Regional seismograms are dominated by the phases Pn, Pg, Sn, and Lg. More often Sn and Lg are used to infer the attenuation structure of the lithosphere. The seismic phase Sn is a high-frequency shear-wave (typically from 1 to 4 Hz and occasionally higher) that travels in the lithospheric mantle above the negative velocity gradient which usually marks the lithosphere-asthenosphere boundary. Sn has been reported out to distances of 35° (e. g. , MOLNAR and OLIVER, 1969; HUESTIS et aI. , 1973). Sn arrives as a high-frequency wave train lasting tens of seconds and up to 1 to 2 minutes. Sn velocities are typically 4. 7 km/s in stable continental and oceanic lithosphere (HUESTIS et al. , 1973) and as low as 4. 3 km/s (KADINSKY-CADE et al. , 1981) in more tectonically active regions. Lg is a complex short period guided wave consisting of high-frequency P and S energy which travels primarily in the earth’s crust at frequencies typically between 0. 5 and 5 Hz. It has been modeled as higher-mode Love and Rayleigh waves as well as a sequence of multiply reflected post-critical S waves trapped in a crustal guide (BOUCHON, 1982; KENNETT, 1986; BOSTOCK and KENNETT, 1990). Lg has been observed not to propagate in oceanic or very thin continental crust (PRESS and EWING, 1952; SEARLE, 1975; ZHANG and LAY, 1995).
