80 



Structure 



on land by detailed mapping in the Santa 

 Monica Mountains (Durrell, 1954) and else- 

 where. At the present stage of information 

 little further speculation about the kind, 

 direction, and amount of movement based 

 on topography alone seems warranted. As 

 age and facies distribution data accumulate 

 and their areas of distribution become bet- 

 ter known, more offsets will probably be- 

 come apparent. 



Seismicity 



Earthquakes were first noted in southern 

 California by the Portola expedition which 

 experienced four violent ones on July 28, 

 1769, near the Santa Ana River and many 

 more the following week in the Los Angeles 

 area (Bancroft, 1884, p. 146; Bolton, 1927). 

 Earthquakes felt during the following 160 

 years are listed by Wood, Allen, and Heck 

 (1934) and by Townley and Allen (1939). 

 Recent destructive shocks in the area of 

 Chart I occurred on June 29, 1925, at Santa 

 Barbara and on March 10, 1933, at Long 

 Beach. Because of the great seismic activity 

 of the region a network of seismograph sta- 

 tions was begun in 1926 by the Carnegie 

 Institution of Washington and California 

 Institute of Technology. At present six sta- 

 tions in southern California are located be- 

 tween Santa Barbara and La Jolla, but three 

 of these are concentrated east of Los 

 Angeles. 



Using data from the seismograph stations. 

 Wood (1947) showed a good correlation of 

 earthquake epicenters with known major 

 faults in California, and in a similar study 

 Clements and Emery (1947) tried to correlate 

 off'shore epicenters with inferred geological 

 structures. During the 10 years since 1947 

 additional earthquake data have accumu- 

 lated. Between January 1, 1934, and Janu- 

 ary 1, 1958, there occurred within the area 

 of Chart I a total of 404 earthquakes of 

 magnitude 3 or greater on the Richter scale 

 (Gutenberg and Richter, 1942), having epi- 

 centers of y4 or B quality (geographical posi- 

 tion known to within 5 km — 3.1 statute 

 miles). The elimination of low-magnitude 



shocks, detectable chiefly only when the epi- 

 center is near a seismograph, is intended to 

 help show the true geographical density of 

 epicenters through the whole continental 

 borderland (Fig. 69). The restriction to A 

 and B quality insures enough accuracy of 

 position to reveal possible relationships to 

 known topographic features. Shocks occur- 

 ring within a month and at the same locality 

 as an earlier one were arbitrarily considered 

 aftershocks and were not counted or plotted. 

 Because of these restrictions the epicenters 

 chosen are considered highly reliable. 



Most epicenters are concentrated in the 

 land region near Los Angeles, doubtlessly 

 more because of greater seismic activity in 

 this region than simply because of nearness 

 to seismographs. Correlation of epicenters 

 with known faults is not obvious, perhaps 

 because of the great number and close spac- 

 ing of large faults; in addition, the average 

 focal depth of 18 km (Gutenberg and Rich- 

 ter, 1949, pp. 33, 89) can result in a wide 

 geographic separation of an epicenter and 

 the surface trace of the fault on which move- 

 ment took place. In the off"shore area epi- 

 centers are too sparse to permit detailed 

 identification with geological structures; 

 nevertheless, it is notable that the greatest 

 concentration occurs near San Clemente Is- 

 land whose northeast scarp is one of the 

 steepest and straightest of the continental 

 borderland. In addition, most epicenters lie 

 within basins, suggesting that the planes of 

 the bounding faults dip basinward. Con- 

 trary to the situation along most parts of the 

 continental slope of North and South Amer- 

 ica (Gutenberg and Richter, 1949, pp. 31, 

 35, 41), no epicenters of A or B quality and 

 of magnitude greater than 3 have been found 

 along the continental slope off" southern Cali- 

 fornia since 1934. However, on November 

 4, 1927, one of intensity X occurred on the 

 continental slope at latitude 34° 32', longitude 

 12r24', just beyond the boundary of Chart 

 I off' Point Conception. The scarcity of reg- 

 istered epicenters along the continental slope 

 may reflect to a certain extent the great dis- 

 tance from seismographs, but it also appears 

 to indicate a truly lower degree of seismic 

 activity there than near Los Angeles. 



