The SmS phase, generated by reflection of downgoing S waves at the Moho discontinuity, was initially identified in only a limited number of earthquakes. Its observability is constrained by several factors, including source characteristics, propagation path, site response, and network density, so the distance range over which it can be clearly recorded is limited. With the continued densification of strong-motion networks and the accumulation of high-quality records, it has gradually become clear that, although the SmS phase is a common seismic phase, a prominent high-amplitude SmS arrival is not observed in every earthquake. When well developed, SmS often exceeds the direct Sg phase in amplitude, so the peak value of a strong-motion record may be controlled more by the reflected phase than by the direct arrival. This paper reviews the current understanding of the effects of SmS on ground motion, with emphasis on reported observations, controlling factors, implications for ground-motion prediction, and recent progress in China.

Reported observations from North America, Europe, and Asia indicate that SmS can significantly enhance recorded amplitudes within specific epicentral distance ranges. The 1989 Loma Prieta earthquake remains a classic example: strong-motion levels in parts of the San Francisco Bay area exceeded those predicted by standard attenuation relations and were later linked to a strongly developed SmS phase. Similar effects have since been reported in eastern North America, the Marmara region, the Taipei basin, the Po Plain of northern Italy, southwestern China, and northeastern China. These cases show that the influence of SmS is widespread but strongly region dependent. Its manifestation varies among regions and records: in some cases, SmS can be recognized as a later phase, whereas in others it is reflected mainly in enhanced peak amplitudes and localized flattening of attenuation curves, even when the phase itself is not clearly separated.

The development of SmS is controlled primarily by crustal structure and variations in the Moho discontinuity. When an earthquake source is located above the Moho, downgoing S-wave energy is partitioned into reflected and transmitted components at the crust-mantle boundary. The strength of SmS therefore depends fundamentally on crustal and uppermost mantle structure, because these parameters govern the reflection and transmission coefficients at the Moho. As epicentral distance increases, the incidence angle at the Moho changes, and once reflection approaches or enters the critical regime, reflected energy can increase markedly, allowing SmS to develop into a strong and coherent phase. This explains why strong SmS is usually observed only within a limited distance range. Variations in crustal complexity and Moho geometry further influence this process. In simple and laterally homogeneous crust, less energy is lost through repeated reflection, transmission, and scattering at internal interfaces, so SmS is more likely to preserve coherent waveforms and strong amplitudes. In contrast, structurally complex crust and irregular Moho geometry can redistribute seismic energy into multiple wave components and shift the distance window for critical reflection, thereby weakening or obscuring SmS. Other factors mainly modulate, rather than fundamentally control, the observed characteristics of SmS. Source depth affects the incidence angle of the downgoing S wave at the Moho, so deeper crustal earthquakes are generally more favorable for strong SmS development over shorter critical-distance ranges. Focal mechanism may introduce azimuthal differences in amplitude through source radiation effects, and attenuation structure may alter the amplitude ratio between direct S and reflected SmS because the two phases follow different ray paths. Site response adds another layer of complexity at the recording stage, because the amplification associated with SmS depends not only on local site conditions but also on the frequency relationship between the reflected phase and site resonance.

Beyond its seismological significance, SmS is also important from an engineering perspective because its impact ultimately depends on whether the reflected wavefield is strong enough to modify peak ground motion. When SmS develops into a strong phase, the reflected energy may equal or exceed that of the direct S-wave arrival over specific distance ranges, thereby altering the expected attenuation pattern and, in some cases, controlling the peak amplitude of the record. This makes SmS directly relevant to earthquake engineering, because peak ground motion is a key parameter in seismic hazard assessment, ground-motion prediction, and performance-based design. In regions where SmS is strongly developed, neglecting its contribution may lead to systematic underestimation of actual shaking levels. This issue also reveals an important limitation of conventional ground-motion prediction equations （GMPEs）. Most classical GMPEs represent path effects using simple distance-dependent attenuation functions, sometimes combined with magnitude scaling, and therefore cannot explicitly account for Moho reflections and related complex propagation effects. As a result, ground motions within the characteristic distance range of strong SmS development may be underestimated by standard models. This has motivated the introduction of segmented geometrical-spreading terms and other path-sensitive modifications in several regional studies.

The problem is particularly relevant in China. With the rapid expansion of the national strong-motion network, increasing observations have shown that SmS can exert strong control on attenuation behavior and peak ground motion in regions such as Sichuan-Yunnan and the Bohai Rim. These findings indicate that SmS should be considered not only in the physical interpretation of strong-motion records, but also in regional seismic hazard assessment, ground-motion modeling, and engineering applications. Taken together, current studies suggest that SmS is not merely a phase-identification issue, but a regionally significant path effect that should be incorporated more explicitly into future ground-motion analyses.