15 - Earthquake dynamics

Summary

Preamble

After more than a century of scientific observation, earthquakes have given a detailed picture of the structure of the Earth, but only a rudimentary idea of the physical processes in the fault zones where they occur. We know that earthquakes are local hiccups in convectively driven tectonic motion and the pattern of their occurrences is a central component of the theory of plate tectonics. Elasticity theory explains the geometry of stress release during earthquakes but not the mechanism that triggers them. Although this depends on the physical properties of materials in the fault zones, we are still far from a level of understanding that might lead to reliable prediction. It is possible that the detailed information that would be required is beyond reach. Nevertheless prediction has been the ultimate target of much seismological research. This chapter is concerned with the information and ideas that the work has produced, with a brief discussion of the possibility of prediction itself in the final section.

A basic problem is that earthquakes are very diverse phenomena. The range of sizes is extremely wide, probably bounded at the upper end of the scale by magnitudes not much greater than that of the Chile 1960 event (Section 14.7) and with no observed lower bound. The range of energies exceeds 10¹⁷:1. The range of speeds of fault movement during earthquakes is also very wide, bounded at the upper end by the accelerations that can be produced by stresses of order 10 MPa if fault friction suddenly vanishes.