SH waves: Theory and observations for epicentral distances greater than 90 degrees

Details SH waves: Theory and observations for epicentral distances greater than 90 degrees SH waves: Theory and observations for epicentral distances greater than 90 degrees

Sep 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977pepi...15...46m&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 15, Issue 1, p. 46-59.

Physics

6

Scientific paper

SH waves observed at epicentral distances greater than 90° contain considerable information on the lowest 200 km of the mantle, the D'' region. Seven event L.P. WWSSN station combinations at which SH was clearly observed have been analyzed. For each event the stations chosen were confined to a small azimuth range from the source. Assuming lateral homogeneity the differences between the received signals are only due to the different paths through the D'' region. Although the upper mantle and crust are certainly not laterally homogeneous, they are not known accurately enough to compensate for the differences beneath the stations. This, however, results in large standard errors of the decay parameters determined. After Fourier transformation the decay with distance for each frequency component (period range 64-68 s) was calculated assuming linearity with distance. The SH-wave potentials for the same frequencies were calculated using a point source and Langer's approximation for the calculation of the radial factor, for several models: PEM-C and related models with lower velocities in the D'' region. The decay with distance was also calculated assuming linearity. Direct comparison between the decay values of the calculated wave field and the observations was allowed at the level of approximation used. It was found that a velocity decrease with depth - i.e., a negative velocity gradient, in terms of the usual convention - of the order of 2.4% over a depth interval of a good 100 km explained the data satisfactorily. Lateral inhomogeneity was indicated by the observations but has to be corroborated by more data.

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