5. From phenomena to their integration in the justification of structures

Volume waves tend to straighten out (rais theory) as they pass through layers of increasingly weaker mechanical characteristics: compressional P-waves cause vertical vibrations, while shear S-waves cause horizontal vibrations. S waves have greater amplitudes than P waves.

For these reasons, an earthquake is generally felt as an initial vertical vibration, followed a few seconds later by a horizontal movement of greater amplitude (figure 40 ).