Earthquakes and shifting tectonic plates factor into coastal erosion along the US West Coast.

The oceans and winds do not bear all of the blame for coastal erosion: tectonic movements also play a powerful role. Researchers analyzed decades of wave, tide and geological data from along the US West Coast and found that the slow rise and sudden fall of land driven by tectonic activity significantly shapes how fast coastal cliffs erode. These findings suggest that current coastal hazard models not accounting for the effects of tectonic movement on cliff erosion may be incomplete. 

More than half of the world’s coastlines are rocky, shaped by tectonic movements (like earthquakes) and wave erosion. Most research has focused on millennia-scale effects, with tectonic activity driving land rising and destructive waves driving erosion. However, the impact of these factors on a shorter, more human timescale of decades to centuries has gone understudied. 

Cesar Lopez and Claire Masteller, researchers at Washington University in St. Louis, USA, studied how geological forces influence coastline erosion over decades along the wave-battered rocky bluffs of the US West Coast. They assessed the individual and joint contributions of tectonic uplift, wave energy, rock strength and tides in coastal erosion and shoreline retreat.

To calculate wave power, the duo used hourly records of wave heights and periods going back 43 years, recorded at 51 virtual buoys within 25 km of the shore, from the USACE Wave Information Study, 2025. The researchers determined shore platform width – the width of the flat rocky shelf left behind by an eroding cliff – using bathymetric maps and specifics on the limits of waves breaking, both towards the coast and towards the sea. To learn how resistant the cliffs were to wave power, they measured rock strength using regional geological maps and laboratory analyses. Data on tidal ranges were gleaned from NOAA Tides and Currents, while the researchers analyzed shoreline measurements using USGS reports from the late 1800s onwards to assess retreat. Furthermore, they took into account uplift rates from three different timescales: millennial from dating marine terraces, decadal from sea level trends from tidal gauges, and daily from GPS stations on the ground.

Lopez and Masteller analyzed all these data using various statistical methods as well as a machine learning model, which assigned each variable an importance score, to identify which variable would more closely predict how quickly the coastline would erode. They found that only 32% depended on millennial-scale and everyday uplift rates as well as rock strength; daily sea level changes, decadal changes in land uplift and wave power accounted for 68% of the model’s ability to explain why some coastlines erode faster than others.

This means that where and how hard waves hit are more significant factors than how hard the rock is, and decadal tectonic movements greatly influence what those waves can reach.

As tectonic plates move, land rises and waves cannot hit as high, slowing the process by which waves cut into the cliff base and erode it landward. A slower-eroding cliff means less new shore platform gets exposed, keeping it narrow. Tectonic movements on longer timescales have the opposite effect: when earthquakes strike, the land dramatically drops, exposing more of the cliff face and rock to wave impact, leading to wider rocky platforms.

These tectonic cycles of land uplift and earthquake drive cycles of less and more coastal erosion.

“Our findings demonstrate that rocky coast evolution along the US West Coast is not shaped by waves and tides alone, but by the repeated rise and fall of the Earth’s surface driven by the earthquake deformation cycle,” write the researchers. “This seismic ‘memory’ highlights tectonics as a governing force that amplifies or damps the effects of marine processes, depending on the timescale.”

Along the West Coast in the US, the population is dense, with homes and infrastructure right on the rocky cliffs. In the Pacific Northwest, the Cascadia Subduction Zone is a major fault system and is expected to produce a major earthquake in the future. Among other impacts, an earthquake of large magnitude would cause land to drop suddenly, exposing the coastline to waves that speed up erosion. Moreover, as sea levels rise, the limited land uplift between earthquakes offers a minimal buffer against coastal erosion.

“Current coastal erosion forecasts rarely account for the geomorphic consequences of rapid land-level change, leaving hazard assessments incomplete,” write the researchers. “Our results provide a process-based framework for linking tectonics, wave dynamics, and shoreline evolution, which should be integrated into future models of rocky coast change and coastal risk.”

Reference: Cesar G. Lopez et al., Tectonics as a Regulator of Shoreline Retreat and Rocky Coast Evolution Across Timescales, AGU Advances (2026). DOI: https://doi.org/10.1029/2025AV002065

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