The idea of California dramatically sinking into the Pacific Ocean is a persistent myth. The reality is more nuanced than a catastrophic drop into the sea, as the state is not plunging downward in a single, massive tectonic event. California’s relationship with the ocean is complex, involving horizontal tectonic shifts, a vertical rise in sea level, and localized sinking in certain inland areas. These simultaneous actions create the relative effect of the ocean encroaching on the coast.
Tectonic Movement: Sliding, Not Submerging
California’s coastline is defined by the boundary between the Pacific Plate and the North American Plate. This boundary is the San Andreas Fault system, a transform fault that accommodates horizontal, side-by-side motion, known as strike-slip movement. The Pacific Plate, which includes the westernmost sliver of California, is slowly grinding northwestward past the North American Plate.
This horizontal sliding is the defining motion of the region, occurring at an average rate of about 46 millimeters (1.8 inches) per year. This movement is not a vertical sinking that would submerge the land. If current rates continue, the landmass containing Los Angeles will eventually slide past the landmass containing San Francisco in approximately 20 million years.
This type of transform boundary differs fundamentally from a subduction zone, which is where one plate is forced underneath another, often leading to vertical land changes. The San Andreas Fault’s motion is purely shear, meaning the plates are tearing past each other, not one sinking beneath the other. The tectonic forces driving California’s major plate boundary are sliding the land northwest, not pulling it down into the ocean basin.
The Impact of Rising Global Sea Levels
The most significant factor making the ocean appear to be advancing on California is eustatic sea-level rise. This worldwide phenomenon is independent of the state’s local land movement and is caused by two main processes related to global climate change. The first is the thermal expansion of seawater, where the ocean absorbs excess heat and the water molecules spread out, increasing the total volume of the ocean basin.
The second and increasingly dominant factor is the addition of water volume from the melting of land-based ice. This includes mountain glaciers and the vast continental ice sheets of Greenland and Antarctica. Between 1993 and 2022, about 60 percent of the global sea-level rise came from this added mass of meltwater.
The combination of warmer, expanding water and added meltwater creates a measurable rise in the global average sea level, occurring at an accelerating rate. This results in a higher baseline for the ocean, causing it to encroach on low-lying coastal areas. This phenomenon creates a relative sea-level rise effect where the land seems to be sinking, even if the crust is stable.
Localized Sinking: Understanding Land Subsidence
While the entire state is not tectonically submerging, certain large regions are experiencing localized vertical sinking, known as land subsidence. This is not a natural tectonic process, but a consequence of human activity, primarily the withdrawal of groundwater. The Central Valley, a vast and agriculturally productive area, has been particularly affected.
When water is pumped out of underground aquifers faster than it can be replenished, the clay and silt layers that held the water compact under the weight of the overlying earth. This compaction is often irreversible, causing the land surface to gradually sink. In parts of the Central Valley, the land has sunk by as much as 28 feet since the 1920s.
This localized sinking has profound consequences for infrastructure, including damaging roads and the massive canals of the State Water Project. The subsidence has already reduced the water-carrying capacity of some major aqueducts. This vertical land collapse also exacerbates the effect of global sea-level rise in low-lying coastal regions, such as the Sacramento-San Joaquin River Delta, increasing the risk of flooding.