Faults and Earthquakes - Earthquake Engineering for Structural Design - page 180

each other due to its peculiar typology. Starting from the basic features, given by
the source, the overlap of different influence factors can change these features by
creating amplification or attenuation of the ground motions, by increasing or
reducing the movement duration, etc. Therefore, the main characteristics in a site
can differ very much from the initial ones and this is the main reason why, in some
cases, a moderate magnitude earthquake can be metamorphosed into a devastating
one.
6.1.2 Mechanical and Thermo-dynamical Processes Producing Earthquakes
There are two types of phenomena producing earthquakes:
- The first phenomenon is due to a mechanical process occurring in the Earth's
crust, where the equilibrium is based on the friction between two plates and the
earthquake is the result of the loss of this equilibrium, or, when new faults are
forming, is due to the brittle fracture of rocks.
- The second phenomenon, dominated by a thermo-dynamic process, is not due
to friction between two plates, but is produced at a deep depth where temperatures
and pressures are very high and there is only one plate.
Due to the first phenomenon, the earthquakes occur at shallow depths of 0 to
50 kilometers in the Earth's crust, where the friction forces between tectonic plates
normally assure the stability of these plates, being in contact along the fault. Some
dislocations exist due to previous movements or other dislocations can occur at the
end of a fault already produced, being characterized by a new brittle rupture in the
continuation of the existing one.
The second phenomenon gives rise to a particular category of earthquakes
which is largely ignored by the scientific community. The earthquakes occur at
depths of 50 to 300 kilometers beneath the Earth's crust and are caused by different
processes. Extreme heat and pressure in the subduction zone cause the
metamorphoses of rocks or change them into different forms. During the change,
the rocks release water, which essentially lubricates the fault, facilitating it to
move. The mechanism whereby earthquakes are produced, because of the high
pressures occurring below 100 kilometers of depth, remains until today an
enigmatic and controversial problem (Persh and Houston, 2004). But the main
unsolved problem is to understand the earthquake features produced below 300
kilometers, where the slab disappears in the mantle. It is very important to mention
that the deepest recorded earthquake exceeds 600 kilometers.
6.1.3 Source Types
A large number of earthquakes daily occur on the Earth. So, earthquakes can strike
any location at any time. But the history shows that they principally occur in some
specific zones of the Earth. Many earthquakes occur in narrow regions around the
plate boundaries. This is evident by comparing the Figure 2.2, showing the
distribution of earthquakes in the world, with Figure 5.1 indicating the main
boundaries of the main tectonics plates. But this is not a general rule, because a lot