TIME-FREQUENCY RELATIONS FOR SEISMIC WAVE AMPLITUDES IN SOUTHERN IRAN

Posted on 2020-03-02 in Events
Mar 6, 2020

Please join us for a Graduate student seminar this Friday March 6 at 3:30 pm in rm 155 Geology presented by Maryam Safarshahi, PhD Candidate:

TIME-FREQUENCY RELATIONS FOR SEISMIC WAVE AMPLITUDES IN SOUTHERN IRAN

In engineering, local, and regional wave studies, frequency- and distance-dependent amplitudes of seismic waves are described by several empirical parameters: the power-law (v), spectral decay (ꓗ), quality factor (Q), and cumulative attenuation t*. Because all these measures are derived from the same data, they are interrelated, trade off with each other, and are sensitive to model assumptions. Because of these uncertainties, it is often difficult to compare results from different studies or geographic areas. In this study, all of the above empirical parameters are derived from a common, unified parameterization of seismic-wave amplitude dependencies on source-receiver distance and wave frequency. Compared to the existing approaches, several new features are also included in this model: 1) joint inversion for time/distance and frequency dependencies, source spectra, site responses, kappas, and Q values, 2) possible non-monotonous time/distance dependencies, 3) use of kappa terms for the sources as well as for receivers, 4) use of three-component amplitudes and adaptive filtering reducing noise effects, and 5) removal of spurious correlations of receiver coupling and data residuals with source-receiver distances and frequencies. The most important improvement of the existing procedure consists in an estimation of model uncertainty by using model bootstrapping and principal-component analysis. The approach is illustrated for regional S waves from two earthquakes in southern Iran. Comparisons with previous analyses of the same and similar datasets show strong biases of the conventional models due to limited parameterizations and subjective model assumptions. From the data, the frequency-independent spreading of S waves is much faster than usually assumed. For example, for transverse-component amplitudes, the decay with travel time t as about t–1.8 at distances below 90 km and t–2.6 beyond 115 km. Between these distances, the amplitude increases by a factor of about three, which is explained by contributions from Moho and crustal reflections, and scattering within the crustal waveguide and on the free surface. In contrast to previous studies, a much higher Q factor above 2000 is found for the crust.