on that side of a fault which has been displaced relatively and not absolute- 

 ly upward or downward. 



Geologic faults in Mobile and Baldwin Counties are the result of 

 irregularities in Cretaceous and Tertiary deposits, which are caused by 

 movement of the underlying Louann Salt. In fact, some of the peripheral 

 faults in north Baldwin County may represent salt movements on the very edge 

 of the Mississippi interior salt, dome basin. Salt under great pressure 

 (i.e., at great depths below the surface) behaves as a plastic substance and 

 moves in response to sedimentary loading. Positive features, such as salt 

 swells and domes and collapse-type features such as grabens where salt was 

 displaced and overlying rock receded, are both found in the study area. 

 Positive features in Mobile and Baldwin Counties include the Citronelle and 

 Wiggins uplifts. The major fault features include the Mobile graben and the 

 peripheral faults associated with the Pollard fault zone. 



Although there are several faults in the area, the probability of damage 

 from an earthquake is extremely unlikely. The U.S. Coast and Geodetic Survey 

 ranks all areas of the United States in regard to the possibility of damage 

 from earthquakes on an arbitrary scale of to 3. These rankings are based 

 on the intensity of possible earthquakes and anticipated damage, but not 

 their frequency. These rankings are based on historical data and are revised 

 as damaging earthquakes occur. Coastal Alabama lies in Zone 0: 



Zone - no damage 



Zone 1 - minor damage 



Zone 2 - moderate damage 



Zone 3 - major damage 



Because coastal Alabama has had no damaging earthquakes (as of October 

 1982) since the rankings were first compiled in 1948, its ranking has 

 remained unchanged (Algermissen 1969). 



Although a significant earthquake originating in Alabama is unlikely, 

 there is a slight possibility that powerful, low- frequency tremors 

 originating at a distance (e.g., Louisiana has regular tremors) could cause 

 damage in the study area. Buildings in the Mobile area could suffer 

 li qui faction of their supporting soil, particularly if they are built on 

 fluvial sediments. As most of the structures in the study area are probably 

 reinforced to withstand wind stress from hurricanes, they could probably 

 withstand any likely earthquake tremor (D. Perkins, National Earthquake 

 Information Center, Boulder, CO, 15 September 1982; pers. comm.). 



A complex of peripheral faults running across southwest Alabama includes 

 the Pollard fault zone on the northern edge of Baldwin County. Faults 

 connected to this complex curve through Florida and reappear in southern 

 Baldwin County. The faulting in this complex has resulted in a series of 

 narrow grabens, and subsequent faulting occurred between the two major 

 faults. The northern fault is the dominant one and is downthrown to the 

 southwest. Fault zones can be detected in this area only by subsurface 

 indications (such as well log data and sonic testing) or very subtle surface 

 indications. Vertical displacement of the fault surfaces ranges from 30 to 



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