ScS reverberation studies

A major source of information on the layered structure of the Earth is the study of ScS reverberations. For the ScS reverberation studies horizontally polarized shear waves (SH-waves) reflected at the core-mantle boundary (CMB) and the Earth's surface are used. The rays of ScS (a wave reflected once at the CMB) and some first order reverberations (upper-side and underside reflections at the discontinuities) are displayed in Figure 3.1.

Figure 3.1: In panel I the standard phase ScS is displayed. Panel II shows an upperside reflection at the 660-km discontinuity (ScSS660$^+$SScS) (compare Revenaugh and Jordan (1989)) between source (star) and receiver (triangle). Panel III shows an underside reflection at the 660-km discontinuity (S660$^-$SScS$_2$). Panel IV shows an upperside reflection at the source side (S660$^+$ScS$_2$).

The free surface and the CMB are perfect reflectors for SH waves. These waves bounce back and forth through the mantle for a long time following an earthquake. The energy is partly reflected at the discontinuities within the upper mantle and the crust (Figure 3.1 II-IV). The reverberations can be identified easily in the recordings of an earthquake.
A good overview on ScS reverberations and the used processing methods can be found in Revenaugh and Jordan (1989), (1991) and (1991a-c).
Due to the long-period nature of ScS the sharpness of the discontinuities cannot be resolved. A vast number of different regions have been studied with this method and evidence for several discontinuities has been found.
The interpretation of the ScS reverberation is not unambiguous, because of a mathematically exact mirror plane symmetry of the reverberation integral kernel (Revenaugh and Jordan, 1989). This symmetry of the equations makes it impossible to decide whether a reflector is located in the upper mantle or in D'' in the lower mantle. The reflections in the lower mantle for the 410 and 660 are neglected because the impedance profile in these depths is constrained to be very smooth by other seismological data (Dziewonski and Anderson, 1981). For shallower discontinuities other seismological data must be used to constrain the interpretation as a discontinuity in the upper mantle.
The ScS reverberation studies give estimates of the reflection coefficients across the discontinuities and therefore, give good insight into the velocity and density contrasts. Recently, a joint inversion of ScS reflectivity profiles, frequency dependent travel times of body waves (S, sS, SS, sSS, SSS) and surface waves (Rayleigh R1 and Love G1) were used to produce high resolution seismic models of the layered structure, the discontinuity depths and the anisotropic structure in different corridors under the Pacific, Australia and the Philippine sea presented in Figure 2.3 (Gaherty et al., 1999).