Question

what kind of waves are produced in an earthquake before the main shock wave begins.

Answer 1

Infrasonic waves are produced in an earthquake before the main shock wave begins.

Answer 2

Preceding an earthquake, stresses accumulate in the earth's crust over time, causing energy to be stored in the form of elastic strain (like that in a compressed spring). Ultimately this stress exceeds the shear strength of the crust in the fault zone causing a sudden brittle failure or rupture. This in turn causes movement and a sudden release of the stored elastic strain energy in the form of seismic waves (or shock waves).