We use teleseismic P waves recorded at Piñon Flat Observatory (PFO) to constrain the three-dimensional crustal and upper-mantle velocity structure beneath the station. By forward modeling radial receiver function waveforms, we construct a one-dimensional crustal model that includes a significant shear-velocity inversion at 9 km in depth. Arrivals on the tangential components indicate dip of at least the uppermost discontinuity. Complicated Moho topography, deepening to the northwest of PFO, is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal P-wave reverberations. Although fine details cannot be resolved, each of those sets of observations plus mislocation vectors provide strong indications of abrupt Moho topography, possibly including step offsets of several kilometers. This is not only consistent with gravity data in implying Airy isostasy with compensation at Moho depth but extends that model to a much finer length scale than had been resolved.

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