Ellipticity of Rayleigh Waves and Shallow Crustal Structure

The axes ratio of elliptically-polarised Rayleigh-wave particle motion may provide information about shallow crustal structure beneath a seismographic station. This seldom-used parameter, often called ellipticity, or H/V, is different from the popular horizontal to vertical amplitude ratio of ambient seismic noise used for microzonation and similar studies, as we refer to deterministic measurements on Rayleigh-wave, fundamental-mode, wave groups originated by distant earthquakes. This measure is not influenced either by the source, or the propagation, and may yield robust information on crustal structure at shallower depth than group or phase velocity at the same frequency. Ellipticity is a potentially excellent tool to retrieve seismic parameters in sedimentary basins. We intend to examine feasibility and robustness of ellipticity measurements, and their application to retrieval of shallow crustal structure in northern Italy. The most striking structural element is the Po Plain sedimentary basin, but also sedimentary and crystalline rocks in the Northern Apennines and Alps. The Po Plain hides a complex system of active thrusts and folds that caused the 2012 seismic sequence. Our aim is to set a reliable system to measure and use H/V ratio to study the shallow crustal structure of this region. We implement and test automatic measurements, for 95 seismic stations in northern Italy. Comparison between observations and predictions from a reference crustal model show substantial fit, particularly for T ? 38s data. Discrepancy for shorter periods suggests that slight modifications of the model are needed. Analysis of synthetic and real data indicates the possibility of prograde (inverse) particle motion, but the influence of ambient noise at the predicted, short, transition periods, makes such observations less reliable. We further invert ellipticity curves to retrieve Vs profiles beneath each station, and finally build a high-resolution crustal model of the Po Plain.