Chatpter 8

Question 1

Subsidence and Collapse

Answer 1

-Downward settling or sinking of earth surface
-Destructive, BUT very slowly like landslide
-YET costs US some 10s of millions of $/ year

Question 2

Causes of Subsidence: name both

Answer 2

Natural OR - Human induced

Question 3

Causes of Subsidence: Natural

Answer 3

Natural: Earthquakes, volcanic Activities, Dissolution of CaCO3, dolomites, salts

-Earthquakes cause subsidence by displacement along faults

-Ground shaking to cause liquifaction of soil e.g., MISSISSIPPI R. Valley

-Volcanic Activities cause subsidence by emptying magma chambers
-Dissolution of CaCO3 results in Caves and Caverns that may collapse to depressions =Sink holes

Question 4

Causes of Subsidence :Human Induced Subsidence

Answer 4

-From disturbance of environment by humans
e.g: -Groundwater pumping

-Extraction of oil and gas
-Costly/Expensive
-Cost US 10s of millions $/year
-Change drainage patterns previously in place to protect people, plants, animals

Question 5

Classification of Subsidence: Name Both

Answer 5

-DEEP SUBSIDENCE: at considerable depth
-SHALLOW SUBSIDENCE: close to ground surface

Question 6

Classification of Subsidence: Shallow Subsidence First

Answer 6

Collapsing Soil OR Hydro-compaction:
-Due to heavy application of irrigation of H2O on loose, dry soils
-Caused by rearrangement of soil grains

Question 7

Classification of Subsidence: Shallow Subsidence Second

Answer 7

Sinkhole Collapse:
-Soil and rocks at surface collapse into caverns formed from
dissolution of CaCO3, dolomite, rock salt, gypsum
-Usually accompanied by disappearing streams, lakes, caverns
– results in KARST TERRAINES

Question 8

Classification of Subsidence: Shallow Subsidence Third

Answer 8

Shallow Mines:
-Due to mining, i.e., removal of solid from subsurface + resultant
collapse due to weight of overburden

Question 9

Major Subsidence/Collapse Events

Answer 9

Many homes in Florida suddenly disappeared
- results of SINKHOLES
-Failure of Baldwin Hill Dam
-due to withdrawal of oil + non-tectonic movement
along faults
-Subsidence in Mexico City
-due to withdrawal of ground water from ancient lake
sediments.

Question 10

MITIGATION

Answer 10

-Difficult because possible areas cannot all be predicted in advance
-We learn more about collapse/subsidence from occurrences

Question 11

Best Strategy for Migration

Answer 11

-BEST STRATEGY:
- Restrict human activities in susceptible areas????

• limit withdrawals
  -back injection of fluids to replace
  -use surface water as much as possible
  -use pillars to support mine openings