332 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



destroyed 100 buildings at a cost of $5.5 million and left cracks in 

 practically all masonry structures — cracks now proudly preserved by 

 the owners of surviving houses as a kind of social distinction. The 

 most famous, if not the strongest, of all American quakes was certainly 

 the one that rocked San Francisco on April 18, 1906. Broken water 

 mains contributed to the wide spread of a fire which caused the larger 

 part of the 700-odd casualties and the half-billion dollar damage toll. 

 San Franciscans insist on the distinction — there seems to be a question 

 of civic pride in tliese matters — between the quake and the resulting 

 fire. This is the only one of the three quakes to have a Gutenberg- 

 Richter rating based on modern instrumental readings. It was 8.25, 

 just barely ahead of the Fairweather shock. 



The Gutenberg-Richter scale has no relation to the number of people 

 killed or houses wrecked but there are other types of rating scales that 

 have. Seismologists call them intensity scales; instead of rating the 

 energy of a quake, they furnish an evaluation of its local effects at a 

 given place regardless of its distance from the shock. These scales are 

 concerned with such things as public alarm, the falling of objects, 

 cracking of plaster, destruction of buildings — even cracks in the 

 ground. Such effects excite people, of course, and newspaper stories 

 deal with them pretty much to the exclusion of such prosaic facts as 

 their energy. The intensity scale most used in America is the Modi- 

 fied-Mercalli scale, from a rating system devised by the Italian Mer- 

 calli. Its highest grade is 12, an ominous number that signifies total 

 destruction. Evidence is lacking for the explicit intensity rating of 

 the Fairweather shock; had such ground movements occurred in a 

 built-up locality, however, there can be little doubt that the destruction 

 would have been essentially complete. 



Seismologists point to the fact that neither the New Madrid nor 

 the Charleston earthquake occurred in an area known as dangerous, 

 although nearby parts of South Carolina do, in fact, exhibit occasional 

 minor sliivers. This supports the standard warning of the profession 

 that no place on earth can be said to be really safe from damaging 

 shocks. The 1906 event in California, on the other hand, was in a 

 highly seismic area, directly on the well-known San Andreas Fault, a 

 source of recurring shocks throughout a notorious past, and probably 

 of many more to come. The Coast and Geodetic Survey has long 

 made periodic repetitions of geodetic measurements in the vicinity 

 of the San Andreas, finding slow movements of the land, which are 

 proceeding, in fact, on a truly grand scale — a great region west of the 

 San Andreas is slowly creeping northwestward in relation to the 

 country east of the fault. This distortion amounts to some 2 inches 

 a year, certainly enough in the course of centuries to place great 

 stresses in the crustal rocks. Historical records are too short to 



