Lithosphere and asthenosphere structure beneath the Gulf of California from SCOOBA and NARS-Baja surface wave data

The Pacific-North America plate boundary, a young oblique rift system running lengthwise along the Gulf of California (GoC), is an excellent modern example of continental breakup and drifting. Because of the high concentration of seismometers surrounding the gulf, research in this region may answer the question of whether upwelling centers are characteristic of rift systems, and, if so, related questions about the cause of upwelling centers and the role they play in seafloor spreading. Wang et al. (2009) suggests that there is mantle upwelling in three, and possibly four, locations along the GoC. Our purpose is to obtain better resolution of the hypothesized low-velocity anomalies in the gulf, and to test whether a fourth upwelling center (located at the mouth of the gulf) exists. We use the data from eight ocean-bottom seismometers (OBS), part of the 12-month deployment of the Sea of Cortez Ocean-Bottom Array (SCOOBA) seismic experiment, and the data from onshore seismometers of the NARS-Baja experiment, to estimate the phase velocities of Rayleigh surface waves propagating through the crust and upper mantle beneath the GoC. For the larger teleseismic events, the OBS stations provide good data to periods of ~150 seconds or more, but in general have a signal-to-noise ratio that is slightly lower than the land stations. A tomographical inversion, using the two-plane wave method, is then performed on the Rayleigh waves to create a shear velocity structure of the upper 200 km of the mantle.