Characterizing Broadband Seismic Noise in Central London

Recordings made at five broadband seismometers, deployed in central London during the summer of 2015, reveal the wideband nature (periods, T, of between 0.01 and 100 s) of anthropogenic noise in a busy urban environment. Temporal variations of power spectral density measurements suggest transportation sources generate the majority of the noise wavefield across the entire wideband, except at the secondary microseismic peak (2< T 20 s) which are recorded across the city. We record a unique set of signals 30m above a subway (London Underground) tunnel interpreted as a short-period dynamic component, a quasi-static response to the train moving underneath the instrument, and a very long period (T>30 s) response to air movement around the tunnel network. A low-velocity clay and sand overburden tens of metres thick is shown to amplify the horizontal component wavefield at T ∼1 s, consistent with properties of the London subsurface derived from engineering investigations. We provide tabulated median power spectral density values for all stations, to facilitate comparison with any future urban seismic deployments.