An earthquake radiates two body-wave types from the hypocentre: faster compressional P-waves and slower shear S-waves, which cannot travel through the liquid outer core and shake the ground transversely. The S-P arrival-time gap is the classic way seismologists estimate distance to a quake.
Vp = sqrt((K + 4/3*mu) / rho)
Vs = sqrt(mu / rho)
distance ~ (t_S - t_P) * (Vp*Vs)/(Vp - Vs)
- Magnitude - sets the initial rupture energy, shown as the size/brightness of the expanding wavefronts and the seismometer trace amplitude.
- Focal depth - moves the hypocentre deeper, changing travel path length and arrival timing to the surface station.
- Crust stiffness - scales the effective elastic moduli, directly changing both P and S wave speeds together.
- Trigger quake - fires a new rupture from the hypocentre; P and S wavefronts expand as separate transparent shells at their respective speeds.
Real-world application: this is the same physics seismic networks use for earthquake early warning - the faster P-wave gives seconds to minutes of lead time before the more damaging S-wave and surface waves arrive.