Answer:
There might not be any S waves on the seismogram of a big earthquake because S waves cannot travel through liquids, and the intense shaking of a big earthquake can cause the liquid-filled areas to expand, preventing the propagation of S waves through them. As a result, the absence of S waves on a seismogram can indicate that the earthquake occurred in a region with a large amount of liquid-filled material, such as an oceanic trench or sedimentary basin.