Researchers are tuning in to urban seismic noise, the man-made signals from human activity, to view geologic structures and track the rhythms of cities. Until now, scientists often tossed away data containing the pesky vibrations created as humans scurry from one place to another. Urban seismic noise often plagues scientists who study earthquakes by overwhelming seismometers, the instruments that detect earthquakes. Because of this interference, these detectors are typically placed far from airports, train tracks and freeways in order to avoid the urban buzz. "For seismologists, the focus was, 'If a train is passing, let's make sure we can remove those trains,'" said Nima Riahi, a researcher and seismologist at Scripps Institution of Oceanography at UC San Diego.