On the improvement of SKS splitting measurements by the Simultaneous Inversion of Multiple Waveforms (SIMW)

The birefringence of core-refracted shear waves (e.g. SKS or SKKS) is often used to study seismic anisotropy in the Earth. However, depth resolution and multilayer anisotropy is generally poor for many regions on Earth. This is primarily due to SKS or SKKS phases that are not observable for different backazimuths either because of missing seismicity at the required distance range or because of a too low signal-to-noise ratio (SNR). We propose a new method called Simultaneous Inversion of Multiple Waveforms (SIMW), which allows the joint inversion of multiple core-refracted shear waves from different earthquakes within the same source region, observed by either the same seismic station or by a seismic network. The waveforms are concatenated into a combined signal, which is then inverted with the Silver & Chan method to determine the two splitting parameters: time delay δt, and fast polarization direction Φ. We apply our method to recordings at the large aperture Norwegian NORSAR Array and the German Gräfenberg array (GRF). Our results demonstrate that SIMW allows a stable determination of splitting results for low-amplitude or noisy SKS signals. Splitting parameter uncertainties can be reduced and reliable results are obtained for both arrays. Moreover, new backazimuth directions can be explored, enabling a more accurate derivation of two-layer anisotropy models. Our new methodology is particularly helpful for temporary station deployments with limited recording times in order to utilize as many as possible signals including such with low-amplitude and small SNR

Application of Surface wave methods for seismic site characterization

Surface-wave dispersion analysis is widely used in geophysics to infer a shear wave velocity model of the subsoil for a wide variety of applications. A shear-wave velocity model is obtained from the solution of an inverse problem based on the surface wave dispersive propagation in vertically heterogeneous media. The analysis can be based either on active source measurements or on seismic noise recordings. This paper discusses the most typical choices for collection and interpretation of experimental data, providing a state of the art on the different steps involved in surface wave surveys. In particular, the different strategies for processing experimental data and to solve the inverse problem are presented, along with their advantages and disadvantages. Also, some issues related to the characteristics of passive surface wave data and their use in H/V spectral ratio technique are discussed as additional information to be used independently or in conjunction with dispersion analysis. Finally, some recommendations for the use of surface wave methods are presented, while also outlining future trends in the research of this topic