||High Resolution Mapping of the Upper Mantle Using Interstation Phase and Amplitude Data of Surface Waves
A new approach to the enhancement of regional-scale surface wave to- mography incorporating inter-station phase and amplitude data is proposed, utilizing a high-density broadband seismic array. The use of observed am- plitude anomalies for the reconstruction of tomographic models can be of great benefit for the further advancement of conventional techniques of sur- face wave tomography, most of which have been based primarily on the phase information of surface waves. We have developed a method of fully non-linear waveform fitting between two stations located on a common great-circle path using the Neighborhood Algorithm as a global optimizer. By applying this technique to the Trans- portable Array (USArray) in the United States, we collect a large-number of inter-station phase speed data (about 130,000 for Rayleigh and 85,000 for Love waves) and amplitude ratio data (about 75,000 for Rayleigh waves) in a period range from 30 to 130 s for fundamental-mode surface waves. The inter-station distances of the measured dispersion data are mostly in a range between 300 and 800 km, which are nearly comparable to the wave- length of surface waves in the target periods of this study, and are much shorter than the epicentral distances between source and receiver for typi- cal single-station dispersion measurements. Massive measurements of inter- station phase speeds and amplitudes of surface waves for such short paths can be of help in enhancing the horizontal resolution of a tomography model.The phase information of surface waves, which reflects the average phase speed perturbation along a propagation path, has been the major source of information for the reconstruction of the lateral heterogeneity in the upper mantle. Amplitude anomalies of surface waves, on the other hand, have rarely been used in tomographic studies, since they are affected by a variety of uncertain factors such as seismic moment and source mechanism, local am- plification at receiver, elastic focusing/defocusing due to lateral heterogeneity and anelastic attenuation. In the inter-station analysis, the effects of seismic source can be canceled out, so that we need to consider the other factors only. With an appropriate correction for anelastic attenuation, we can focus on the effects of the focusing/defocusing as well as the receiver amplification factors. Since the effects of elastic focusing/defocusing on amplitude anoma- lies depend on the second derivatives of phase speed perturbation across the ray path, amplitude data should be more sensitive to the shorter-wavelength structure than the conventional phase data. This indicates that the ampli- tude measurements of surface waves are effective in improving the lateral resolution of velocity models. In this study, the measured inter-station phase speeds and amplitude ratios are inverted simultaneously for phase speed maps as well as local am- plification factor at each receiver locations. The phase speed maps derived from both phase and amplitude data exhibits better recovery of the strength of velocity perturbations of small-scale heterogeneities than those from phase data only, with enhanced local-scale tectonic features characterized by strong velocity gradients. Furthermore, the spatial distribution of receiver amplifica- tion factor shows a clear correlation with the velocity structure, particularly in the longer period. Isotropic and anisotropic 3-D S wave speed models in the United States are then constructed from the phase speed maps of fundamental-mode sur-face waves. In the isotropic S wave model derived from both phase and amplitude data for Rayleigh waves, smaller-scale tectonic features with the size of about 200 km are emphasized clearly; e.g., Colorado Plateau in the western region and New Madrid Seismic Zone in the central region at the depth of around 100 km. These results indicate that inter-station amplitude ratios can be used as additional constraints on high-resolution mapping of the upper mantle structure. A preliminary radial anisotropy model derived from the simultaneous use of Rayleigh and Love waves has shown faster SH wave speed anomalies than SV above the depth of 100 km, particularly in tectonically active regions in the western and central U.S., suggesting the effects of current and former tectonic processes on anisotropic properties in the lithosphere.
Hokkaido University（北海道大学）. 博士(理学)