ARRAY MEASUREMENTS OF PHASES USED IN RECEIVER-FUNCTION CALCULATIONS -IMPORTANCE OF SCATTERING

Receiver-function methods utilize three-component teleseismic P coda t o determine structure beneath seismographs. Most methods treat coda co mplexity as mode conversions and reflections off subhorizontal interfa ces beneath the station, such as the Moho. This assumption is evaluate d beneath a broadband array in the Kopet Dagh foothills, at Geyokcha, Turkmenistan, by applying array processing techniques to P coda waves at frequencies near 1 Hz. Beneath the Geyokcha array, horizontal compo nents show signals that deviate significantly in slowness and bearing from predictions of a plane-layered Earth. Large phases arrive at the same time as the predicted Ps conversion from the Moho yet systematica lly come from a single azimuth (from NNE) and travel at phase velociti es of 3 to 5 km/sec. The coda waves appear to be P-to-S converted wave s from a near-surface scatterer. An inversion for scatterer location i s developed and applied to 29 events. Most coda energy comes from a sc atterer similar to 16 km NNE of the array. This location is coincident with the range-bounding faults that separate Kopet Dagh mountains fro m adjacent basins. Thus, array analysis shows that signals potentially interpreted as Moho conversions are instead scattered from a near-sur face structure. Although these results suggest caution is necessary in interpreting single-station receiver functions, particularly at high frequencies, they also demonstrate the power of array sampling in reso lving lateral structure variations.