ABSTRACT

Empirical models of geometrical‐, Q‐, t‐star, and kappa‐type attenuation of seismic waves and ground‐motion prediction equations (GMPEs) are viewed as cases of a common empirical standard model describing variation of wave amplitudes with time and frequency. Compared with existing parametric and nonparametric approaches, several new features are included in this model: (1) flexible empirical parameterization with possible nonmonotonous time or distance dependencies; (2) joint inversion for time or distance and frequency dependencies, source spectra, site responses, kappas, and Q; (3) additional constraints removing spurious correlations of model parameters and data residuals with source–receiver distances and frequencies; (4) possible kappa terms for sources as well as for receivers; (5) orientation‐independent horizontal‐ and three‐component amplitudes; and (6) adaptive filtering to reduce noise effects. The approach is applied to local and regional S‐wave amplitudes in southeastern Iran. Comparisons with previous studies show that conventional attenuation models often contain method‐specific biases caused by limited parameterizations of frequency‐independent amplitude decays and assumptions about the models, such as smoothness of amplitude variations. Without such assumptions, the frequency‐independent spreading of S waves is much faster than inferred by conventional modeling. For example, transverse‐component amplitudes decrease with travel time t as about t1.8 at distances closer than 90 km and as t2.5 beyond 115 km. The rapid amplitude decay at larger distances could be caused by scattering within the near surface. From about 90 to 115 km distances, the amplitude increases by a factor of about 3, which could be due to reflections from the Moho and within the crust. With more accurate geometrical‐spreading and kappa models, the Q factor for the study area is frequency independent and exceeds 2000. The frequency‐independent and Q‐type attenuation for vertical‐component and multicomponent amplitudes is somewhat weaker than for the horizontal components. These observations appear to be general and likely apply to other areas.

You do not currently have access to this article.