ALBERT

Notes: In this paper, a new procedure, called the coherent-signal-subspace method (CSS), for broad-band multiple-signal detection and slowness-vector estimation is discussed and applied to a surface seismographic array. the major improvements in estimation accuracy and resolution given by the CSS method, which was developed in the fields of radar and sonar, result from the use of extended-frequency components within the data bandwidth and a high-resolution algorithm. the former is made possible through a frequency focusing transformation that for each frequency component corrects for the phase-delay difference between that frequency and a reference frequency ωo. the result is the condensation of a broad frequency band into a narrow band at ωo. the high resolution algorithm used in the CSS method is the MUSIC (multiple signal characterization; Schmidt 1986) algorithm, which is based on the eigen property of the data cross-covariance matrix that the signal phase-delay vectors lie within the subspace spanned by the signal eigenvectors. the frequency transformation improves the singularity of the estimated cross-covariance matrix and the accuracy of the estimated signal eigenvectors at ωo, which are often serious problems in seismic array analysis. Combination of these two features in CSS ensures a superior array performance over the widely used beam steering and minimum-variance (Capon 1969) methods. Approximate methods to estimate the mean and variance of the estimated slowness vector are also presented in this paper. the estimation biases introduced by deterministic arrival-time deviations of array data from a plane wavefront are derived for the single signal case. It is shown that, in the case of a single signal, significant reduction in the estimation bias may be achieved if a large enough reference frequency is used in the CSS method. Finally, with observed and synthetic ground motions, tests are performed to illustrate the utility of CSS in resolving two closely separated signals. In both cases, CSS successfully resolved two separate peaks at almost the correct slowness vectors, while the conventional beam steering and minimum variance estimation methods either failed to resolve the two signals or gave an incorrect slowness estimate.

Notes: The propagation of Rayleigh waves across the regions of subduction of Japan and of the North Island of New Zealand is examined in the frequency domain by the finite element method. Study of the Japan region of subduction ESE of the island of Honshu has shown that, for perfectly elastic 2D models, although Love waves decrease in velocity across the region, Rayleigh waves of periods of 35–60 s increase slightly in velocity. The material properties of the predominantly crustal material being subducted appear to slow the Love waves, whereas the Rayleigh waves of periods of from 35 to 60 s tend to retain their oceanic velocities, which are higher than the velocities of Rayleigh waves of those periods for the island of Honshu. Amplitude calculations have shown that, at periods of 60 s down to 10 s, Love waves are increasingly forward scattered into higher Love modes, whereas, at a period of 20 s, 98 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. This result, together with that of Drake for the continental margin at Berkeley, California, confirms the practice of using Rayleigh waves at a period of approximately 20 s measured at seismographic stations near continental margins and near regions of subduction to estimate the magnitude of teleseisms. Study of the New Zealand region of subduction SE of Lake Taupo on the North Island of New Zealand has shown similar results to those for the Japan region of subduction. Although Love waves decrease in velocity across the region, Rayleigh waves of periods of 60 s down to 20 s increase slightly in velocity. Amplitude calculations have shown that at periods of 60 s down to 15 s, Love waves are increasingly forward scattered into higher Love modes. On the other hand, at a period of 20 s, 96 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. At a period of 10 s, in contrast to the results for the Japan region of subduction and for the continental margin at Berkeley, 73 per cent of the energy of Love waves is transmitted as the Love fundamental mode and 97 per cent of the energy of Rayleigh waves is transmitted as the Rayleigh fundamental mode. Amplitude and phase changes of Love and Rayleigh waves at continental margins and in regions of subduction need to be allowed for when waveform inversions of waves which have traversed these regions are made.