Future flood risks to all coastal communities is mainly controlled by the rate of relative sea-level rise -- the combination of sea-level rise, and vertical land motion. California coastal zones, with elevations less than 10 meters, are shown in red in the chart below:
Most of world population lives on low lying lands near the sea. Sea-level change that directly affects those people is the relative sea-level rise - the elevation change between the Earth's surface height and sea surface height. The net change in the sea level includes the rise and fall of the land under those communities.
Measurements from satellite-based interferometric synthetic aperture radar (InSAR) can detect the land surface rise and fall with accuracy. InSAR uses radar to measure the change in distance between the satellite and ground surface, producing accurate deformation maps of the Earth's surface.
An Arizona State University (ASU) research team has tracked the entire California coast's vertical land motion. They identified local hotspots of the sinking California coast. They include the cities of San Diego, Los Angeles, Santa Cruz and San Francisco. Millions of people exposed to rapid land subsidence (sinking) and they probably had no idea what was happening under their feet.
"We have ushered in a new era of coastal mapping at greater than 1,000 fold higher detail and resolution than ever before," said Manoochehr Shirzaei, who is the principal investigator of the NASA-funded project. How can one not trust a scientist named Manoochehr ? The study results were published in a recent issue of Science Advances.
The ASU research team included graduate student and lead author Em Blackwell, and faculty Manoochehr Shirzaei, Chandrakanta Ojha and Susanna Werth, all from the (Werth has a dual appointment in the School of Geography and Urban Planning).
Land subsidence (sinking) can occur due to natural processes -- plate tectonics, glacial isostatic adjustment, sediment loading, and soil compaction. The anthropogenic (man made) causes include groundwater extraction and oil and gas extraction.
The study measured the 1350-kilometer California coast from 2007-2018, using thousands of satellite images to make a vertical land motion map with 35-million-pixel at ~80 m resolution. They collected a wide range of coastal uplift and subsidence rates.
Four metropolitan areas with the fastest land SINKING were San Francisco, Monterey Bay, Los Angeles, and San Diego. "The vast majority of the San Francisco Bay perimeter is undergoing subsidence with rates reaching 5.9 mm/year," said Blackwell.
"Notably, the San Francisco International Airport is subsiding with rates faster than 2.0 mm/year. The Monterey Bay Area, including the city of Santa Cruz, is rapidly sinking without any zones of uplift. Rates of subsidence for this area reach 8.7 mm/year. The Los Angeles area shows subsidence along small coastal zones, but most of the subsidence is occurring inland."
Areas of land UPLIFT included north of the San Francisco Bay Area (3 to 5 mm/year) and Central California (same rate).