Open AccessDiffracted ScS and the Shear Velocity at the Core Boundary
Author(s) -
Bolt Bruce A.,
Niazi Mansour,
Somerville M. R.
Publication year - 1970
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1970.tb06048.x
Subject(s) - geology , seismology , diffraction , shear (geology) , mantle (geology) , surface wave , geophysics , shear waves , wave velocity , core–mantle boundary , core (optical fiber) , geodesy , optics , physics , paleontology
Summary SH waves from the surface‐focus Iranian earthquake of 1968 August 31, have been analysed to study the structure at the base of the mantle. The long‐period E‐W component records from western North American stations (91° < Δ < 114°) recorded essentially only the SH polarized wave as a pulse; high frequency but less energetic precursors (perhaps SKS ) were also present. The gradient of the arrival‐times was estimated to be 8.68±0.13 s deg −1 . The result is close to a similar estimate of J. Cleary. For a radius of 3479km, assuming the waves are S and diffracted ScS , the S velocity at the boundary is 6.99±0.10 km s −1 which suggests a slight velocity decrease downwards at the mantle bottom. Values of log A / T , used in magnitude determination from S waves, show a monotonic decrease with distance for 100° < Δ < 114°. The unadjusted SH amplitude attenuation (Δ+ 105°) for period 35 s is −0.065 (In A versus Δ°). The frequency spectra of the waves do not seem to vary with distance at the low frequencies in the manner predicted by simple diffraction theory for a homogeneous mantle.