NOTE(2): Many seismograms like this will initially be displayed in the conventional NEV (or NEZ) or North-East-Vertical coordinate system . But, to optimize many kinds of analyses and displays, one needs to rotate (or transform) this system into another - LQT. Read here to understand what that means.

NOTE(3): How does one know or estimate seismic phases like these? You need an earth model! From the source location and at the instantaneous moment of when the seismic event occurs, a corresponding seismic wavefield is generated and travels throughout the earth - interacting with its fluid and/or elastic mediums - all the while responding to (or convolving) its complex, 3-D velocity (and density) structure. As the seismic wavefield passes through different mediums via refraction, reflection, dispersion, attenuation, etc., it changes and subsequent various phases are produced and then recorded at a station. The phases - i.g. 'SKS', 'P', and many others - basically tell a story of how/where the wavefield changed, which is great for analyzing the crust, all sections of the mantle, and even the core!

*I've included a lot here but you've asked an outstanding question that you (and I) can really learn from!