Lecture 29 Subsidence Types

Subsidence Less obvious than catastrophic sinkhole formation Develops gradually and is typically widespread You can’t tell by looking that the San Joaquin Valley floor was once 35 ft higher in some locations Mapping is critical: InSAR (inteferometric synthetic aperture radar) uses repeat-pass radar images from Earth-orbiting satellites to measure subsidence at sub-centimeter resolution

Ground subsidence Can occur owing to the compaction of material as Water is drained from the soil, often caused by a reliance on groundwater supplies. Organic material decays Oil is removed

In the arid southwest, a visible sign may be fissuring of the earth.

Groundwater pumping and land subsidence

Amount of subsidence West of Phoenix: 18 feet Las Vegas: 6 feet Albuquerque: 1 foot California Lancaster: 6 feet Davis: 4 feet Santa Clara Valley: 12 feet Houston, TX: 9 feet

San Joaquin Valley

The future? Increasing population in the southwest means more groundwater pumping, and continued subsidence, possibly spreading to metropolitan areas where damage will be great.

Well head protrusion in Las Vegas.

Subsidence…free home remodeling. Las Vegas

Some of the most spectacular examples of subsidence-related earth fissures occur in south-central Arizona.

Subsidence in the LA Basin? Causes Tectonic activity Water and oil withdrawal Examples include Pomona (water); Beverly Hills (oil) San Pedro and Long Beach (probably oil)

Groundwater withdrawal, Lancaster

Texas: oil related subsidence. Bay waters intruded the oil field.

Historic Houston subsidence Historic Houston subsidence. Over 3,200 mi2 of Houston has sunk at least one foot. The Houston ship canal has sunk 10 ft. Cause: groundwater withdrawal.

Drainage of organic soils Occurs when soils rich in organic carbon are drained for agriculture or other purposes. The most important cause is microbial decomposition, which, under drained conditions, readily converts organic carbon to carbon-dioxide gas and water. Compaction, desiccation, erosion by wind and water, and prescribed or accidental burning can also be significant factors.

Two important areas of subsidence: Sacramento-San Joaquin Delta Everglades Continuing organic-soil subsidence threatens agriculture Affects engineering infrastructure and water transfers Complicates ecosystem restoration Threatens the integrity of levees

Santa Clara Valley

Expansive soils Soils which shrink in dry periods and swell during wet periods. Causes differential movement which damages foundations. Clay-rich

Expansive Soils

Expansive Soils Check the soil survey for your area.

Tectonic Subsidence Land subsidence due to earthquakes Fault movement Crustal deformation

Tectonic Subsidence Not to scale! Subsided region can be ~ 100 miles wide!

Permafrost Melting of frozen soils Common at higher latitudes Why not lower latitudes? Frost heaving (opposite of subsidence) also a hazard

Permafrost

Permafrost

Permafrost Extent

Permafrost

Permafrost

Permafrost Mine in permafrost

Permafrost

Deflation of Magma or Lava Reservoirs Lava tubes may collapse much like caves Common in Hawaii ”Caldera collapse”

Toba Indonesia

Lava Tube Collapse

Lava Tube Collapse

Links to other Natural Hazards Earthquakes How? Volcanoes Floods

Benefits Karst provides unique and seldomly explored frontier Caves contain many rare endemic species Troglobites New sinkholes create new lakes

Sinkhole lakes

Florida has most sinkholes in nation

Caves One of the last frontiers for exploration Many provide fascinating irreplaceable features and rare species Kentucky New Mexico Sierra Nevada – Sequoia/Kings Canyon NP Hurricane Crawl Cave Lilburn Cave Ursa Minor Cave Crystal Caverns

Hurricane Crawl Cave One of Park’s most decorated

Lilburn Cave Park’s longest (~20 miles)

Crystal Cave Tourist Cave

Ursa Minor Cave Park’s newest (non-alpine)

Mineral King Best alpine karst in nation Hundreds of sinkholes ~130 cave entrances Numerous springs and disappearing streams Not fully mapped until last summer

Troglobites Cave-adapted species