We used local earthquakes recorded in central Italy during the 1997– 1998 Umbria-Marche sequence to analyze the frequency dependence of the S-wave radiation pattern. We rotated the north–south and east–west ground-motion components into radial and transversal components to study the S-wave energy partition into sv and sh waves. We separated source and path effects using a generalized spectral inversion technique in the frequency band from 0.23 to 30.7 Hz. We found that the quality factor Q for both sv and sh waves have approximately the same value and show the same frequency dependence reported in previous studies in central Italy.
We found that at 0.34 Hz the fraction of sh energy is similar to that expected from a double-couple source. Thus, we used this frequency as reference to investigate the variation of the radiation pattern at other frequencies (0.26–23.55 Hz). We observed that the sh-wave energy approaches to the expected sh radiation at low frequencies (f < 0.5 Hz) but that varies randomly at higher frequencies (f > 0.5 Hz). This observation suggests that the radiation pattern of sv and sh waves are stochastic in a wider frequency band than previously reported for other regions like Japan (Takenaka et al., 2003).