Another Kind of Depression

For many Americans, the word depression means more than economic or psychological problems. Under their homes or places of business, the ground is literally sinking. Parts of the Houston-Galveston region, for instance, have dropped as much as 8 ft. In California's fertile agricultural valleys, the sink rate has reached 1 ft. per year. Indeed, geological depression is so serious, reports the New York Academy of Sciences, that it has already caused millions of dollars in damage across the U.S. The toll has ranged from broken sewer lines and cracked pavements to an increased incidence of lowland flooding.

Such sinking, called subsidence by geologists, can occur naturally. In river deltas, for example, as muddy sediments pile up, their weight often grows great enough to press down the land beneath them. Subsidence can also take place on a larger scale as a byproduct of the creeping movements of the giant, continent-sized plates that make up the earth's surface. Whatever the cause, natural subsidence is extremely slow and almost imperceptible. It is subsidence caused by humans that is taking place with alarming speed in many parts of the U.S. and elsewhere.

Shrinking Underpinning. As water, oil and gas are pumped in increasing quantities from deeply drilled wells, the upper layers of clay, shale or silt often dry out and contract. The surface of the earth then subsides on its shrinking underpinning. California's San Joaquin Valley, pocked with irrigation wells, has sunk up to 29 ft. since intensive farming began there in the 1940s. Only recently have engineers finally managed to halt the subsidence by piping in water from elsewhere in the state.

Near Los Angeles, a 20-sq.-mi. depression has formed around the Wilmington oilfield after 35 years of exploitation. At the center of the great bowl lies the Long Beach Naval Shipyard, where a 29-ft. decline in the land level has forced the Navy, oil companies and others to build flood-control dikes. Besides twisting railroad tracks, crushing oil-well casings and undermining buildings, the slumping of the ground has also triggered small earthquakes. To jack up the sunken terrain, the city of Long Beach has been forcing water back into the ground.

Even more spectacular collapses have occurred in parts of Alabama, which is crisscrossed by underground limestone caverns.

As the water table lowers, the clay covering over the caverns becomes more compact and weaker; sometimes it collapses completely, creating gaping craters known as sinkholes. In one Birmingham industrial park, more than 200 such collapses have occurred in recent years, turning a half-sq.-mi. area into a facsimile of the lunar landscape. No end to the problem appears to be at hand. As the New York Academy's journal, The Sciences, points out, "With man's seemingly unquenchable thirst for Earth's fluids, the land will continue to sink."

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