Time-domain inversions of radial receiver functions and forward modeling of radial and tangential receiver functions were used to determine the velocity structure beneath DWWSSN station COL (Fairbanks, Alaska). Intermediate-period waveforms from 57 events recorded between February 1982 and November 1985 were used in the final analysis.
Receiver functions from similar backazimuths and distances are very uniform and were stacked to increase the signal-to-noise ratio. At some azimuths (particularly from the west), significant energy is observed in the tangential component, indicating the existence of heterogeneous structure. The calculated velocity profiles are complicated and contain several distinct features that are consistent between the different stacks. The most obvious features are two strong velocity increases or boundaries in the lower crust. The first of these velocity gradients is between 27 and 32 km, a depth that is often associated with the crust-mantle transition in this region. Forward modeling of the receiver function arrivals associated with this boundary indicates that it has a strike of ∼ 280° and a dip of at least 10° to the northeast. The velocity profiles and forward modeling also indicate a low-velocity layer between depths of about 15 and 28 km. This layer is consistent with TACT results that show a layer at approximately this depth, with high electrical conductivity and a lower-than-expected seismic velocity, interpreted to be Mesozoic flysch.