Intrinsic and Scattering Attenuation of Shear Waves: Depth- and Frequency-Dependent Attenuation Insights using the QEST Code

In local earthquake seismology depth-dependent elastic models of P- and S-wave velocities are indispensable, e. g. to locate earthquakes. If not only travel times but also amplitudes of seismic waves are important, elastic Earth models are insufficient and visco-elastic models are required to include intrinsic absorption of seismic waves. This applies e. g. to the estimation of moment magnitudes and to physics-based ground motion modeling in seismic hazard analysis. The estimation of seismic attenuation parameters of the Earth is significantly more difficult than the estimation of velocities for two reasons: (1) Scattering attenuation as well as intrinsic absorption contribute to the attenuation of seismic waves and it is essential to separate these two effects. (2) Seismic attenuation parameters are inherently frequency dependent, whereas the frequency dependence of seismic P- and S-wave velocities can be neglected in almost all cases. Due to the lack of information, attenuation is often completely neglected in seismic wave simulation, standard values for seismic Q are used, or frequency dependence and depth dependence are ignored. To solve this issue, we develop a computer code, 'QEST - Q estimation'. The code is based on a forward modeling using radiative transfer theory in depth-dependent velocity and attenuation models and a global inversion scheme based on a genetic algorithm. Besides frequency and depth dependent intrinsic as well as scattering attenuation parameters, earthquake source spectra and frequency dependent site amplifications are also a result of the inversion with QEST. We applied the technique to seismograms of earthquakes in three regions: the Upper Rhine Graben (Germany), the Leipzig-Regensburg fault zone (Germany) and the Alaska Subduction Zone (Alaska). These regions were selected to represent exemplary areas with thick sedimentary layers, without thick sedimentary layers and the lithosphere and asthenosphere, respectively. Results show a clear depth and frequency dependence of both, scattering attenuation as well as intrinsic absorption, within the thick sediments of the graben. In contrast, the Leipzig-Regensburg fault zone exhibits a clear frequency dependence of the attenuation parameters, albeit only a smaller depth dependence, while the results of the Alaska Subduction Zone show a depth and frequency dependence that is particularly evident in the scattering attenuation.