Oceanic Sa

• Published in: Geophysical Journal International
• Main Authors: Kausel, E. G., Schwab, F., Mantovani, E.
• Format: Text
• Language: English
• Published: Oxford University Press 1977
• Subjects: Articles; Kamchatka Peninsula; Kamchatka
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/50/2/407; https://doi.org/10.1111/j.1365-246X.1977.tb04181.x

The seismic phase Sa on an oceanic structure is interpreted using evidence from both group velocity stationary phases and excitation functions of higher mode Love waves. This frequency-domain information, combined with theoretical time series, establishes the identification of oceanic Sa with Love wave stationary phases. Two types of higher-mode stationary phases are identified: La(i, j) and Lb(i, j) , La(i, j) phases are confined to propagating almost entirely above the ‘400-km’ discontinuity, whereas Lb(i, j) phases sample the structure down to, and slightly below, the ‘650-km’ discontinuity. Sa thus appears to be a useful tool for the study of regional variations of these features and the associated regions of the upper mantle. An analytically correct model for Sa on a spherical earth is a flat structure composed of two homogeneous layers over a homogeneous half-space. Long-period SH seismograms for an event occurring at the foot of the Kamchatka peninsula, and recorded at Port Moresby (PMG) and Honiara (HNR), are analysed. The SH component of Sa is satisfactorily explained as a superposition of the La(i, j) and Lb(i, j) stationary phases for Love waves. Qualitative analysis of the experimental data indicates that Sa may be valuable as a means of determining whether or not the ‘400-km’ discontinuity is present beneath a given position on the surface of the earth. Direct comparison of the Sa portion of experimental and theoretical seismograms indicates whether ‘normal’ attenuation, due to the anelasticity of the earth, is taking place over a given propagation path; this provides an indirect method of testing for the existence of a ‘normal’ low-velocity channel in the upper mantle. A direct inversion scheme for the La ( i , 1) stationary phase is developed. Universal curves are given which illustrate normalized periods, and group and phase velocities for the La ( i , 1) phases, as functions of β 1 /β 2 , for a simple layer-over-a-half-space model. These curves are convenient for the determination of regional ...