Graduate Thesis Or Dissertation
 

Finite difference wavefield modeling of large-aperture data from the 1993 Mendocino Triple Junction Seismic Experiment

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  • This thesis presents elastic finite difference simulations of two dimensional wavefield propagation for line-9, recorded during the 1993 part of the multi-institutional, multi-year Mendocino Triple Junction (MTJ) Seismic Experiment. Line-9 is a high-resolution, large-aperture seismic profile oriented in north-south direction, extending from the subduction regime north of the MTJ to the transform regime south of the MTJ. Snapshots of the wavefield are recorded during the simulation to create a movie which assists with the analysis of the synthetic seismograms. The simulations are computed on a 64 processor CM-5 parallel supercomputer. First a velocity model obtained by the Mendocino Working Group is examined. The model is based on information from depth-migrated single-fold reflection profiles and tomographic inversion of first arrivals. The synthetic seismic sections show acceptable first arrival traveltime fit but fail to correctly model secondary arrivals and amplitude variations of the wavefield. Based on the shortcomings of the initial model an alternative model for the north-south line is proposed. The new model introduces several distinct structures, like a subducted Gorda plate, a slabless window, and a wedge shaped low velocity zone between Franciscan material and Gorda plate. The big difference between the two models is that the initial model is too smooth to contain any discontinuities and cannot account for any reflection arrivals. In addition all structures in the lower crust of the alternative models are located about 4 km deeper than the equivalent velocities in the initial model. The seismic sections of the alternative model show much improved traveltime and amplitude fit. Also, some characteristics of the wavefield observed in the field data (e.g. traveltime oscillations, amplitude variations, and discontinuous arrivals) are also observed in the synthetic seismograms.. The position of the southern end of the Gorda plate (SEDGE) is at 80 km in the model (39.7°N). Because of the limited resolution it is not possible to resolve whether the Gorda plate just north of the SEDGE is fragmented or continuous. To add attenuation by scattering or coda waves generation to the model, the effects of a stochastic velocity description for Franciscan rocks on the wavefield are explored. The best velocity contrast between sandstones and melange units in a bimodal, seifaffine, sinuous connected model is determined as 0.5 km/s. Stochastic models of this kind successfully model background noise and coda waves and to some degree discontinuous phases, amplitude variations, and traveltime oscillations. They cannot, however, account for the full range of characteristics observed in the field data. For this, additional large scale velocity variations must be added to the model. For the final model, the (deterministic) alternative model and the statistical description of Franciscan rocks are combined. The resulting wavefield shows good amplitude and traveltime fit and the observed wavefield characteristics are similar to the characteristics of the field data.
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