Interpretation of Sa for a Shield Structure

Abstract

The seismic phase §a, on a shield structure, is interpreted using evidence from both group velocity dispersion and amplitude excitation functions.§a on a shield is explained in terms of stationary phases of higher-mode Love waves. It does not appear to be a low-velocity zone channel wave, but rather a phenomenon mainly influenced by the presence of a rapid increase in S-wave velocity at a depth of about 400-450km. The energy associated with the Sa stationary phases propagates much as though it were propagating through a single, homogeneous, low-velocity layer overlying the high-velocity material below the velocity increase.

Interpretation of Sa on a continental structure

“…Inspection of the higher-mode group velocity curves shows the same set of Sa stationary phases, from a period of about 60 s to about 10 s, as we found in our work with a standard shield structure (Schwab et al 1974a). Thus the entire period range which Caloi associates with Sa, from experimental evidence, can be accounted for by these higher-mode Love wave arrivals (stationary phases) for our present structure.…”

“…This appears to remove any question of the validity of the interpretation given by Schwab et al (1974a): the displacement-depth functions (Schwab et al 1974a, Fig . 3 ) and the stress-depth functions (Fig.…”

“…In our first interpretive work on Sa in continental structures (Schwab, Kausel & Knopoff 1974a), we extended this identification of Sa with higher-mode Love wave propagation by including amplitude excitation functions, as well as group velocities, in the interpretation for a shield structure. The basic conclusion of that work is that Sa does not appear to be a low-velocity zone channel wave, but rather a phenomenon mainly influenced by the presence of the rapid velocity increase at a depth of about 400-450 km; the energy associated with the Sa stationary phases propagates much as though it were propagating through a single, homogeneous, low-velocity layer overlying the high-velocity material below the velocity increase at a depth near 400-450 km.…”

Oceanic Sa

“…Since La(i, l), i z 1, sample the region above the '400-km' discontinuity in a uniform manner, and penetrate only slightly below this discontinuity, the associated energy appears t o propagate much as though it were travelling in a single, homogeneous layer over a homogeneous sphere of higher velocity, where the velocity change occurs at the depth of the '400-km' discontinuity. This interpretation was originally proposed by Schwab et al (1974), and the quality of this two-layer interpretation was later evaluated by direct numerical testing (Nakanishi et al 1976, Fig. 12).…”

“…For periods between 30 and 60 s, the group velocity data indicated values varying between 4.48 km/s for paths through shield-like structures, to about 4.33 km/s for mixed, tectonic-continental type structures. For shorter periods, the group velocity increased, reaching an average value of 4.5 km/s at a period of around 20 s. Schwab, Kausel & Knopoff (1974) extended the interpretation of Sa (SHcomponent), in terms of higher-mode Love wave propagation, by using evidence from both group velocity dispersion and amplitude excitation functions, where their analysis was based on a realistic model of a shield structure. They identified Sa as the superposition of stationary phases of higher-mode Love waves for periods between 10 and 60 s. One important conclusion from their work (see also Nakanishi, Schwab & Kausel 1976) is that the existence of a lowvelocity channel in the upper mantle is not necessary for the propagation of Sa; rather, it appears that the presence of a rapid increase in S-wave velocity, at a depth of 400-450 km, is the governing structural feature.…”