August 6, 1920] 



SCIENCE 



121 



THE RECENT EARTHQUAKES AT LOS 

 ANGELES, CALIFORNIA 



To the late Homer Hamlin more than any 

 one else is due the credit for identifying the 

 connection between certain local structural 

 lines and the earthquakes which have affected 

 the region about Los Angeles, Cal., during the 

 past month. Hamlin's work, to the writer's 

 knowledge, covered a period of over fifteen 

 years prior to his death a few weeks ago. 

 He, single handed, studied the cause of more 

 than twenty earthquakes of varying degrees 

 of intensity which have occurred in southern 

 California during this period. Hamlin's con- 

 clusions, few of which unfortunately ever 

 were put in print, were that the line of struc- 

 tural disturbance along which the epicentrums 

 of most of the earthquakes were located, was 

 that which extends from the Santa Monica 

 Mountains, north of the Soldier's Home 

 (about ten miles northwest of the business 

 district of Los Angeles), in a southeasterly 

 direction through the Baldwin Hills, Domin- 

 guez Hill, El Cerrito (near Long Beach), an^ 

 thence easterly to the San Joaquin Hills 

 northeast of ISTewport. The section along this 

 line which has been the greatest oilender is 

 that extending several miles southeasterly 

 from the Baldwin Hills. From a study of the 

 intensity records, Hamlin was inclined to be- 

 lieve that the actual epicentrums were coin- 

 cident in general with a fault which paral- 

 leled the anticline forming the Baldwin and 

 Dominguez Hills, and extending along the 

 northeast base of these hills. This may be 

 true, but the writer is inclined to the theory 

 that the actual crustal movements which pro- 

 duced the shocks took place along the Bald- 

 win Hills-Dominguez Hill line, and that the 

 maximum surface reaction might have been 

 greater to the east of the hills because of 

 the more unconsolidated character of the sedi- 

 ments in this direction. 



In the shocks that occurred in the middle 

 of Jtme last, the greatest intensity was at 

 Inglewood, a town lying ten miles southwest 

 of Los Angeles, and only a very short distance 

 southwest of the axis of the Baldwin Hills- 

 Dominguez Hills fold. This would confirm 



the theory that the main line of disturbance 

 is along the axis of this fold. In the shocks 

 of July 16, the newspaper reports indicate 

 only slight damage in Inglewood with the 

 principal damage in the city of Los Angeles 

 proper. These reiwrts being true, it seems 

 probable that this last tremor originated along 

 the very pronounced fault, that extends east 

 and west through the northwest residential 

 district of Los Angeles, or along one of the 

 lines of disturbance associated with this fault. 

 It is this fault wliich marks the northern 

 boundary of the oil-producing area of the Los 

 Angeles city field, and is believed to act as a 

 barrier to the northward migration of the oil 

 in the sands on the down-thrown block of 

 Pliocene sediments on the south side of the 

 fault. This fault is part of a zone of dis- 

 turbance which extends eastward past Whit- 

 tier and is responsible for the structurally 

 complex Puenta Hills north and west of 

 Whittier. This last named town is men- 

 tioned in the dispatches as having been sub- 

 jected to sharp shocks on July 16; further 

 evidence of the probability of the cause of 

 this earthquake being in the east-west line of 

 disturbance just described. It would be nat- 

 ural to suppose that a readjustment of stresses 

 along the Baldwin Hills-Dominguez Hills line 

 in the earthquakes in June might develop 

 stresses in the east-west line north of Los 

 Angeles that relieved themselves by move- 

 ments which caused the disturbances of 

 July 16. 



In connection with the earthquake history 

 of the Los Angeles region, attention is called 

 to the very recent earth movements that are 

 recorded in the topography thereabouts. San 

 Pedro Hill, over 1,000 feet in height, which 

 marks the southwest corner of the Los 

 Angeles Plain, has eleven wave cut terraces 

 on its southern or ocean side, all of which 

 are believed to be of Pleistocene age. Beds 

 along the fianks of the Baldwin Hills-Domin- 

 guez Hills-El Cerrito fold, dipping over 30°, 

 are known to be of Pleistocene age. Pleisto- 

 cene fossils are found at a depth of over 1,000 

 feet in a well at Bells Station on the Los 

 Angeles plain south of Los Angeles. At least 



