similarity between stations, indicating that the sources are persistent oceanic or atmospheric perturbations across a large area. The daily amplitude for the > 2 Hz band, however, is low, indicating a local source that changes on shorter length scales. Human activities responsible for the 2–40 Hz energy in the city, we discovered, are able to produce amplitudes approximately 2 to 1500 times larger than natural sources.