Analysis of regional seismic data by means of higher-order spectra

Abstract: The use of higher-order statistical and spectral estimators as complementary analysis for conventional regional seismic data processing is studied. The issues addressed are; the properties of the complex bispectrum of regional seismic phases, the experimental detection performance of higher-order statistical detectors, discrimination between local earthquakes and explosions, statistical tests and bicoherence, biphase and spatial bispectrum analysis of regional P-, S, and Lg-phases. The complex bispectral measures show a more antisymmetric waveform for mine explosions. Third- and fourth-order measures can add information about seismic phases for the first regional P- and S-phase. The fourth-order cumulant for the first P-phase outperforms the second- and third-order cumulants. A master-event correlation methodis used for discrimination between explosions and earthquakes. A library of seismic events with an a priori knowledge of the source type is employed. The method compares unknown events with the library through second-, third- and fourth-order cross-correlations. The discrimination by the third- and fourth-order master-event correlation outperforms by a factor of two the second-order correlation. Statistical tests based on bispectra can also provide information about seismic noise and seismic phases important for both conventional analysis and detection. Ambient seismic noise recorded with an ocean bottom seismometer is characterized as stationary, symmetric and linear. Prior to spectral estimation, bispectral preconditioning of the data is recommended. The first P-phase is mainly characterized as non-symmetric (non-Gaussian), about one third of recorded second P-phases, and S-phases and one quarter of the Lg-phase are non-Gaussian. This means that conventional analysis of regional phases, will gain by using higher-order statistics and spectra. All of the non-Gaussian phases are characterized as linear. Thus, linear models for regional, even non-Gaussian, phases can be used. The bicoherence and the biphase were estimated for twelve events. First P-phases exhibit considerable values useful for a frequency-wavenumber analysis, which represents additional information about the arriving wave field in the complex frequency-wavenumber bispectrum.