。。
That one just now was a P-wave. An S-wave is coming.
Literal
Earlier [attributive-の] one [topic-は] P-wave was-[polite-past-でした]. S-wave [subject-が] comes-[polite-ます].
Seismology jargon, clipped and urgent. 先程 is the formal/written 'a moment ago' (vs. conversational さっき); the ~の after it is the pronominal nominalizer '(the) one,' standing in for an omitted noun — 'the one that was a moment ago.' Ｐ波 (primary/compressional) and Ｓ波 (secondary/shear) are the two main seismic wave types; the P-wave arrives first and is the early-warning signal, the stronger S-wave follows. The full-width Roman letters are standard in Japanese scientific writing.