The Land Under These CA Cities Is Sinking; Here's Why

CALIFORNIA — The land under big California cities is slowly sinking, threatening buildings, roads and other infrastructure, according to a study published in the journal Nature Cities.

The study looked at the phenomenon known as land subsidence, or the gradual sinking of Earth’s surface, in the nation’s 28 largest cities, including Los Angeles, San Diego, San Francisco and San Jose.

All exhibited some level of subsidence, with two-thirds of them seeing sinking across three-fourths of their land. About 34 million people — 10% of the U.S. population — live in areas experiencing subsidence, and 29,000 buildings are at risk.

Subsidence is often thought of as a coastal problem caused by rising sea levels, but this study shows it’s a problem for inland cities as well, threatening both the stability of buildings and other infrastructure, but also worsening flooding during storms. Among interior cities with at least 98% of their land subject to subsidence are Chicago; Dallas; Columbus, Ohio; Detroit; New York; Indianapolis; Charlotte; Denver; Houston; and Fort Worth.

On average, 25 out of the 28 cities are experiencing sinking at varying rates, according to the study. San Diego is sinking by about 1.1 millimeters per year, San Francisco is sinking by about 0.9 millimeters per year, and Los Angeles is sinking by about 0.9 millimeters per year.

San Jose was an exception. The city is rising by about 0.2 millimeters per year, according to the study.

The causes differ from city to city, but pumping groundwater from aquifers to supply public drinking water systems and agricultural needs is the dominant cause of subsidence in 80% of them, according to the authors. Natural causes are glacial isostatic adjustments — that is, the effects of melting from the last Ice Age — and the interaction of Earth’s tectonic plates, which causes earthquakes and volcanic eruptions.

The authors noted their findings underscore the need for cities to incorporate subsidence data in infrastructure planning and to adopt sustainable groundwater extraction practices. Satellite-based monitoring can also aid in early detection of subsidence and the development of proactive mitigation strategies in rapidly growing cities, the researchers said.

To be sure, the change is gradual, measured in millimeters per year, but the effects can accumulate over time, the authors noted. Also, some cities, notably Houston. Dallas and Fort Worth have subsidence rates exceeding 4 millimeters per year, with some areas surpassing 10 millimeters a year.