S velocities in D'' from diffracted SH-waves at the core boundary

Abstract

Summary. The slowness of the diffracted SH-wave around the core boundary is re-examined. The criticism that measurements from the Iranian earthquake of 1968 August 31 are affected by the focal mechanism is shown to be unjustified theoretically and by comparison with analogous recordings from the Imperial Valley earthquake of 1979 October 15. Evidence is presented that an SH ray parameter estimated from first impetus SH diffracted onsets does not represent a shear velocity averaged over a hundred kilometres depth but rather the velocity within a few tens of kilometres of the core–mantle boundary. In addition to a re-examination of the seismograms previously used for the 1968 August 3 1 Iran earthquake, long-period seismograms for 80° < ▵ < 150° have been studied for two Tonga earthquakes, 1967 October 9 and 1977 August 11. Exclusion of readings with ▵ < 105° from the Iranian earthquake increases the gradient from 8.68 ± 0.13 to 9.48 ± 0.26s deg−1. The exceptionally large ray parameter is not fully explained, but a plausible hypothesis is anisotropy in shear velocity near the core–mantle boundary. When observations are restricted to clear arrivals at distances exceeding a few degrees from the shadow boundary, reliable new estimates are obtained for the SH slowness, namely 8.41 ± 0.07s deg−1 for Tonga 1967 and 8.39 ± 0.06 for Tonga 1977. The statistical uncertainty of the estimates indicates no significant average change in SH velocity at the base of the mantle along two easterly profiles 3000 km apart under the east Pacific and North America. For a core radius of 3485 km and a slowness of 8.4s dec−1, the average velocity near the core–mantle boundary is 7.24km s−1. This value is in agreement with estimates in D″ by Mula & Muller. The observed attenuation of the diffracted SH ground motion amplitudes at periods of 15–20s is similar for the three profiles considered, with a reduction of an order of magnitude within a distance of 25° in the shadow zone. This corresponds to an equivalent decay parameter γ= 0.10 at the core boundary.