318 



Figure 241. Dynamometer records of 50-cm waves at 

 Zuma Beach. Solid line shows measurements about 0.6 

 meter below the surface; dashed line those 1.5 meters 

 below the surface a few hours later. From Lindsay 

 (1957fl). 



fronting a harbor is that of subsidence at 

 Los Angeles-Long Beach (Gilluly and 

 Grant, 1949; Grant, 1954; Jones, 1955). 

 Beginning about 1937 an elliptical area of 

 Terminal Island and adjacent mainland 

 began to sink. By 1941 subsidence in the 

 middle of the area had reached 1.2 feet; by 

 1947, 6 feet; by 1951, 16 feet; and by 1958, 

 25 feet (Fig. 242). According to Jones the 

 ultimate subsidence may reach 35 feet un- 

 less water injection is practiced. The an- 

 nual rate attained a peak of 2.5 feet in 1951, 

 decreasing thereafter to about half in 1958. 

 Accompanying the vertical movements are 

 radially inward horizontal displacements 

 amounting to as much as 5 feet. Also as- 

 sociated with the subsidence are local earth- 

 quakes and shear along small faults, one of 

 which in 1947 resulted in destruction or 

 damage to about 100 oil wells. The con- 

 tours of subsidence correspond closely to 

 the outline of the Wilmington Oil Field. 

 As of August 1958 the total crude-oil pro- 

 duction of the field (the fourth largest of the 

 United States) amounted to about 750 mil- 

 lion barrels, or about 4.2 billion cubic feet 

 (0.12 billion cu meters). The volume of 

 subsidence is less well known, owing to in- 

 complete data on the position and nature of 

 the outer margin of the affected area. How- 

 ever, a reasonable extrapolation of the areas 

 of the various contours of Figure 242 yields 

 a figure of subsidence of about 5 billion 

 cubic feet (0.14 billion cu meters), not far 

 from the volume of oil produced. The sim- 



Economic Aspects ^ 



ilarity of shape, volume, and time of subsi- 

 dence to oil production leads naturally to 

 the belief that the two are related. Prob- 

 ably the subsidence resulted from compac- 

 tion of shales interbedded with the thick 

 sequence of oil sands when fluid pressure 

 was decreased during recovery of oil from 

 the field. Repressuring with water injection 

 wells will reduce the rate of subsidence and 

 the ultimate amount of subsidence as well 

 as increase the ultimate amount of oil to be 

 recovered, but political and ownership prob- 

 lems conspired against effective repressur- 

 ing until late 1958. The effect of subsidence 

 reduces the amount of dredging needed to 

 keep ship channels clear, but this advantage 

 is more than offset by serious results of sub- 

 sidence. Bridges have had to be jacked up 

 and straightened, dikes have been built 

 around low-lying areas that would have be- 

 come submerged, buildings have been 

 damaged and probably some will require 

 hfting, and a large navy drydock is in danger 

 of being flooded according to press releases. 

 Although the value of the oil produced is 

 great, the ultimate costs of combating ef- 

 fects of subsidence are also great. 



Figure 242. Contours of subsidence (2-foot interval) in 

 area of Wilmington Oil Field for period between 1928 

 and 1958. Redrawn from survey of August 1958 by Har- 

 bor Department. City of Long Beach. 



