Deep-ocean sediments chronicle thousands of years of Cascadia megathrust earthquakes

Powerful megathrust earthquakes along the Cascadia subduction zone have repeatedly destabilized the seafloor at the edge of North America, generating widespread submarine landslides whose deposits form a continuous long-term record of past seismic events.

A detailed analysis of abyssal sediments offshore northern California and southern Oregon shows that these deep-sea deposits directly record earthquake rupture, slope failure, and deformation over the past 7 500 years.

The study shows that many Cascadia paleoseismic records do not originate from sediment transported through submarine canyons, as previously assumed. They form locally at the base of the continental slope, where repeated uplift and steepening of the accretionary wedge prepare the seafloor for failure during each major earthquake cycle.

The results revise existing interpretations of marine turbidites, earthquake recurrence estimates, and the identification of shallow megathrust rupture in the geologic record.

The discovery focuses on the abyssal plain seaward of the deformation front, at depths of about 2 650–3 090 m (8 700–10 140 feet). High-resolution mapping identifies a narrow, strongly deformed zone where thrust faults uplift sediment into asymmetrical folds up to 100 m (330 feet) high. On the seaward faces of these folds, slopes often exceed 10°, above the threshold at which unconsolidated marine sediments become prone to failure during strong shaking.

Using autonomous underwater vehicles, remotely operated vehicles, and ship-based coring, researchers mapped seafloor features at resolutions of about 1 m (3.3 feet), nearly ten times finer than standard shipboard bathymetry. The data revealed extensive landslide scarps and stacked mass-transport deposits that are absent in coarser datasets but represent repeated, margin-scale processes.

At the base of the slope, at least five distinct mass transport deposits were identified, some thicker than 11 m (36 feet) and extending more than 700 m (2 300 feet) across the abyssal plain. These chaotic, block-rich units record large submarine landslides sourced directly from the seaward limbs of thrust folds at the deformation front. Seaward, the deposits thin and transition smoothly into laterally continuous turbidites, only a few centimeters to a few tens of centimeters thick, laid down by turbidity currents generated during the same earthquakes.

Sediment cores penetrating these layers reveal sharp basal contacts and normally graded sequences that fine upward from very fine sand into silt and clay. Many event beds consist of closely spaced doublets or triplets rather than a single layer, indicating that multiple slope failures were triggered nearly simultaneously during individual earthquakes. These composite deposits represent cascading failures from different parts of the accretionary wedge rather than a single point source.

Radiocarbon dating of foraminifera beneath individual turbidites enabled the construction of Bayesian age–depth models covering most of the Holocene. The chronology records at least 10 full-margin megathrust earthquakes and up to 12 smaller events that affected only parts of the margin during the past 7 500 years.

Several earthquakes proposed in earlier compilations do not appear in this record, while others shift in timing, showing how sensitive recurrence estimates are to assumptions about sediment sources and correlation.

The most recent, well-constrained events align with known margin-wide ruptures, but some older deposits previously interpreted as full-margin earthquakes appear to represent smaller, localized events. Earlier reconstructions likely overestimated rupture uniformity due to an incomplete understanding of sediment sources.

The study identifies a bottom-up sediment recycling system at the deformation front. As subduction continues, abyssal plain sediments are uplifted and folded into the accretionary wedge. Continued convergence steepens these folds until their seaward faces fail during strong shaking. The displaced material spreads across the abyssal plain as mass-transport deposits and turbidity currents, forming new layers later uplifted and recycled.

This conveyor-like process maintains a continuous sediment supply for turbidite formation, even in areas without river input or submarine canyons. It explains why abyssal plain sites, often secondary in past studies, can preserve long, undisturbed earthquake records with minimal influence from storms or bottom currents.

The regular and widespread nature of these landslides clarifies earthquake-related deformation processes. If the base of the slope deformed mainly through aseismic creep, failures would occur irregularly under local sedimentation or pore pressure changes. The observed pattern instead indicates repeated coseismic deformation of the outer accretionary wedge during megathrust ruptures reaching shallow depths near the trench.

Comparable behavior has been recorded during recent great earthquakes elsewhere, where large shallow slip caused significant seafloor displacement and extensive submarine landsliding. In Cascadia, the distribution of lower-slope failures and related turbidites may indicate shallow slip, a parameter essential for estimating tsunami potential.

The study reveals limitations in traditional marine paleoseismic methods. Techniques that correlate turbidites across canyon networks or classify distinctive sedimentary layers assume simple sediment pathways. The Cascadia record instead represents a complex system with multiple distributed sources, strong local geomorphic control, and deposits that vary over short distances. In such settings, the absence of a turbidite at one site does not mean that strong shaking did not occur.

By linking seafloor deformation, landslide sources, and abyssal turbidites, the research establishes a physical basis for interpreting deep-sea earthquake records. The same framework can be applied to other subduction zones, including regions once considered unsuitable for marine paleoseismic studies.

The deep ocean floor offshore the Pacific Northwest preserves both a chronological record of Cascadia earthquakes and evidence of how the margin deforms and renews during each seismic cycle. The results support the reliability of abyssal turbidites as indicators of great earthquakes and expand understanding of fault behavior along the margin.

References:

¹ Widespread abyssal turbidites record megathrust earthquake-triggered landslides and coseismic deformation in the Cascadia subduction zone – Jenna C. Hill et al. – Science Advances – January 13, 2026 – DOI: 10.1126/sciadv.adx6028 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

This author does not have any more posts.