SYNTHETIC SEISMOGRAMS THROUGH SYNTHETIC FRANCISCAN - INSIGHTS INTO FACTORS AFFECTING LARGE-APERTURE SEISMIC DATA

In spite of an order of magnitude increase over the past 15 years in s patial sampling of the wavefield, a major uncertainty in the analysis of active source seismic data remains phase identification. This uncer tainty results in part from the wide range of spatial scales of veloci ty heterogeneity in the crust. Smaller scale variations than those whi ch can be deterministically resolved given the design of a particular seismic experiment can be modeled statistically using geologic constra ints. Here we present synthetic seismograms generated from several dif ferent realizations of a stochastic model describing the velocity hete rogeneity of Franciscan terrane rocks. We compare the results to obser ved data and to synthetic seismograms generated for a model derived fr om tomographic inversion of the data in order to obtain qualitative in sights into the relative importance of large and small scale velocity heterogeneity. Not surprisingly, the synthetic data for the tomographi c model best reproduce observed small-scale variations in first arriva l time, which only occur for particular realizations of the stochastic model. The synthetic seismograms generated for the stochastic models best reproduce the level of signal-generated noise and suggest that th e amplitude of velocity variation locally within the Franciscan is app roximately 1 km/s. They also illustrate the effect of a strongly heter ogeneous upper and mid-crust on the amplitude-versus-offset pattern of arrivals from the lower crust and upper mantle. These effects may som etimes be interpreted deterministically, leading to biased models or a n overly optimistic estimate of lower crustal resolution.