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- Sparsity-promoting approach to polarization analysis of seismic signals in the time-frequency domainPublication . Mohammadigheymasi, Hamzeh; Crocker, Paul; Fathi, Maryam; Almeida, Eduardo; Silveira, Graça; Gholami, Ali; Schimmel, MartinTime - frequency (TF)-domain polarization analysis (PA) methods are widely used as a processing tool to decompose multicomponent seismic signals. However, as a drawback, they are unable to obtain sufficient resolution to discriminate between overlapping seismic phases, as they generally rely on a low-resolution time-frequency representation (TFR) method. In this article, we present a new approach to the TF-domain PA methods. More precisely, we provide an in-detailed discussion on rearranging the eigenvalue decomposition polarization analysis (EDPA) formalism in the frequency domain to obtain the frequency-dependent polarization properties from the Fourier coefficients owing to the Fourier space orthogonality. Then, by extending the formulation to the TF domain and incorporating sparsity promoting TFR (SP-TFR), we improve the resolution when estimating the TF-domain polarization parameters. Finally, an adaptive SP-TFF is applied to extract and filter different phases of the seismic wave. By processing earthquake wave-forms, we show that, by combining amplitude, directivity, and rectilinearity attributes on the sparse TF-domain polarization map of the signal, we are able to extract (or filter) different phases of seismic waves. The SP-TFF method is evaluated on synthetic and real data associated with the source mechanism of the M-w = 8.2 earthquake that occurred in the south-southwest of Tres Picos, Mexico. A discussion on the results is given, verifying the efficiency of the method in separating not only the Rayleigh waves from the Love waves hut also in discriminating them from the body and coda waves. The codes and datasets are available at https://github.com/SigProSeismology/SP-TFF, contributing to the geoscience community.
- Imaging the crust and uppermost mantle structure of Portugal (West Iberia) with seismic ambient noisePublication . Silveira, Graça; Dias, Nuno; Kiselev, Sergey; Stutzmann, Eleonore; Custodio, Susana; Schimmel, MartinWe present a new high-resolution three-dimensional (3D) shear wave velocity (Vs) model of the crust and uppermost mantle beneath Portugal, inferred from ambient seismic noise tomography. We use broadband seismic data from a dense temporary deployment covering the entire Portuguese mainland between 2010 and 2012 in the scope of the WILAS project. Vertical component data are processed using phase correlation and phase weighted stack to obtain Empirical Green functions (EGF) for 3900 station pairs. Further, we use a random sampling and subset stacking strategy to measure robust Rayleigh wave group velocities in the period range 7-30 s and associated uncertainties. The tomographic inversion is performed in 2 steps: First, we determine group velocity lateral variations for each period. Next, we invert them at each grid point using a new trans-dimensional inversion scheme to obtain the 3D shear wave velocity model. The final 3D model extends from the upper crust (5 km) down to the uppermost mantle (60 km) and has a lateral resolution of similar to 50 km. In the upper and middle crust, the Vs anomaly pattern matches the tectonic units of the variscan massif and alpine basins. The transition between the Lusitanian Basin and the Ossa Morena Zone is marked by a contrast between moderate and high velocity anomalies, in addition to two arched earthquake lineations. Some faults, namely the Manteigas-Vilarica-Braganca fault and the Porto-Tomar-Ferreira do Alentejo fault, have a clear signature from the upper crust down to the uppermost mantle (60 km). Our 3D shear wave velocity model offers new insights into the continuation of the main tectonic units at depth and contributes to better understanding the seismicity of Portugal.