BROAD-SCALE VELOCITY MODEL FOR THE GULF OF CALIFORNIA REGION (MEXICO)

The Gulf of California is one of only a few newly forming ocean basins, where seafloor spreading, the final stage of crustal extension is still not fully established. Improved knowledge of crustal composition and thickness in this region is needed for a more complete understanding of how the current plate boundary has evolved and how the apparently slow, but imminent transition to seafloor spreading is manifested in its main characteristics. Whether the crust is mainly oceanic, continental or transitional in composition has broader implications for studies on magmatism, structure and deformation. Furthermore, the constraints provided by reliable crustal velocities on earthquake locations and determinations of crustal thickness are invaluable. A lithosphere/upper-mantle velocity model is determined for the Gulf of California region from surface wave group velocities and receiver functions, computed from data recorded by the NARS-Baja and RESBAN seismic networks. We have also used receiver functions to look at lateral variations in the 400 km seismic discontinuity and with less resolution the 670 km discontinuity. Surface wave studies are key to facilitating a better understanding of the evolution of this complex plate boundary, since they utilize the dispersive characteristics of the largest amplitude and mostly easily recognizable waves recorded in seismograms to determine the 1st order velocity structure along event-station paths. The surface wave group velocities of regional earthquakes are determined at periods from 10 to 100 secs, thus providing sampling from the crust to about 200-300 km into the mantle. Fast lithosphere is observed at 15-20 km in depth in the northern Gulf of California; relatively low shear-wave velocities are observed in the central and southern Gulf of California; NARS-Baja stations show Moho depths in the range 22-33 km, becoming shallower from North to South.