TAILIEUCHUNG - Marine Geology Phần 7

flats and raise them to the level where vegetation can grow once , repeated earthquakes produce alternating layers of lowland soil and tidal flat mud | MARINE GEOLOgy Figure 142 Submerged coastline north of Portland Maine. Photo by J. R. Balsley courtesy USGS flats and raise them to the level where vegetation can grow once again. Therefore repeated earthquakes produce alternating layers of lowland soil and tidal flat mud. Earthquake-induced subsidence in the United States has occurred mainly in California Alaska and Hawaii. The subsidence results from vertical displacements along faults that can affect broad areas. During the 1964 Good Friday Alaska earthquake more than 70 000 square miles of land tilted downward more than 3 feet causing extensive flooding in coastal areas of southern Alaska. Flow failures usually develop in loose saturated sands and silts. They originate on land and on the seafloor near coastal areas. The Alaskan earthquake produced submarine flow failures that destroyed seaport facilities at Valdez Whittier and Seward. The flow failures also generated large tsunamis that overran coastal areas and caused additional casualties. Some of the most spectacular examples of nonseismic subsidence in the United States are along coasts Fig. 142 .The Houston-Galveston area in Texas has experienced local subsidence of as much as feet and subsidence of 1 190 COASTAL GEOLOgy foot or more over an area of 2 500 miles mostly from the withdrawal of groundwater. In Galveston Bay the ground subsided 3 feet or more over an area of several square miles following oil extraction from the underlying strata. Subsidence in some coastal towns has increased susceptibility to flooding during severe coastal storms. The pumping of large quantities of oil at Long Beach California caused the ground to subside forming a huge bowl up to 25 feet deep over an area of about 20 square miles. In some parts of the oil field land subsided at a rate of 2 feet per year. In the downtown area the subsidence was upward of 6 feet causing severe damage to the city s injection of seawater under high pressure into the .

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