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The Tracy Stockton Fault traverses Middle Roberts island in a north-easterly direction. Although the fault has shown no surface movement, history of the area indicates that the fault may indeed be active. Three earthquakes of magnitude approximately 5+ on the Richter Scale have been recorded near the fault zone. Experts believe that these quakes were generated by this fault. Major damage from earthquakes results from groundshaking. The Delta is generally in a zone with moderate shaking. According to the Modified Mercalli Scale, damage to structures would be slight although levees to several islands would be subject to failure. While no failures have been recorded as a result of earthquakes, several old or poorly constructed levees probably are vulnerable. In addition, earthquake induced seiches, or oscillations of the water surface, also could develop in the network of sloughs or river channels during a major earthquake, causing overtopping of the levees. 7. Fish and Wildlife: The Delta serves as habitat for a variety of animals. Today, the Delta contains : - 5 species of anadromous fish -10 species of resident freshwater fish - 2 species of shellfish - 4 species of migratory waterfowl - 5 species of aquatic birds - 4 species of raptors - Hundreds of species of other non-aquatic birds, mostly migrant (songbirds) -39 species of mammals -19 species of reptiles - 8 species of amphibians Table 1 includes a list of these species. Seven endangered or rare species are included in this list. Preserving the fish and wildlife is equivalent to preserving the habitat. The decline in some species is directly due to the loss of marshland that existed in the Delta before the widespread construction of levees. Since then, these marshlands have been con- verted into agricultural lands and some species, such as migratory waterfowl, have adapted to this change while others have not. For example, species such as elk and antelope have been eliminated. - 9 -