= Solution
The \b[northern station is closer], under comparable <P wave> and <S wave> paths through the same <velocity model>. In the actual Figure 4 the northern P and S pulses are separated by about \b[6 s], whereas the southern separation is roughly \b[8–8.5 s]. The <S–P time> removes the unknown <earthquake origin time>:
$$
\Delta t=t_S-t_P=D\left(\frac1{V_s}-\frac1{V_p}\right).
$$
Both northern pulses also occur earlier on the shared absolute clock, consistent with the smaller distance. Earlier arrival alone would not be sufficient if clocks or <velocity models> differed.
For reproducible graph readings, the northern positive-to-negative zero crossings are approximately $13.3\ \mathrm s$ for P and $19.3\ \mathrm s$ for S; the southern crossings are approximately $16.7\ \mathrm s$ and $25.0\ \mathrm s$. These are convenient corresponding pulse landmarks, not sharply defined first-break onsets. The first visible motion begins earlier: approximately 12 s and 18 s in the north, and 14–15 s and 22–23 s in the south, depending on the amplitude threshold. The horizontal axis is \b[seconds since midnight on 1 January 2000], not elapsed time since rupture initiation.
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