Juke 29, 1906.] 



SCIENCE. 



963 



the same general straight trend past San 

 Fernando and along the base of the remark- 

 ably even fault scarp at the foot of which 

 lies Lake Elsinore. But, leaving the south- 

 ern extension of the line out of considera- 

 tion as somewhat debatable, we have a very 

 remarkable physiographic line extending 

 from Point Arena to Mount Pinos which 

 affords every evidence of having been in 

 past time a rift, or line of dislocation, of 

 the earth's crust and of recurrent differ- 

 ential movement along the plane of rup- 

 ture. The movements which have taken 

 place along this line extend far back into 

 the Quaternary period, as indicated by the 

 major, well degraded fault scarps and their 

 associated valleys; but they have also oc- 

 curred in quite recent times, as is indicated 

 by the minor and still undegraded scarps. 

 Probably every movement on this line pro- 

 duced an earthquake, the severity of which 

 was proportionate to the amount of move- 

 ment. 



The cause of these movements in general 

 terms is that stresses are generated in the 

 earth's crust which accumulate till they 

 exceed the strength of the rocks composing 

 the crust and they find a relief in a sudden 

 rupture. This establishes the plane of 

 dislocation in the first instance, and in 

 future movements the stresses have only to 

 accumulate to the point of overcoming the 

 friction on that plane and any cementa- 

 tion that may have been effected in the 

 intervals between movements. 



The earthquake of the eighteenth of 

 April, 1906, was due to one of these move- 

 ments. The extent of the rift upon which 

 the movement of that date took place is at 

 ■ the time of writing not fully known. It is, 

 however, known from direct field observa- 

 tions that it extends certainly from the 

 mouth of Alder Creek near Point Arena to 

 the vicinity of San Juan in San Benito 

 County, a distance of about 185 miles. 

 The destruction at Petrolia and Perndale 



in Humboldt County indicates that the 

 movement on the rift extended at least as 

 far as Cape Mendocino, though whether the 

 rift lies inland or offshore remains as yet 

 a matter of inquiry. Adding the inferred 

 extension of the movement to its observed 

 extent gives us a total length of about three 

 hundred miles. The general trend of this 

 line is about N. 35° W., but in Sonoma 

 and Mendocino counties it appears to have 

 a slight concavity to the northeast, and if 

 this curvature be maintained in its path 

 beneath the waters of the Pacific it would 

 pass very close to and possibly inside of 

 Capes Gordo and Mendocino. Along the 

 185 miles of this rift where movement has 

 actually been observed the displacement has 

 been chiefly horizontal on a nearly vertical 

 plane, and the country to the southwest of 

 the rift has moved northwesterly relatively 

 to the country on the northeast of the rift. 

 By this it is not intended to imply that the 

 northeast side was passive and the south- 

 west side active in the movement. Most 

 probably the two sides moved in opposite 

 directions. The evidence of the rupture 

 and of the differential movement along the 

 line of rift is very clear and unequivocal. 

 The surface soil presents a continuous fur- 

 row generally several feet wide with trans- 

 verse cracks which show very plainly the 

 effort of tortion within the zone of the 

 movement. All fences, roads, stream 

 courses, pipe lines, dams, conduits and 

 property lines which cross the rift are dis- 

 located. The amount of dislocation varies. 

 In several instances observed it does not ex- 

 ceed six feet. A more common measure- 

 ment is eight to ten feet. In some cases 

 as much as fifteen or sixteen feet of hori- 

 zontal displacement has been observed, 

 while in one case a roadway was found to 

 have been differentially moved twenty feet. 

 Probably the mean value for the amount 

 of horizontal displacement along the rift 

 line is about ten feet and the variations 



