of the ground due to natural
causes. Examples are as follows:
Subsidence due to groundwater withdrawl. Las Vegas, with its increasing population, has been lowering elevation ever since 1935. Las Vegas gets only 20% of its water supply from groundwater, but the water table has lowered up to 90 m in some parts of the valley Las Vegas lies in. With the loss of pore water pressure, the silt and fine grain sands begin to settle or fill in the pore spaces that the water previously occupied. This causes stress on the pre-existing faults causing more cracks to occur. Consequently, areas of Las Vegas have subsided up to 2 m since 1935.
Other examples of subsidence by groundwater withdrawl:
- Mexico City subsided up to 7.5 m between 1891 and 1959. The opera house (which weighs 54,000 metric tons) has settled more than 3 m, half of its first floor is now below ground level.
- New Orleans, Louisiana is now 4 m below sea level due to groundwater pumping.
Subsidence due to mining.This is easy to imagine. Large man made holes in the ground (mostly abandoned) with a few columns to hold up the immense weight of the rock above. Eventually, mines collapse leading to the settling of the earth above.
Subsidence due to tectonics. earthquake that occur along normal fault cause both uplift and subsidence. The areas that subside are called graben or a downthrown crustal block of land bordered by areas of uplift or horsts. An example of this occurred during an earthquake near Prince William Sound in Alaska on March 28, 1964. The town of Portage, which lies on the shores of the Cook Inlet, subsided 1.8 meters in a matter of seconds during the earthquake. The lowering of elevation inundated the town during high tide.
- Subisidence due to withdrawl of other subsurface fluids. Long Beach, California has subsided 9 m as a result of 40 years of oil removal resulting in millions of dollars worth of damage to buildings, wells, pipelines and other structures.