I. Introduction

• A. Subsidence is widespread in California
  
  • 1. 30 feet of subsidence has occurred in the San Joaquin Valley in 30 years
    
• B. Subsidence is human caused in 37 states
  
• C. Subsidence affects 15,000 sq. miles of the United States
  
• D. The cost nation wide is $100 million/year
  
• E. The world wide economic impact is astounding

II. Causes And Occurrences Of Land Subsidence

• A. Tectonic Activity
  
  • 1. Easily recognized in coastal areas - gradual inundation
    
  • 2. Vertical offset in Owens Valley 1872 Earthquake was 23'
    
  • 3. Surveys in the LA area began in the 1930's and reveal that sedimentary basins are going down and hills are going up
    
    • a. San Fernando Valley -0.045 feet/yr
      
    • b. Santa Monica Mountains +0.013 feet/yr
      
    • c. San Gabriel Mountains +0.02 feet/yr
      
  • 4. Tectonic subsidence is generally slow compared to other causes
    
• B. Dewatering Sediments
  
  • 1. Occurs where water table is lowered due to drainage or pumping
    
  • 2. Peats have been drained in the Sacramento - San Joaquin Delta area since 1850
    
    • a. 12 to 14' subsidence
      
    • b. This is a problem when the areas are close to sea level
      
  • 3. Solution: reduce pumping rate to equal recharge rate
    
• C. Cave Collapse / Sinkholes
  
  • 1. This is often caused by lowering of the water table and reduction in buoyant support of the rocks above the cave.
    
  • 2. Occurs in regions underlain by carbonate rocks
    
    • a. Limestone, dolomite, and marble
      
    • b. All contain CaCO3 which is easily dissolved in water
      
  • 3. Examples:
    
    • a. Winter Park, Florida 1981:
      
      • 1) Cause: karst topography
        
      • 2) 100 m depression 13 m deep
        
      • 3) $2 million in damage.
        
    • b. Hershey, Pennsylvania in 1949
      
      • 1) Cause: Dewatering at a limestone quarry created a cone of depression in water table
        
      • 2) 100 sinkholes appeared in an area over 26 km2
        
      • 3) 2-7 m in diameter, 8 m deep.
        
      • 4) No new sinkholes after pumping stopped
        
  • c. Near Johannesburg, S. Africa
    
    • 1) Cause: Dewatering of a gold mine
      
    • 2) 1962 - 1966: water table lowered 450 m
      
    • 3) Eight sinkholes > 50 m in diameter x 30 m deep formed in cone of depression
      
  • d. Yucatan Peninsula - The Cenotes of Chichén Itzá
    
    • 1) Stages in the Formation of a Cenote (say-NO-tay)
      
      • a) SOLUTION CAVERN - Naturally acidic groundwater seeping through cracks in the limestone bedrock dissolves areas of softer rock lying beneath the hard surface crust. Over time, this process creates large underground caverns roofed with only a thin layer of surface limestone.
        
      • b) YOUNG CENOTE As erosion continues, this thin roof eventually collapses, leaving an open, water-filled hole.
        
      • c) MATURE CENOTE Over thousands of years, erosion gradually fills the cenote with organic and mineral debris, reducing its depth. The Cenote of Sacrifice is currently in this stage.
        
      • d) DRY CENOTE As erosion continues, the cenote may completely fill, becoming a dry, shallow basin supporting trees and other vegetation.
        
    • 2) Sacred Cenote
      
      • a) Used for ritual offerings
        
• D. Application of Water to Soils of Low Water Content
  
  • 1. Irrigation water applied to low density alluvial sediments may cause subsidence
    
  • 2. This is most spectacular on the west side of the San Joaquin Valley
    
    • a. Test plots subside 10.5 feet in 27 months
      
      • 1) Flat surfaces become very irregular
        
    • b. Extensive damage to structures occurs
      
      • 1) P. Gas & Electric transmission lines
        
      • 2) Gas and oil pipelines
        
      • 3) Drainage ditches
        
      • 4) Freeway requires special protection
        
  • 3. Solution: Use sprinklers to get more even irrigation
    
• E. Pressure Decline in Artesian Aquifers and Oil and Gas Fields
  
  • 1. Occurs in partially consolidated sedimentary deposits
    
    • a. Fluids are removed and overburden support decreases
      
  • 2. As much as 1 foot of subsidence for each 10 - 25 feet of water level decline
    
    • a. -2.8 feet in Lancaster between 1926 & 1965
      
  • 3. Solutions:
    
    • a. Reduction of pumping rate to equal recharge rate
      
    • b. Injection of water in oil and gas fields or even artesian aquifers
      
      • 1) Costly
        

I. Introduction | II. Causes And Occurrences Of Land Subsidence | III. Long Beach Subsidence | Links | top | Geol 357 Lecture Home

III. Long Beach Subsidence - A Special Case History

• A. General Character of Subsidence
  
  • 1. It greatly affected the L.B. port and naval shipyard
    
    • a. Subsidence exceeded 29 feet
      
    • b. Also included 10 feet of horizontal shift
      
    • c. Caused very extensive damage
      
  • 2. Significant subsidence began with oil field development in 1938 & 1939
    
    • a. East end of Terminal Island had subsided 4 feet by 1945
      
  • 3. Extensive diking and reconstruction has been necessary
    
    • a. Cost exceeded $100,000,000
      
• B. Postulated Causes
  
  • 1. Oil reservoir compaction due to oil field pumping
    
  • 2. Surface loading by buildings
    
  • 3. Vibrations due to land use
    
  • 4. Tectonic movement
    
• C. Mechanics of Movement
  
  • 1. Subsidence produced compression along axis of a bowl-shaped depression
    
    • a. Tension occurred on the flanks
      
  • 2. Sudden slippages on clay beds 1750' deep set off small earthquakes
    
    • a. Oil wells were badly damaged
      
    • b. 5 such earthquakes occurred
      
• D. Subsidence Arrest
  
  • 1. The field was repressured with water injection
    
    • a. Subsidence decreased drastically
      
      • 1) Some areas even rebounded
        
  • 2. Bench marks are now constantly watched
    

I. Introduction | II. Causes And Occurrences Of Land Subsidence | III. Long Beach Subsidence | Links | top | Geol 357 Lecture Home