Seismic Waves
Part of the accumulated potential energy in the form of rocks' deformation is released through seismic waves which leave the starting point of rupture and propagate towards all directions. This region confined within the subsurface where the waves drive from is called Focus or hypocenter. Its projection to the Earth's surface corresponds to the earthquake's epicenter.
The seismic waves' speed and characteristics of propgation are defined by the properties of the medium through which they travel. They are classified as Internal Waves when they propagate through the interior of the Earth (Body Waves) and Surface Waves when their propagation is close to the earth surface. In the first case we have P and S Waves and in the second one we have the Love and Rayleigh waves.
Types of seismic waves
P or Primary, Longitudinal or Compressional-dilatational wave: the sound wave is an example of a P wave. It is the fastest one and can propagate through all mediums. The particle vibrations are parallel to the wave's propagation direction and correspond to the successive compressions and dilatations of the medium through which they pass, as illustrated in the picture below:
S, Secondary, Transversal or Shear waves: They can't propagate through liquid mediums (pasty). The particle vibrations have shear movements, which are perpendicular to the direction of the wave' propagation. See the picture below:
Love Wave:The vibrations occur on the horizonatl plan, which is perpendicular to the direction of the wave's propagation. See the example below:
Rayleigh Wave: The movement of the particles is on a plane vertical to the direction of the wave' propagation:
In the earth crust, the average speed of the P waves is approximately 5 km/s and the S Waves' is about 3 km/s. The surface waves are slower and their penetration into greater depths depends on its wave length. It takes a P wave aproximately 20 min to travel across the Earth diametrically. The 1960 earthquake in Chile (magnitude 8.0 MS or 9,5 mW) relased so much energy that the surface waves kept being registered for 60 hours after the event.
Stages for the earth-crust-propagating waves:
*Pg P waves which cover shorter distances (0,1°< distance < 8°). These waves propagate only inside the crust. When travelling greater distances, they cause multiple reverberations within the crust. This wave reaches an average speed of 5.8 km/s.*Sg S waves which cover shorter distances (0,1° < distance < 8°). These waves propagate only within the crust. Greater distances cause superposition and multiple reverberation within the crust. They also occur in the conversion of P into SV or SH. This wave reaches an average speed close to 3,5 km/s and can also be called Lg.*P or Pb, S* or Sb: P and S waves refracted in the Conrad Descontinuity (2° < distance < 8°)*Pn and Sn: P and S waves refracted in Moho Descontinuity (Mohorovicic) which separate the crust from the earth mantle (2º < Distance < 20º).*PnPn and SnSn: Pn and Sn waves with free reflection in the surface.*PgPg and SgSg: Pg and Sg waves with free reflection in the surface.*PmP: P waves with reflection in the outside of the Moho Descontinuity.*PmS: P waves with reflection in the outside of the Moho Descontinuity and being converted into a S wave.*SmS: S waves with reflection in the outside of the Moho Descontinuity.*SmP: S waves with reflection in the outside of the Moho Descontinuity and being converting into a P wave.*Rg: Rayleigh waves (Surface) of a short period with propagation in the Crust.
The Picture below shows the waves' trajectory in the earth crust:
Core-propagating waves
*PKP: It is the direct P wave which propagates within the core (Distance < 100º).*SKS: It is the direct S wave which propagates within the core.*PKPbc (PKP1): It is the P wave which propagates in the inner part of the outer core.*PKPab (PKP2): It is the P wave which propagates in the upper part of the outer core.*PKPdf (PKIKP): It is the P wave which propagates within the outer core.*PKPdif: It is the P wave diffracted on the limit between the outer core and the inner nucleus.*PKS: It is the P wave not specified which propagates in the core and which turns into an S wave at the Mantle-Core limit.*SKS: It is the S wave not specified which propagates in mantle with an S-phase and, on entering the outer core, turns into a P wave, keeping its course.* PKKP, SKKS, PKKS, SKKP: They are the P or S waves not specified which reflect one another once they are at the Mantle-Core limit within the outer core. In the case of the S waves, because of their not propagating in the inside of the outer core, they have the capacity of turning into P waves as they travel across the outer core.*PkiKP, SkiKS: They are the P or S waves reflected at the limit of the inner core. In the case of S waves, as they cannot propagate through the inside of the outer core, they have the capacity of turning into P waves while they travel across the outer core.*PkiKS: It is the P wave which is reflected at the limit of the inner core and which, after that, turns into an S wave.*PKJKP, SKJKS: They are the P or S waves which propagate in the outer core with an P-phase and within the inner core with an S-phase.