A5 Subsidence and Differential Collapse

Topic

Subsidence is the sinking of the Earth's surface; it often occurs differentially, with some points sinking more than others within the same region. This subsidence can stem from various mechanisms: the karst dissolution of carbonate and evaporite rocks, which creates underground cavities prone to catastrophic collapse; the extraction of subsurface fluids—such as water, oil, or gas—which compacts the remaining sediment as the fluids are removed; natural compaction in river deltas; the collapse of a volcanic caldera; or tectonic subsidence within a rift zone.

Subsurface fluid extraction causes subsidence proportional to the product of three factors: the compressibility of the aquifer itself, the thickness of the water-saturated layer within that aquifer, and the drop in pore-water pressure as the fluid is extracted. When pore pressure decreases, the effective pressure borne by the sediment increases, causing the sediment to compact. In Mexico City, groundwater extraction caused up to ten meters of subsidence during the twentieth century.

In the Nile and Po deltas, a combination of fluid extraction, natural sediment compaction, and a lack of new sediment input from upstream results in sinking rates of five to fifteen millimeters per year—a process further amplified by the rising sea levels these deltas simultaneously face.

Karst terrain is particularly hazardous due to its sudden and unpredictable nature: underground cavities grow silently as dissolution progresses—without visible surface signs—until the cavity roof eventually collapses abruptly. The safety factor of that roof depends on three inherent factors: the thickness of the roof itself, the radius of the cavity it must span, and the mechanical properties of the rock composing it.

The expected behavior in karst zones is the sudden appearance of collapse sinkholes—ranging from one to a thousand meters in diameter—that emerge without warning, thereby linking the silent dissolution of the subsurface to the sudden surface subsidence with which this description of subsidence and differential collapse began.

ID:910