Origin of Earthquakes and Seismic Waves
During a disturbance deep in the Earth’s crust or mantle such as faults, tectonic plate shifts, volcano eruptions or underground explosions, a large amount of energy is released as a result of which seismic longitudinal (P-waves), shear (S-waves), and surface waves propagate, causing an earthquake. This cause-and-effect process seems simple and easy to understand. However for describing step by step the generation and propagation of various types of seismic waves, many factors should be taken into consideration. Deep seismic disturbances cannot be directly studied, but observations of man-caused explosions causing tremors point to the generation of short, powerful, pressure pulses reaching Mega or even Giga Pascal ranges in the focus of the earthquake. However, a pressure pulse is not a wave. A wave is a periodic transient motion in a medium of propagation that requires a continuous driving force. The question is how a pressure pulse becomes the origin of a seismic wave? Of course, in an earthquake’s focus not one, but a raw of pressure pulses can occur. They follow each other at random intervals of time and the question of the origin of seismic waves is still relevant.