July 19, 1906] 



NA TURE 



285 



and Geodetic Survey and the Weather Bureau, the lines 

 of research of which, existing or proposed, do not seem 

 wholly distinct from those indicated in the programme. 

 Even European magneticians might feel some slight un- 

 easiness lest a department of so novel a kind, and display- 

 ing such unusual readiness to "pay the piper," might not 

 display a corresponding proclivity to " call the tune.' 

 One can thus understand the occasion for the director's 

 assurance that the general policy of the department is 

 " not to supplant any existing organisation ... but rather 

 to supplement and ' to cooperate in the most effective 

 manner." 



REPORT UPON THE CALIFORNIAN 

 EARTHQUAKE OF APRIL i8. 

 A PRELIMINARY report of the commission appointed 

 "■ by the Governor of California on April 21 to obtain 

 information concerning the earthquake of April 18 has 

 reached us. The commission includes Prof. A. C. Lawson, 

 State University of California, chairman ; Prof. G. K. 

 Gilbert, U.S. Geological Survey ; Prof. Fielding Reid, 

 Johns Hopkins University; Prof. J. C. Branner, Stanford 

 University; Profs. A. O. Leuschner and George Davidson, 

 State University ; Prof. C. Burkhalter, Chabot Observatory ; 

 and Prof. W. W. Campbell, director of the Lick Observ- 

 atory. 



The scope of the work of the commission in its pre- 

 liminary stages embraced the questions as to the origin, 

 position, and character of the disturbance in the earth's 

 crust which gave rise to the Californian earthquake. 

 Having decided upon the scope of its work, the commission 

 appointed three subcommittees to deal respectively with 

 isoseismals, coseismals, and the geophysics of the earth- 

 quake. The cooperation of the San Francisco branch of the 

 American Association of Civil Engineers was secured, and 

 the work of its members greatly simplified the collection 

 of data. 



The committee on coseismals, being concerned with the 

 records of times at which the earthquake was felt, had to 

 depend largely upon correspondence for information, and 

 for times automatically registered the committee is in- 

 debted to seismologists in many countries. Numerous other 

 observations were supplied by officers in the various public 

 services. The committee on isoseismals has also received 

 assistance from many sources. 



Subjoined is a summary of the chief results obtained up 

 to the present. 



One of the remarkable features of the Coast Ranges of 

 California is a line of peculiar geomorphic expression which 

 extends obliquely across the entire width of the moun- 

 tainous belt from Mendocino County to Riverside County. 

 The peculiarity of the surface features along this line lies 

 in the fact that they are not due, as nearly all the other 

 features of the mountains are, to atmospheric and stream 

 erosion of the uplifted mass which constitutes the moun- 

 tains, but have been formed by a dislocation of the earth's 

 crust, or rather a series of such dislocations, in time past, 

 with a differential movement of the parts on either side of 

 the plane of rupture. In general, this line follows a system 

 ■ of long, narrow valleys, or where it passes through wide 

 valleys it lies close to the base of the confining hills, and 

 these have a very straight trend ; in some places, however, 

 it passes over mountain ridges, usually, at the divide 

 separating the ends of two valleys ; it even in some cases 

 goes over a spur or shoulder of a mountain. Along this 

 line are very commonly found abrupt changes in the normal 

 slope of the valley sides giving rise to what are technically 

 known as scarps. These scarps have the appearance of 

 low, precipitous walls, which have been usually softened 

 and rounded somewhat by the action of the weather. 

 Small basins or ponds, many having no outlet, and some 

 containing saline water, are of fairly frequent occurrence, 

 and they usually lie at the base of the small scarps. 

 Trough-like depressions also occur, bounded on both sides 

 by scarps. These troughs and basins can only be explained 

 as due to an actual subsidence of the ground, to an 

 uplift of the ground on one side or the other, or on both 

 sides. The scarps similarly can only be ascribed to a 



NO. IQI6, VOL 74] 



rupture of the earth with a relative vertical displacement 

 along the rupture plane. Frequently small knolls or sharp 

 little ridges are found to characterise this line, and these 

 are bounded on one side by a softened scarp and separated 

 from the normal slope of the valley side by a line of de- 

 pression. In many cases these features have been so 

 modified and toned down by atmospheric attack that only 

 the expert eye can recognise their abnormal character ; but 

 where their line traverses the more desert .parts of the 

 coast range, as, for example, in the Carissa Plains, they 

 are well known to the people of the country, and the 

 aggregate of the features is commonly referred to as the 

 " earthquake crack." 



This line, which can be traced from Point Arena to 

 Mount Pinos, in Ventura County, has a length of 375 

 miles, is remarkably straight, and cuts obliquely across the 

 entire breadth of the Coast Ranges. To the south of 

 Mount Pinos the line either bends to the eastward, follow- 

 ing the general curvature of the ranges, or is paralleled by 

 a similar line offset from it en echelon ; for similar features 

 are reported at the Tejon Pass, and traceable thence, 

 though less continuously, across the Mojave Desert to 

 Cajon Pass and beyond this to San Jacinto and the south- 

 east border of the Colorado Desert. The probability is 

 that there are two such lines, and that the main line 

 traced from Point Arena to Mount Pinos is continued with 

 the same general straight trend past San Fernando and 

 along the base of the remarkably even fault scarp at the 

 foot of which lies Lake Elsinore. But, leaving the southern 

 extension of the line out of consideration as somewhat 

 debatable, we have a very remarkable physiographic line 

 extending from Point Arena to Mount Pinos which affords 

 everv 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 rupture. The move- 

 ments 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 occurred in quite recent times, as is 

 indicated by the minor and still undegraded scarps. Prob- 

 ably every movement on this line produced an earthquake, 

 the severity of which was proportionate to the amount of 

 movement. 



The cause of these movements in general terms is that 

 stresses are generated in the earth's crust which accumulate 

 until they exceed the strength of the rocks composing the 

 crust, aiid 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 cementation that may have been effected in the intervals 

 between movements. 



The earthquake of .April 18 was due to one of these 

 movements. The extent of the rift upon which the move- 

 ment of that date took place is at the time of writing not 

 fully known. It is, however, known from direct field 

 observations 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 Ferndale, 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 off-shore 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 300 

 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 north-east, 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 1S5 miles of this rift 

 where movement has actually been observed, the displace- 

 ment has been chiefly horizontal on a nearly vertical plane, 

 and the country to the south-west of the rift has moved 

 north-westerly relatively to the country on the north-east 

 of the rift. By this it is not intended to imply that the 

 north-east 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 



