Re: How do earthquakes originate, and why is there always an aftershock?

Earthquakes are generated by an instantaneous energy release in the deep
that originates elastic waves that propagate through all the planet. The
energy released may be accumulated by several processes, but the usual is
that this is due the continuous movement and interaction of the litospheric
plates, but it can be by a volcanic eruption, a nuclear explosion, and so on.

Before the main event (the earthquake itself), numerous small quakes can
occur, showing that the Earth has too many energy accumulated and the rocks
can't support it. After the main shock, some energy still remains, and it
is usual to have some smaller events to dissipate this residual energy.

Best regards

Eder C. Molina
Dept. of Geophysics
Institute of Astronomy, Geophysics and Atmospheric Sciences
University of Sao Paulo - Brasil