On Regional Differences in Dispersion of Mantle Rayleigh Waves

Abstract

Rayleigh waves generated by the Peru-Bolivian border earthquake of 1963 August 15 have been analysed at 35 WWSSN stations in the period range belween 150 and 300 seconds. Thirty-one of these stations were grouped into six narrow azimuth windows as a result of an analysis of observed phase velocities. In each of these windows the phase velocity dispersion shows similar characteristics, and the composition along the paths (in terms of the shield, oceanic and tectonic fractions of a path) was approximately the same. An error analysis indicates that differences in phase velocities obtained for individual windows are statistically meaningful, thus proving that the Rayleigh wave dispersion measured on world-circling paths shows regional variations. These six areal windows cover a third of the Earth's surface.Group velocities were measured for the six azimuthal windows by summing the auto-correlograms of individual seismograms for stations within each azimuth range. A statistical test has shown that the measurements of phase and group velocities are mutually consistent proving for the first time that the lateral inhomogeneities in crust and mantle can also be detected by measurements of group velocities for world-circling paths. The simultaneous least-squares condition for both phase and group velocity observations for each window was used to derive a polynomial approximation for the frequency of free oscillations as a function of order number measured as the excess over a reference model. ' Pure path ' phase and group velocities derived according to the approach of Toksoz and Anderson indicate substantial differences in dispersion for each basic type of mantle. Pure path phase velocities derived in this report differ considerably from the corresponding results of an analysis by Kanamori, whereas world-wide averages are very similar for both reports.Group velocities can be separated much more effectively than phase velocities, especially at the longest periods. This effect may be explained by world-wide variations of depths to the so-called 400 km and 650 km discontinuities. These variations appear to be unrelated to the regionaiization adapted in this paper.

Rayleigh wave group velocities and attenuation coefficients

“…All events have focal mechanism solutions which show that the fault motion is predominantly dipslip. Q n which are calculated.from global average attenuation coefficients n and group velocities are similar to the largest Q n computed from n fundamental mode spheroidal oscillations by Jobert and Roult [7] and are plus island arc, and ocean basin plus mid-ocean ridge provinces which are in good agreement with group velocity dispersion observed at periods shorter than 100 seconds and which also agree with the pure path group velocities derived from great circle measurements of Dziewonski [3] average group velocities and attenuation coefficients has also been regionalized successfully for 2 and 3 region combinations. The resulting pure path specific attenuation for continents is much lower than that for ocean basins and ocean basin plus mid-ocean ridge provinces.…”

Earth Structure from Free Oscillations and Travel Times

“…For these modes and for oSJ we have used the periods given by Derr (1969b). In addition, we included in our data set the average periods of the modes o S 5 1 -0 S 6 3 given in Table 3 of Dziewonski (1971). These data are listed with the eigenperiods of our final model in Tables A1 and A2.…”

“…The mean eigenperiods and standard errors in the means taken from Tables 2 and 3 (all data) and Tables 4 and 5 of Dziewonski & Gilbert (1972) are given in Tables A1 (spheroidal modes) and A2 (toroidal modes) of this paper, along with Derr's (1969b) averages for oSz, oS3, and oT, and Dziewonski's (1971 , Table 3) values for oS,, through oS, , . Also listed are the computed periods for model B1 and the relative errors of this model with respect to the data (computed minus observed/observed).…”

Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

“…These relatively long waves are useful for constraining the physics and geometry of the layers at crustal and mantle depths (OLIVER, 1962;DZIEWONSKI, 1971;KNOPOFF, 1972) as they transport the greatest part of the seismic energy, and key information about the elastic and inelastic properties of the Earth's crust and mantle RITZWOLLER et al, , 2001XU et al, 2000;YANG et al, 2004). The surface waves observed on records of earthquakes are predominantly fundamental mode Rayleigh-and Love-waves and their group velocities along their respective great-circle propagation paths are mainly controlled by variations in the velocity structure of the crust and mantle.…”