Deep Sea Landslides Unlock Secrets of Pacific Northwest Megaquakes

Breakthrough Mapping Reveals Hidden Earthquake History

Scientists have discovered a revolutionary way to decode the timing of massive earthquakes along the Pacific Coast by studying underwater landslides buried deep beneath the ocean floor. Using autonomous underwater drones and tethered robots, researchers created high-definition maps of the seafloor off Crescent City, California, revealing that turbidites can be reliably linked to earthquake-triggered landslides, not storms.

Marine turbidites are layers of mud and sand deposited on the deep ocean floor by massive underwater landslides and are often used as a historical record for reconstructing earthquake histories. However, they can be unreliable because it is difficult to show they were not triggered by a storm or flood rather than a quake. This uncertainty has long plagued earthquake researchers attempting to understand when the next "Big One" might strike.

In a new study published in the journal Science Advances, researchers detail a new way to link these mud layers to the specific landslides that caused them. This could mean much more accurate earthquake timelines.

Deep Ocean Holds the Key

The breakthrough came from studying sediment deposits in the deepest parts of the ocean, far from coastal influences. Because these hills are located in the deep ocean, surface storms have no effect. Therefore, the researchers can now have much greater confidence that when this mud slides, the cause is an earthquake, not the weather.

These deposits, known as abyssal turbidites, form when earthquakes destabilize sediment on the continental slope, sending landslides downslope to settle on the abyssal ocean floor. The study identifies the lower continental slope as the primary and recurring source of these earthquake-triggered sediments. As tectonic forces steadily deform the plate margin, thrust folds grow and become oversteepened, recycling the old seafloor and creating a perpetual supply of unstable material.

The researchers found evidence of at least 10 events in the past 7,500 years, which enabled them to link historical quakes, landslides and resulting turbidites. "We are able to clarify how and where the turbidites are generated," Hill told Live Science. "So we know they're coming from landslides that we know are triggered by earthquakes."

Timing the Next Catastrophic Quake

Using this method, the team was able to create a precise timeline of massive earthquakes that occurred across the entire 600-mile stretch of coastline from Northern California up to Canada. Their findings confirmed that these giant quakes occur on average every 500 years. Since the last big one happened in the year 1700, they now have a much clearer window for when the next one might hit the Pacific Northwest.

The Cascadia subduction zone — which extends from northern California to Vancouver Island, British Columbia — is capable of quakes of at least magnitude 9.0, according to the Pacific Northwest Seismic Network. It's not clear how large a quake has to be to trigger deep-sea turbidites, Hill said, but it probably has to be large enough to cause damage. She and her colleagues also saw signs of seafloor shaking corresponding with the earthquake turbidites, which could additionally raise the risk of tsunamis from this type of quake.

Global Implications for Earthquake Prediction

As they state in their study, this new approach may change the way we study ocean history: "Our results present a paradigm for marine turbidite paleoseismology and have important implications for site selection and seismoturbidite interpretations along subduction zones globally."

The lead researcher believes these findings extend far beyond the Pacific Northwest: "We think they're happening most everywhere along subduction zones," she said, "so we should be able to find these landslide deposits and marine turbidites globally in places where we have never looked for them before." This discovery could transform earthquake hazard assessment worldwide, offering coastal communities better tools to prepare for the inevitable return of nature's most destructive forces.