Measurement of the spatial decay of spectral amplitudes of the higher‐mode seismic surface wave train Lgindicates that in the Great Basin the apparent seismic quality factor,Q, is a function of frequency. Observations along a 300 km profile recording of a nuclear explosion yield the functionQ(ƒ)=206ƒ0.68over the band 0.3 ⩽ ƒ ⩽ 10.0 Hz. Similar analysis using numerous recordings of earthquakes along paths which provide a good average sampling of the Great Basin crust givesQ(ƒ) = 214(± 15)ƒ0.54(±0.09), for 0.3 ⩽ ƒ ⩽ 5.0 Hz. If the crustal sampling by Lgenergy from nuclear explosions (surface sources) is primarily in the shallow crust, as has been suggested, then these results indicate a greater frequency dependence of apparentQth