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 (PSD) measurements suggest that transportation sources generate the majority of the noise wavefield across the entire wideband, except at the secondary microseismic peak (2 < T < 6 s). The effect of road traffic is greatest at short periods (T < 0: 4 s) for which acceleration noise powers are similar to 20 dB larger than the new high-noise model; at T similar to 0: 1 s daytime root mean square acceleration amplitudes are 1000 times higher in central London than at an observatory station in Eskdalemuir, Scotland. Overground railways generate observable signals both at short periods (T < 0: 3 s), which are recorded in close proximity to the tracks, and at very long periods (T > 20 s), which are recorded across the city. We record a unique set of signals 30 m 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 lowvelocity clay and sand overburden tens of meters thick is shown to amplify the horizontal-component wavefield at T similar to 1 s, consistent with properties of the London subsurface derived from engineering investigations. We provide tabulated median PSD values for all stations to facilitate comparison with any future urban seismic deployments.