- © 2008 by the Seismological Society of America
Global Positioning System (GPS) geodesy is a well-known tool to measure the long-term surface deformation of the Earth at various temporal and spatial scales, and it is widely used in studies of geodynamics. GPS dual-frequency geodetic-quality receivers are typically sampled at rates of 15 to 30 s and analyzed in a batch mode over intervals from one hour to 24 hours. Recently, 1-Hz or greater rate sampled continuous GPS has been recognized as useful in combination with seismometers to capture the long-period seismic waves from large earthquakes and to detect volcano inflation and tsunamigenic landsliding (Bock et al. 2000; Larson et al. 2003; Bock et al. 2004; Mattia et al. 2004). Modern GPS receivers are capable of sampling at rates as high as 50 Hz (Genrich and Bock 2006).
The impact of heavy pedestrian movement on a highway suspension bridge has not yet been documented by precise observations, nor has it been modeled, although the impact of vehicles and wind was observed and used to constrain theoretical analysis (Nakamura 2000; Roberts et al. 2004; Guo et al. 2005). A large crowd can gather on a highway suspension bridge under extreme natural or human-generated conditions like earthquakes or power blackouts (Julavits 2003); an unpredicted response to the crowd was observed on a foot-bridge (Strogatz et al. 2005). Using GPS observations, here we evaluate the deflection of a large suspension bridge that connects Brooklyn with Staten Island at the mouth of upper New York Bay under the load of marathon runners and under the load of vehicles. Furthermore, by combining GPS and accelerometer data, we find that the runners excited the vibrations of the bridge at a modal frequency of 2.8 Hz, vastly different from the first vertical frequency of 0.13 Hz excited by vehicles. Our …