Interference.
Congregation, if not confluence.
diffraction
Interference. Constructive Interference, which is where the waves increase amplitudes and troughs when they overlap. Destructive Interference, which is where the waves cancel each other out when they overlap.
When two waves meet, their interaction is known as interference.
Reinforcement of waves that are in phase with eachother.
Modulation
Presuming you mean earthquakes, L waves are known as Long waves. P waves are known as primary waves, and S waves are known as secondary waves.
P-waves and primary waves are the same thing. They may also be known as pressure waves (or compression waves) due to their mode of propagation, the way they move through the Earth.
primary waves or compressional waves
diffraction
Yes
P-waves and S-waves. P waves are known as Primary waves as they have a high velocity and so are recorded at seismometer stations before Secondary or S-waves. P waves may also be known as pressure waves as they are in fact compression or longitudinal waves. S-waves may also be known as shear waves as they are transverse waves.
Compression Waves
Shear waves