The amplitude (height) of the surface waves are measured by the Ritcher scale [log scale].

Earthquakes can cause ground displacements from less than a millimeter to several meters.

Several hundred tremors occur around the world every day.

Moderate earthquakes, about 7.0 in Ritcher scale, occur about 20 times per year.

Earthquakes can cause damage to civil engineering structures such as buildings, bridges, roads, dams, canals, etc.

Seismic design seeks to build structures that will be better able to sustain earthquake loadings.

In California, since the Loma Prieta [S.F. Bay and Oakland] earthquake of 1989 and the Northridge earthquake of 1993, important bridges have been retrofitted to sustain earthquakes.