Relation of Kappa to Coda Q and Their Differences From “In-Situ” Q

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The spectral parameter kappa and coda Q (Q_c) are important empirical properties commonly used for site characterization in earthquake hazard analysis. A number of approaches for measuring  have been developed, including those from coda measurements from which Q_c is obtained. However, despite their popularity, the physical meanings of both kappa and Q_c are still poorly understood. The principal difficulty of understanding these properties seems to be in the common assumption that both of them are caused by the Q-factor of the near surface. Nevertheless, this relation appears to be not true, at least in many common cases. It is rarely noted that the logarithmic decrements of coda envelopes (from which Q_c is inferred) are usually nearly frequency-independent, which means that the Q of the subsurface should be high. Consequently, coda envelopes and the frequency-dependent Q_c are caused not by Q but by the layering and other elastic structures within the crust, such as topography and velocity heterogeneities within the near surface. For example, similar to seismic traces studied in reflection seismology, coda can be naturally explained by upward reflections from subsurface layering. These reflection records can be produced by downward free-surface reflections of direct waves from earthquakes. Therefore, coda waveforms may simply represent the subsurface layering and spatial distributions of scatterers. Also similarly to a seismic reflection record, kappa would then represent the slope of the Fourier spectrum of this layering. Thus, kappa and Q_c indeed appear to be closely related but caused almost entirely by the elastic structure of the subsurface and unrelated to attenuation. In addition, large scatters in kappa values measured from closely-spaced events and at the same stations suggest that kappa should likely contain significant contributions from the sources. The above observations are illustrated on coda records and spectra from several earthquake recordings in Iran.

Presenting Author: Igor Morozov

Authors