Preamble

Elucidation of the details of the tectonic pattern is one of the incentives for seismicity studies. Another is earthquake prediction. In the 1960s and early 1970s there was a widespread, but not universal, expectation that a decade or so of intensive research would yield a methodology for predicting the times, places and magnitudes of earthquakes. The task was underestimated because the physical mechanism of earthquakes is not as simple as was supposed. In spite of the lack of success we now have a somewhat clearer perception of the earthquake process, although rather little encouragement to believe that detailed prediction is possible. Nevertheless, research with this aim continues, so we can expect further improvement in our understanding of earthquake mechanisms.

The underlying driving power for earthquakes is derived from thermal convection of the mantle, the subject of a thermodynamic analysis in Chapter 22. We can compare the energy released by earthquakes with the energy that is shown thermodynamically to be available. Except for a few shallow shocks, for which fault displacements are directly observed, earthquake energies are estimated from the radiated elastic waves. Energy is directly related to magnitude and, given the numbers of earthquakes as a function of magnitude, we can integrate over all earthquakes to estimate the total energy release. Typically, the efficiency for conversion of static elastic energy to seismic waves is estimated as 6%.