The seismic velocity structure beneath Texas Gulf Coastal Plain (GCP) is imaged by migrating Ps receiver functions with a seismic velocity model found by fitting surface wave dispersion. We use seismic data from a linear array of 22 broadband stations, spaced 16-20 km apart. A Common Conversion Point (CCP) stacking technique is applied to earthquake data to improve the S/N ratios of receiver functions. Using an incorrect velocity model for depth migration of a stacked CCP image may produce an inaccurate image of the subsurface. To find sufficiently accurate P and S-velocity models, we first apply a nonlinear modeling technique to fit Rayleigh wave group velocity dispersion via Very Fast Simulated Annealing. Vs ranges from 1.5 km/s in shallow layers of the GCP to 4.5 km/s beneath the Llano uplift and just outboard of the Balcones Fault Zone (BFZ). The BFZ is characterized by slow velocities that persist to nearly 100 km depth. In the stacked image, the largest amplitude positive-polarity event ranges from the surface, at the Llano uplift, to a maximum depth of 16 km beneath Matagorda Island. We attribute this event to the sediment-basement contact, which is expected to produce a large impedance contrast. Another large-amplitude and positive-polarity event at 35 km depth, which likely marks the Moho, disappears outboard of the Luling Fault Zone. The disappearance of the Moho beneath the GCP may be due to serpentinization of the upper mantle, which would reduce the impedance contrast between the lower crust and upper mantle dramatically.
- Texas Gulf Coast
- receiver functions