Recovering Absolute Seismic Displacements Through Combined Use of 1 Hz GPS and Strong Motion Accereometers

Measuring absolute ground displacement with 1 Hz GPS observations has high potential for correcting some of the most troublesome errors in seismic records from near source ground motions. We have studied ground motions from a recent earthquake to develop statistical models of the noise sources for seismic and 1 Hz GPS observations in order to retrieve highly accurate displacement records from the combined data sets. Retrieving displacement from seismic acceleration records is often difficult because unknown small offsets in the acceleration time series will contaminate the doubly integrated record with large parabolic errors. Baseline corrections that are typically applied may create a reasonable looking record, however the static offset and lower frequency information is not known to be reliable. We use the additional information that is provided by 1 Hz GPS at many of these sites to reliably retrieve the absolute displacement and the amplitude of lower frequency velocity pulse in the presence of these effects. GPS 1 Hz time series and collocated seismic data are available from the 2003 Mw 8 Tokachi-Oki (Hokkaido) earthquake. We use an inversion method to simultaneously solve for ground displacement using both data sets as input constraints that takes into account the presence of unknown offsets in the acceleration record and the relatively large uncertainties in the processed 1 Hz GPS data. This work will impact both earthquake dynamics research and earthquake hazard mitigation. The interpretation of slip on earthquake faults will be aided by providing accurate ground displacement based on this technique. This work will provide to the earthquake engineering community a new collection of near-source time histories that are reliable at low frequencies, for application to the design limits for dynamic response of large structures.