MIGRATION OF WIDE-APERTURE ONSHORE OFFSHORE SEISMIC DATA, CENTRAL CALIFORNIA - SEISMIC IMAGES OF LATE-STAGE SUBDUCTION

We conducted a high shot and receiver density, onshore-offshore seismic experiment in 1986 across the central California transform margin as part of the Pacific Gas and Electric/EDGE Deep Crustal Geophysical Survey. The data set consists of land recordings of 406 marine air gun shots fired along a 70-km track across the Santa Maria basin. The seismic vessel's track was collinear with 154 land seismic recorders deployed over 50 km across the Sur Obispo terrane and part of the Salinian block. The source-receiver offsets range from 2.5 km to 120 km. The unique recording geometry, and high shot and receiver density, permit imaging the deeper crustal structure of the California transform margin but pose serious challenges for conventional seismic imaging methods. Consequently we developed a unified approach to wide-angle imaging which incorporates a tomography technique for obtaining initial migration velocity models, and a prestack Kirchhoff depth migration with depth focusing of the main reflectors for velocity model refinement and imaging. This approach retains the velocity and dip information in the data and allows user control over the imaging process. We applied this processing to subsets of the onshore-offshore data to obtain an upper crustal velocity model, and a 32-km-long, wide-angle depth migrated image of the lower crust, in the coastal region under the Santa Maria basin, and the Sur-Obispo terrane. The migration image shows a part of an underplated oceanic crustal layer, interpreted as the Monterey plate (Meltzer and Levander, 1991; Miller et al., 1992), which dips coastward at depths of 18 to 28 km. The plate appears to be kinked or imbricated beneath the coastline. The image correlates well with existing wide-angle seismic interpretations based on ray tracing (Waiter and Sharpless, 1987; Putzig, 1988; Howie et al., 1993). The migrated lower crustal image has more structural information than is apparent in conventional ray-based interpretations along the same, crossing, and adjacent profiles. The crustal image demonstrates that automated imaging techniques applied to wide-angle data can provide images of crustal structure comparable in fidelity to conventionally processed and migrated near-vertical incidence data.