Living with Earthquakes - Earthquake Engineering for Structural Design - page 32

Chapter2
LivingwithEarthquakes
2.1 WHENTHEEARTH SHAKES
2.1.1EarthasLivingBodyinPermanentMotion
A severe earthquake is a terrifying experience. Relatives, homes and goods can be
lost in only a few minutes. Compared to any other natural event, the earthquake is
the most frightful one, because it undermines the basic stability of human existence
and the confidence that this stability is under control. When this stability is
destroyed by the violence of nature, fright and panic reach an intolerable level.
As natural as wind and snow are in the atmosphere, the seismic movements
develop inside of the Earth. The entire lifetime of the Earth is a continuous
sequence of underground movements with more than a billion quakes per year, an
average of one quake every 30 seconds. Therefore, the Earth is like a living body
in permanent motion. This movement is produced by the convection currents
developed in the viscous mantle due to the prevailing high temperature and
pressure gradients between the crust and the core. These convection currents result
in a circulation of the Earth masses: hot molten lava comes out and the cold rock
mass goes down into the Earth (Fig. 2.1).
Figure 2.1 Convection currents into the Earth (USGS, nd)