Parts of the Los Angeles region are gradually sinking, and that slow descent can compound the flood threat posed by rising seas along the Southern California coast. A NASA-led study mapped vertical land motion across the California coastline, identifying areas where the ground is dropping by measurable amounts each year and showing that local land motion can meaningfully affect relative sea-level rise estimates. The findings underscore why flood planning for low-lying neighborhoods needs to account for both ocean rise and land subsidence.
Satellite Radar Reveals a Sinking Coastline
The NASA study used a remote sensing technique called interferometric synthetic aperture radar, or InSAR, to track where California’s land surface is rising and where it is falling. By comparing repeated radar passes from orbiting satellites, researchers produced detailed maps showing areas of uplift and subsidence at fine spatial scales. The technique detects changes as small as a few millimeters, which means even subtle sinking that would escape ground-level observation can be quantified from orbit.
This matters because subsidence does not need to be dramatic to be dangerous. A patch of coastline dropping just a few millimeters per year may seem trivial in isolation, but over a decade that motion adds centimeters to the effective sea level experienced by roads, storm drains, and building foundations. The NASA-led research found that local land motion can materially change relative sea-level projections and increase uncertainty in those projections. Put plainly, planners who ignore subsidence may be underestimating how soon floodwaters will reach a given neighborhood.
Why the Ground Is Dropping
Subsidence along the California coast stems from several overlapping causes. Groundwater extraction is the most widely documented driver: when water is pumped from underground aquifers faster than it recharges, the overlying sediment compacts and the surface drops. Oil and gas production can trigger a similar effect. The U.S. Geological Survey describes subsidence as a measured geophysical process tied to these human activities, not a speculative risk, and explains why it threatens infrastructure and water management systems alike.
USGS researchers have been applying InSAR to Southern California for more than two decades. A foundational USGS fact sheet published in 2003 detailed how radar interferograms translate into measured deformation and included examples from the Los Angeles and Santa Ana Basin region, establishing an early evidence base that human-induced sinking was already detectable in the metro area. The current NASA study builds on that lineage with far more data and wider coverage.
Palos Verdes: A Visible Warning
While basin-wide subsidence is slow and largely invisible, the Palos Verdes Peninsula on the southwestern edge of Los Angeles County offers a stark, visible example of how quickly the ground can move. NASA’s Jet Propulsion Laboratory used its airborne UAVSAR instrument to measure rapid displacement on the peninsula of up to approximately 4 inches, or 10 centimeters, per week over a four-week period in fall 2024. That rate far exceeds typical subsidence, because the Palos Verdes motion involves an active landslide complex rather than gradual compaction.
The city of Rancho Palos Verdes has documented accelerating land movement at the Portuguese Bend Landslide complex since spring 2023, with movement rates referenced in feet per year, emergency declarations, road damage, and service disruptions. Landslide motion and basin subsidence are distinct processes, but they share a common lesson: the ground beneath greater Los Angeles is not static, and assuming otherwise leads to infrastructure failures and emergency costs that could have been anticipated.
How Sinking Rewrites Flood Maps
Most flood planning models start with a projection of how much the ocean will rise and then overlay that projection onto a fixed land surface. The problem, as the NASA study demonstrates, is that the land surface is not fixed. When the ground drops even modestly, the gap between sea level and the built environment shrinks faster than ocean-only models predict. California’s state government has recognized this gap. The California Ocean Protection Council has draft sea-level rise guidance that identifies vertical land motion, including both uplift and subsidence, as a primary driver of local variation in relative sea-level rise across the state.
That policy language carries real planning weight. Coastal communities that rely solely on ocean-rise projections without accounting for subsidence may be designing seawalls, drainage systems, and building codes to a standard that is already outdated by the time construction finishes. For Los Angeles, where low-lying port areas and river corridors sit near sea level, even a small underestimate in effective sea-level rise can translate into substantially higher flood exposure over the coming decades.
New Tools for Tracking the Problem
One reason subsidence has been slow to enter mainstream flood planning is that the data were historically fragmented. Different agencies used different instruments, time windows, and processing methods, making it hard to compare one location to another. That is beginning to change. NASA announced its OPERA displacement product, a ready-to-use dataset that provides consistent measurements of land motion across North America, including coastal California. The product is designed so that non-specialist agencies can download displacement data and incorporate it directly into their risk models without having to process raw radar imagery.
In California, state agencies are also investing in more accessible InSAR resources. A statewide portal hosts subsidence data derived from commercial radar analysis, giving planners and local governments a way to see where the ground has been dropping and how fast. Combined with the NASA coastal land-motion maps, these datasets create an emerging baseline that can be updated as new satellite missions, such as NISAR, come online.
Communicating what these technical products mean for everyday life remains a challenge. NASA has been expanding its public-facing storytelling through the Plus video series, which includes explainers on Earth science topics and visualizations that turn abstract numbers into scenes of changing coasts, cities, and ecosystems. The broader NASA Plus platform is free to access and can help residents, students, and local officials understand why a few millimeters of motion per year matter when stacked on top of sea-level rise and stronger storms.
Implications for Los Angeles and Beyond
For the Los Angeles region, the emerging science paints a picture of uneven risk. Some stretches of coastline and inland basins are relatively stable or even rising slightly, while other pockets are subsiding at rates that could add several inches of effective sea-level rise within a human lifetime. That patchwork means that citywide averages can be misleading: a neighborhood built atop compacting sediments near a tidal channel may face much higher flood risk than a bluff-top community just a few miles away.
Integrating vertical land motion into planning will require close coordination between scientists, engineers, and policymakers. Flood maps, building codes, and infrastructure design standards can be updated to use relative sea-level projections that combine ocean rise with local subsidence rates. Port authorities and transportation agencies can prioritize investments in areas where the ground is sinking fastest, strengthening levees, elevating roadways, and redesigning drainage systems with extra capacity.
Ultimately, the story of a slowly sinking Los Angeles coastline is not just about geophysics; it is about choices. Groundwater management policies influence how much aquifers compact. Land-use decisions shape where homes and critical facilities are built relative to subsiding ground. Climate policy will determine how quickly global sea levels rise. The new radar-based maps do not dictate those choices, but they strip away the illusion that the land itself is fixed. Planning for a moving baseline is harder, but ignoring that motion increases the odds that future flood risk will be underestimated.
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*This article was researched with the help of AI, with human editors creating the final content.
