23.1.4 Local earthquakes

Any P-phase can be used like Pn and Pg. When few polarities are available, it is an advantage to use both Pg and Pn since these phases have different angles of incidence. Polarities associated with other phases are not used. There is no check if a P-phase has been duplicated.
Amplitude ratios must be determined from the same wave type for example Pg and Sg and the program will only form amplitude ratios from the same wave types. While in principle it should be possible to use ratios determined from refracted waves, generally ratios determined only from direct waves are used since they are easier to identify and have larger amplitudes than refracted arrivals. Particularly the Sn is difficult to identify. This means that the amplitudes readings most often will be made within what is considered the maximum amplitude in the Pg and Sg wave trains. However, the polarity might be read on the first arrival which can be Pn or another refracted arrival.

Distant earthquakes

Polarities of any P-phase can be used (but not pP since first letter is not P ). Using amplitudes require events with clear P and S phases and usually this means reading on broad band records. The amplitude phase names AMPN/SN are used to indicate first arrivals.