THE SAN FEAXCISCO EARTHQUAKE 



103 



The San Francisco earthquake was caused by a new slipping on the 

 plane of an old fault which had been recognized for a long distance in 

 California, and in one place had been named the San Andreas fault. 

 Associated with this fault is a belt of peculiar topography, differing 

 from the ordinary topographic expression of the country in that many 

 of its features are directly due to dislocation, instead of being the 

 product of erosion by rains and streams. One of its characteristics is 

 the frequent occurrence of long lines of very straight cliffs. Another 

 is the frequent occurrence of ponds or lakes in straight rows. The 

 tendency of erosion is to break up such cliffs into series of spurs and 

 valleys and to obliterate the lakes by cutting down their outlets or 

 filling their basins with sediment. Fig. 2 shows one of the fault-made 

 ponds. This line and zone have been recognized by California geol- 

 ogists through a distance of several 

 hundred miles. It was to this line 

 that attention and expectation were 

 especially directed, and it was on 

 this line that the surface evidence 

 of new faulting was actually found. 

 The new movement was not coex- 

 tensive with the line as previously 

 traced, but affected only the north- 

 western portion; and, on the other 

 hand, it extended farther to the north- 

 west and north than the old line 

 had previously been recognized. The 

 accompanying map represents only 

 the line along which the recent 

 change occurred. From a point a 

 few miles southwest of Hollister it 

 runs northwestward in a series of 



valleys between low mountain ridges to the Mussel Rock, ten miles 

 south of the Golden Gate. Thence northwestward and northward it 

 follows the general coast line, alternately traversing land and water. 

 The farthest point as yet definitely located is at Point Delgada, but the 

 intensity of the shock at the towns of Petrolia and Ferndale probably 

 indicates the close proximity of the fault and warrants the statement 

 that its full length is not less than three hundred miles. South of 

 Point Arena its course is direct, with only gentle flexure, but the data 

 farther north seem to imply either branching or strong inflexion. 

 Opposite San Francisco its position is several miles west of the coast 

 line, and it nowhere touches a large town. 



That which occurred along this line was a differential movement 

 and permanent displacement of the rock and earth on the two sides of a 



Fig. 4. Diagrams Illustrating the 

 Dislocation Causing the San Fran- 

 cisco Earthquake. The upper repre- 

 sents an earth block 100 feet square and 

 25 feet thick, with indication of the posi- 

 tion of the fracture. The lower shows the 

 relation of its two parts after faulting. 



