PART II— DYNAMICS OF THE SOLID EARTH 



Figure 11-5— SEISMICITY OF THE EARTH 



Earthquakes occur in well-defined zones where the plates adjoin. The character 

 of the earthquakes varies with the nature of the plates' contact. 



However steady the slow motion 

 of the plates at their centers, the 

 motions are not steady at their edges. 

 As the giant plates move relative to 

 one another, they rub against their 

 neighbors at their common edges. 

 The friction at the edges seizes the 

 plates and allows the accumulation of 

 stress at the contact. When the stress 

 at these contacts exceeds the friction, 

 the contact breaks, a rupture takes 

 place, and an earthquake occurs. (See 

 Figure II-6) 



A map of earthquake locations, 

 therefore, is actually a map of the 

 plates, and the character of earth- 

 quakes varies with the nature of the 

 plates' contact. The character of the 

 earthquakes in the Aleutians and 



along the San Andreas Fault of Cali- 

 fornia are significantly different, for 

 example: the first is a zone of com- 

 pression with surface area being con- 

 sumed, while the second is a zone of 

 relative horizontal motions with con- 

 servation of area. 



Approaches to Earthquake 

 Prediction 



The prediction problem is, there- 

 fore, the problem of finding a way to 

 determine the first breakage of the 

 frictional contact between two plates 

 at a particular point along the plate 

 boundary. This problem can be ap- 

 proached by three methods: a search 

 for premonitors, stress measurement, 

 and historical studies. 



Search for Premonitors — When 

 solids approach the breaking point, 

 they enter a nonlinear regime of plas- 

 tic deformation in which the physical 

 properties of the materials change 

 markedly. Although the stresses con- 

 tinue to accumulate at a constant rate, 

 the strains increase greatly prior to 

 fracture. Indeed, in some cases, much 

 of the deformation observed in earth- 

 quakes is not associated with abrupt 

 displacements in rupture but is due 

 to "creep" — i.e., plastic deforma- 

 tion — certainly occurring after, and 

 probably occurring before, the shock. 

 Pre-shock creep has been observed in 

 laboratory experiments on fracture 

 and has been reported by Japanese 

 seismologists prior to some Japanese 

 earthquakes. 



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