Seismic waves are characterized by their wavelength and amplitude. Shorter-wavelength and higher-amplitude waves are the most destructive (Figure 1).
Earthquakes happen all the time everywhere on Earth. Most are so small that we don’t feel them. The amount of energy released by an earthquake is measured using the Moment Magnitude Scale (MMS; formerly the Richter scale) or using the Modifed Mercalli Intensity scale (Ta The Richter and Mercalli Scales
bles 1 and 2). Intensity scales measure the amount of shaking at a particular location. The Richter scale was superseded in the 1970s by the MMS. Magnitude scales measure the size or energy of the earthquake at its source. As with the Richter scale, an increase of one step on this scale corresponds to a 32-fold increase in the amount of energy released, and an increase of two steps corresponds to a 1000 times increase in energy.
Tremors less than 3 on the MMS are rarely felt and are usually nondestructive, but they can be recorded. Values greater than 4 start damaging surface structures. The amount of damage is a function of the earthquake size, the distance from the focus, the surface materials, and
the type of construction. Steel-reinforced concrete structures on a shock-absorbing foundation suffer less damage than unreinforced stone or adobe structures. Structures built on firm soil or bedrock suffer less damage than structures built on waterlogged soil or landfill.
Liquefaction is when soil liqueies as a result of the vibrations and causes structures to tilt or sink (Figure 2.). In addition to soil liquefaction, earthquakes cause surface displacements that uplift or downdrop large areas, and they can cause breaks (faults) that move the surface laterally, vertically, or open small local issures.They also cause landslides,avalanches, and tsunamis.
G. L. Prost, B.P. Prost