island, which occurred on 21 February 1973, had a magnitude of 5.7, and was 

 located near Oxnard about 31 miles from the island, and the closest earth- 

 quake to the island since its construction, which occurred on 14 July 1958, 

 had a magnitude of 4.7, and was 2.8 miles west. Because both these events 

 were felt at the island, they were of special interest. Careful examina- 

 tion of the island immediately after each quake revealed no detectable 

 damage. Each examination also included a recheck of the level check points 

 on tlie island. 



A current estimate of the island's threshold of damage from liquefac- 

 tion of the core is that damage would occur from a magnitude 6.5 earthquake 

 within 10 miles of the island, a magnitude 7.0 earthquake within 20 miles 

 of the island, or a magnitude 8.0 earthquake within 80 miles of the island. 

 Figure 16 shows the three fault systems of primary concern to the island. 

 Because of the correlation between California earthquakes and the length 

 of the associated fault system, earthquakes on the Santa Inez or Malibu 

 faults would not exceed a magnitude of 7.0 to 7.5, and the nearest earth- 

 quakes of greater magnitude will not occur closer than the longer San 

 Andreas fault, about 45 miles away from the island. Using the same data 

 plotted in Figure 16, it is estimated that there is a 3 percent probability 

 in a 50-year period that an earthquake within 10 miles of the island will 

 have a magnitude of 6.5, a 5 percent probability in a 50-year period that 

 an earthquake within 20 to 30 miles of the island will have a magnitude of 

 7.0 to 7.5, and a 3 percent probability in a 50-year period that an earth- 

 quake within 45 to 80 miles of the island will have a magnitude greater 

 than 8.0. Combining these probabilities indicates an 11 percent chance of 

 an earthquake causing some liquefaction of the island's core within a 50- 

 year period. 



The island's construction of rock rings containing a sand core makes 

 a prediction of the damage resulting from liquefaction of the core a diffi- 

 cult problem. Uneven settlements of the island work surface are the most 

 likely result of core liquefaction, but a maximum possible earthquake on 

 the San Andreas fault at its closest location to the island could cause more 

 serious damage by inducing partial collapse of the upper part of the revet- 

 ment. 



Earthquake engineering is a rapidly improving technology. Design of a 

 similar island today would include a more refined analysis of the core lique- 

 faction problem, and would probably specify either vibratory compaction of 

 the underwater position of the core fill or a single height ring of rock 

 which would remain stable irrespective of core liquefaction, thus ensuring 

 reduced damage should this occur. 



III. RECOMMENDED ADDITIONAL DATA COLLECTION 



1. Wave Gages 



The design of Rincon Island was influenced botli by considerations of 

 the probable effect of the environment on the island and the probable impact 

 of the island on the environment. One major environmental influence was 



54 



