Fundamental Mode Love Waves Research Articles

Variability of seismic ground motion in complex media: the case of a sedimentary basin in the Friuli (Italy) area

A computational hybrid technique is presented to estimate ground motion in complex two-dimensional, anelastic media. The technique combines modal summation and finite-difference methods, and it can take into account the source, path and local soil effects to calculate the local wavefield due to a seismic event. The hybrid technique is applied to study wave propagation in a sedimentary basin in the Friuli region during the September 11, 1976, Friuli aftershock (16h 35m 04s). Special emphasis is given to the understanding of the different features of ground motion in sedimentary basins. The most important effects that can be observed are the excitation of local surface waves at lateral heterogeneities, and local resonances. Within the sedimentary basin studied, the coda of the transverse component is mainly composed of the local, fundamental-mode Love wave, whereas the P-SV-wavefield shows dominant contributions of the higher modes of Rayleigh waves. These differences in wave composition lead, in general, to different dispersion characteristics and attenuation phenomena for SH- and P-SV-waves. A parametric study demonstrates the sensitivity of the computer ground motion to small changes in the subsurface topography of the sedimentary basin, and the velocity and quality factor of the sediments. The results obtained for one- and two-dimensional structural models show that only two-dimensional structural models are suitable for the prediction of complete seismic ground motions in sedimentary basins, because they can account for the generation of local surface waves. To establish the validity of the numerical results, they are compared with observed ground motion. The relative amplitudes, durations, and frequency content of the different components of the synthetic signals agree well with the observations.

Secondary Love Waves Observed by a Strong-Motion Array in the Tokyo Lowlands, Japan.

In order to investigate the generation and propagation of Love waves recorded in the Tokyo lowlands, we analyzed array data obtained for six events that occurred around the Kanto district. From wavenumber analysis, the following conclusions are obtained. (1) Love waves are converted from direct S-waves at the edge of a sedimentary basin. The Hachiohji tectonic line is an especially important zone for the generation of Love waves. (2) Apparent velocities of Love waves for periods between 6 and 9 s are concordant with the phase velocities of fundamental mode Love waves calculated from a three-layered model. The first, second, and third layers in the model have S-wave velocities of 0.5, 0.8, and 1.2 km/s, respectively. The S-wave velocity in the pre-Tertiary basement is assumed to be 2.6 km/s.

Observation and inversion of surface wave group velocities across central India

abstractSurface wave group velocities across central India are measured from computer plots of frequency-time analysis using multiple-filter technique. Periods range from 5 to 56 sec for fundamental mode Rayleigh wave, 5 to 16 sec for first higher mode Rayleigh wave, and 5 to 49 sec for fundamental mode Love wave. The wave paths from Koyna earthquakes to Bokaro cross purely through the Peninsular Plateau. The group velocities for these paths are found to be similar to those of surface waves recorded at Poona from earthquake which lie to the northeast of Bokaro in sub-Himalayan regions. From this, it is concluded that the shield structure of the peninsula extends to the sub-Himalayan regions to northeast of the Peninsular Plateu with a negligible part of sedimentary layer. Group velocities of fundamental mode Rayleigh and Love waves are also obtained for the paths from Koyna earthquakes to Shillong and are found to be similar to the above-mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of Bengal Basin. Northern limit of this basin sedimentary layer has been identified.Mean and standard deviation of observed group velocity at each period are obtained for each of Rayleigh fundamental, Rayleigh first higher, and Love fundamental modes. The group velocities closely agree with those obtained earlier for the path between New Delhi and Kodaikanal and are also found to be similar to those observed in some shield regions.Monte Carlo inversion method is applied and crust and upper mantle structure of the Indian Shield across central India is obtained. Crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study.

Love waves normally incident at a continental boundary

abstract The dispersion curve for fundamental-mode Love waves between periods of 60 and 4.375 sec has been obtained by the finite element method for a path normally incident at a continental margin (California) between a seismometer site on the ocean bottom 120 km from the continental shelf and a coastal seismographic station (Berkeley). Between periods of 60 and 17.5 sec, the computed phase velocity of fundamental-mode Love waves across the continental margin was very slightly greater (less than 1 per cent) than the mean of the phase velocities of fundamental-mode Love waves in the horizontally layered structures at the ends of the model representing the regions of the ocean bottom and continent. Between periods of 15 and 4.375 sec, the phase velocity of fundamental-mode Love waves across the finite element model was, in general, slightly less (about 2 per cent) than the mean of the phase velocities of fundamental-mode Love waves in the horizontally layered bounding structures. Two important new results concerning the propagation of the energy of fundamental-mode Love waves at a continental boundary were obtained. First, between periods of 20 and 11 sec, 90 per cent of the energy of fundamental-mode Love waves incident from the oceanic side is transmitted as the first higher mode. Second, between periods of 15 and 8 sec, only 2 per cent of the energy of fundamental-mode Love waves incident from either the oceanic or the continental side of the continental boundary is transmitted in waves of the fundamental Love mode. From the first of these results, it follows that, at periods of 20 sec or less, measurements on a continent of the phase velocity of fundamental-mode Love waves will be contaminated by the presence of the first higher mode. From the second, it follows that, between periods of 15 and 8 sec, the continental boundary is a barrier to the propagation of fundamental-mode Love waves in both directions, quite independently of damping. This strongly affects estimates of the attenuation of fundamental-mode Love waves. Fundamental-mode Love wave microseisms on the ocean floor from an oceanic source can be expected to be approximately four times as large as fundamental-mode Love wave microseisms from the same source at continental sites.

Analysis to Extended Periods of Rayleigh and Love Wave Dispersion across Australia

Group velocities and phase velocities for Rayleigh waves and Love waves have been measured to periods of almost 200 s for nine paths across Australia. This extended range of dispersion data was obtained by the new process of summing cross-correlograms of a large number of seismograms of earthquakes with moderate magnitudes (average mb was 5.8) prior to the non-linear computation of phase. This is an unbiased averaging process that enhances the signal at longer periods. Western-central Australia has the same dispersion as shield areas of other parts of the world; eastern Australia may be catalogued tectonic or ‘aseismic platform’. Group velocity for eastern Australian paths has a pronounced minimum value, possibly less than 3·4 km s−1, between 100- and 200-s period. Smoothed Rayleigh wave phase velocities were obtained by integration of regionally averaged group velocity curves. A usable estimate of fundamental mode Love wave phase velocities was obtained by Fourier analysis of sums of cross-correlograms, but Love wave group velocities could not be resolved by frequency-time analysis.

Higher mode interference and observed anomalous apparent Love wave phase velocities

For a representative selection of spherical earth models compatible with seismic observations, there is a significant frequency range over which the fundamental and first higher Love mode group velocity curves approach each other closely or actually overlap. Higher Love modes can be excited comparably to the fundamental mode for both shallow and deep sources under a variety of circumstances, and thus higher mode interference is an important factor to be taken into account in the proper interpretation and analysis of Love waves. Simple theoretical computations reveal the nature of the effect of mode contamination on measured phase velocities, and biases in the selection and analysis of surface wave data make it appear likely that fundamental mode Love waves experiencing higher mode interference will exhibit anomalously high apparent phase velocities, as observed in the United States midcontinent and in Japan. Thus it is suggested that it may not be necessary to resort to complex or anisotropic models to explain these observations. Consideration of the effects of mode interference, as well as knowledge of source parameters (type, depth, orientation), are important in determining dependable fundamental and higher Love mode phase velocity dispersion over a broad frequency band. Phase velocity filtering across a large array could effectively separate the modes. Love waves traversing continental regions from the source will suffer little or no contamination from higher modes for wave periods of less than about 60 sec and thus are preferred to oceanic paths in obtaining reliable fundamental mode data in this restricted frequency range.

ESTIMATION OF SEISMIC NOISE STRUCTURE USING ARRAYS

A theoretical study of the use of arrays for the analysis of seismic noise fields has been completed. The frequency‐wavenumber power spectral density [Formula: see text] is defined and techniques for estimating it are given. The estimates require that the auto‐ and crosspower spectral densities be estimated for all elements in the array. Subject to certain asymptotic properties of these auto‐ and crosspower spectral density estimates, expressions for both the mean and variance of the estimates of [Formula: see text] have been obtained. It has been demonstrated that if [Formula: see text] is estimated by the Frequency Domain Beamforming Method, then the estimate has the same stability as the estimates of auto‐ and crosspower spectral density. [Formula: see text] has been estimated from both long‐ and short‐period noise recorded by the Large Aperture Seismic Array in Montana. At frequencies higher than 0.3 Hz, a compressional body‐wave component which correlates with atmospheric disturbances over distant oceans has been detected. In the frequency range of 0.2 and 0.3 Hz both body waves and higher mode Rayleigh waves are observed. At frequencies below 0.15 Hz the organized vertical component of microseisms consists primarily of fundamental mode Rayleigh waves. Appreciable amounts of fundamental mode Love wave energy may also be present on horizontal instruments at these low frequencies.

Long-period love waves in a heterogeneous, spherical Earth

Periods of torsional eigenvibrations have been computed for heterogeneous spheres corresponding to a variety of earth models, and the periods of oscillation are used to calculate phase and group velocities for the fundamental and first higher modes of Love waves. A comparison is made between velocities computed for different spherical models and for equivalent flat earth structures. The comparison shows (1) that the effect of sphericity is more complicated for fundamental mode Love waves than for Rayleigh waves because of the efficient channeling of waves by low-velocity layers and (2) that the first higher Love mode is more affected by curvature than the fundamental mode. The variation with depth of the relative amplitude of the displacements indicates that the first higher Love mode for periods less than 90 seconds is very sensitive to upper-mantle structure in the vicinity of the low-velocity zone. Comparison of the theoretical results with recent phase velocity and torsional oscillation data shows that a Gutenberg type of velocity structure is more satisfactory than either the Lehmann or Jeffreys structures. The use of consistent densities with the Gutenberg model, rather than Bullen A densities, has a small but significant effect on the calculated velocities. For periods greater than 200 seconds the calculated phase velocities for various oceanic and continental structures are all within 2 per cent of each other. The calculated group velocities are within 1½ per cent of each other in the range 150 < T < 400 sec, thereby confirming experimental results. Dispersion measurements must therefore be made with precision if significant conclusions are to be inferred about details of earth structure.