Above is the three component broadband seismogram from station LSA 2550 km from the Pakistani test site (see map).
About a third of the way along the trace (at ~ 10:21:25) the P-wave arrival can be seen, then a larger high frequency arrival is seen towards the end of the trace.
The high frequency arrival is most likely an earthquake local to station LSA for the following reasons.
Usually the P arrival contains the highest frequency data from any event.
The S-wave arrives later than the data shown - based on estimated arrival time for an event 2550 km away.
This suggests it is a small event local to the station LSA.
The data below is the same time window of data as above however it has been bandpass filtered to enhance specific frequencies. The first has been bandpassed at 2-4Hz. It clearly shows the P-arrival from the nuclear test. The second figure has been bandpassed at a higher frequency of 4-8Hz. On it the nuclear P-arrival is not seen but the local earthquake is.