Some of the most powerful landslides on Earth happen where nobody sees them: underwater. Entire mountains of sediment can detach from continental margins, canyon walls, or volcanic island flanks — and when that mass moves fast enough, it can shove water into waves.
This is why “mystery tsunami” headlines appear even when there’s no obvious megaquake on the news: submarine landslides and seafloor collapse can generate tsunamis, amplify earthquake tsunamis, or create confusing wave patterns that don’t match the usual tectonic script.
This guide is built as an evergreen explanation and a 301 sink for short-lived “underwater collapse” stories. It covers: what underwater slope failures are, where they cluster, what triggers them (earthquakes, storm loading, rapid sediment buildup, weak layers, overpressure), how to tell landslide tsunamis from weather-driven lookalikes, and why methane hydrates are often blamed incorrectly.
TL;DR — Submarine Landslides in 60 Seconds
- Submarine landslides are underwater slope failures that can move huge volumes of sediment.
- They can trigger tsunamis or amplify earthquake tsunamis — sometimes creating “mystery” wave events.
- Common triggers include earthquakes, rapid sediment loading, storm-wave pressure pumping, and weak layers in the seabed.
- Methane hydrates are often blamed, but they are rarely the simple “ocean bomb” mechanism people claim.
- Underwater slope failures are real — but most viral claims exaggerate scale, frequency, and global risk.
What Are Submarine Landslides?
A submarine landslide is the downslope movement of sediment, rock, or debris on the seafloor. Instead of a hillside failing into a valley, you get:
- continental margins failing into deep ocean basins
- submarine canyon walls collapsing
- delta fronts slumping after floods
- volcanic island flanks shedding material into the sea
These events range from slow slumps to fast, long-runout flows that travel for tens to hundreds of kilometers. In the real world, a single “event” can evolve: a slump becomes a debris flow, which transforms into a turbidity current.
Why They Matter: The Tsunami Wildcard
Tsunamis are usually associated with subduction-zone earthquakes. But a tsunami can also be generated when a large mass moves water quickly — which is exactly what a fast submarine landslide can do.
Key difference: earthquake tsunamis are driven by seafloor displacement from fault rupture. Landslide tsunamis are driven by mass movement (a moving pile of sediment/rock pushing water).
Why “mystery tsunamis” happen:
- a moderate quake triggers a big underwater slump
- a slump occurs without a felt quake (storm loading + weak layers)
- a local landslide tsunami arrives fast and hits hard near the source
Want the big map of where earthquake-driven tsunami risk clusters? Start here: Global Earthquake & Volcanic Zones and the Pacific Ring of Fire.
Where Submarine Landslides Happen Most
Underwater slope failure clusters in predictable places:
- Subduction margins: thick sediments + frequent quakes
- River deltas: rapid sediment loading (delta-front failure)
- Submarine canyons: steep walls that fail repeatedly
- Volcanic island flanks: unstable piles of lava and debris
- Glaciated margins: large sediment packages left by ice ages
Translation: if a place has steep slopes, fast sediment buildup, weak layers, or regular shaking — the seafloor is not “stable.” It’s just underwater.
Major Types: Slumps, Slides, Debris Flows & Turbidity Currents
The vocabulary looks complicated, but the physics is simple: material fails, moves downslope, and may turn into a fast, watery flow.
1) Slumps (rotational failures)
- Blocks of sediment rotate and move downslope.
- Can be slow (creep) or sudden if destabilized.
2) Slides (translational failures)
- Material detaches along a weak layer and slides.
- Can generate tsunamis if motion is rapid and volume is high.
3) Debris flows
- Chaotic mixture of sediment, rock, and water moving as a dense mass.
- Can travel far across the seafloor and bury infrastructure.
4) Turbidity currents
- Fast-moving, sediment-laden currents (underwater “avalanches”).
- They can break seafloor cables and carve submarine channels.
- They may or may not generate significant tsunamis depending on how the mass moves water.
If you want the on-land version of these mechanics (and warning signs you can actually see), use: Landslides & Mudslides (Surface Failures).
What Triggers Underwater Slope Failure?
Most submarine landslides require two ingredients: unstable sediment + a trigger.
- Earthquake shaking: reduces sediment strength; triggers failure on steep slopes (especially at plate boundaries)
- Rapid sediment loading: floods/deltas pile material faster than it can stabilize
- Storm-wave loading: long-period waves can “pump” pressure into shallow seabeds
- Weak layers: clay-rich horizons or organic-rich layers act like slip planes
- Overpressure: trapped fluids in sediments reduce friction and stability
- Volcanic island growth: unstable piles build outward until parts fail
Reality check: most slides are not “one cause.” They are preconditioned over time, then triggered by a final push.
And when a headline screams “GAS CAUSED IT” — pause and run the fundamentals:
Methane Seeps & Hydrates Explained.
Earthquakes vs Landslide Tsunamis (Why Size Isn’t Everything)
People often ask: “How can a small earthquake cause a big tsunami?”
Because the quake might be the trigger — but the tsunami is produced by the landslide volume and speed. A moderate quake can unleash a very large slump if the slope is already primed.
Clue pattern (common in landslide tsunami scenarios):
- Big local waves, fast arrival times, and highly localized damage
- Earthquake magnitude that seems “too small” for the reported waves
- Complex wave behavior not consistent with a single fault-rupture source
How Big Does a Submarine Landslide Need to Be to Make a Tsunami?
There isn’t a single magic number, because tsunami size depends on how the mass moves water, not just how “big” the slide is in cubic kilometers.
The tsunami controls that matter most:
- Speed / acceleration: fast failures push water efficiently; slow creep usually doesn’t.
- Water depth at the source: shallow-water failures can couple energy into the water column more efficiently (and hit coasts faster).
- Slide geometry: thick, coherent blocks can act like a moving piston; thin drapes may not.
- Directionality: landslide tsunamis can “beam” energy — one coastline gets wrecked, others barely notice.
- Distance to coast: landslide tsunamis are often most dangerous near-source, with very short warning times.
Practical takeaway: a modest quake + a primed slope can outperform a larger quake in terms of local wave impact — because the landslide becomes the real wave engine.
Landslide Tsunami vs Meteotsunami vs Storm Surge (How to Tell)
Not every “sudden water surge” is a tectonic tsunami. Three common impostors are meteotsunamis, storm surge, and seiches.
| Wave type | Main driver | Typical clue pattern | Best “first check” |
|---|---|---|---|
| Earthquake tsunami | Fault rupture displaces seafloor | Regional-scale signals; multiple coasts; clear seismic trigger | Seismic + tsunami center bulletins |
| Landslide tsunami | Mass movement (submarine slump/slide) | Very fast local arrival; uneven damage; quake magnitude may look “too small” | Local tide gauges + nearby seismicity + bathymetry context |
| Meteotsunami | Air-pressure/wind disturbances from fast-moving weather | Coincides with squall lines/fronts; amplifies in bays/harbors | Weather radar + pressure jumps + harbor geometry |
| Storm surge | Sustained winds + low pressure pile water up | Rises with storm; hours-long flooding; wind-driven | Cyclone / wind warnings + tide forecasts |
| Seiche | Standing-wave slosh in enclosed waters | Rhythmic back-and-forth oscillations; harbors/lakes | Local gauge pattern (repeating oscillation) |
StrangeSounds shortcut: if the “tsunami” happens during a fast weather front and is strongest inside one harbor/bay, meteotsunami is often a better first hypothesis than “secret megaquake.”
Methane Hydrates: What’s Real vs Hype
Methane hydrates (gas hydrates) are ice-like crystals that trap methane in seafloor sediments under high pressure and low temperature.
What’s real: hydrates can influence sediment strength and fluid pressure in some settings — and they’re part of the methane cycle. Hydrate-rich zones can sit near pathways where fluids migrate, which matters for overpressure and seepage.
What’s exaggerated: the idea that hydrates form a simple “bomb layer” that detonates and explodes the ocean. Hydrate destabilization is usually tied to temperature/pressure changes and fluid migration over time. Even when hydrate systems shift, the dominant hazards are typically slope stability, seepage, and sediment failure mechanics — not instant Hollywood chaos.
Deep dive here: Methane Seeps & Hydrates Explained.
Seafloor Scars, Canyons & “Mystery Craters”
Modern seafloor mapping reveals dramatic features that look like impacts or “engineering”:
- headwalls and slide scarps (the detachment zone)
- chaotic debris fields (the runout)
- submarine canyon channels carved by turbidity currents
- pockmarks from fluid escape (gas/water venting)
StrangeSounds reality check: many “mystery seafloor craters” are simply pockmarks (fluid escape) or collapse features in soft sediment — not impacts or ancient structures.
Famous Examples (What They Taught Us)
These anchor events are why “no megaquake headline” doesn’t automatically mean “no tsunami mechanism.” (Also: they make this pillar linkable, which is how you win.)
1929 — Grand Banks (Newfoundland): quake-triggered failure + turbidity current clues
Why it matters: the sequential breaking of undersea cables became a famous “fingerprint” showing how fast and far underwater sediment flows can run.
~8150 years BP — Storegga Slide (Norwegian margin): paleotsunami scale
Why it matters: a deep-time reminder that giant slides exist — but the biggest ones typically live on geological timescales, not doom-week timelines.
1998 — Papua New Guinea: near-source devastation + complex source interpretation
Why it matters: widely discussed because observed waves pushed investigators to consider offshore mass movement as a major contributor (not a simple “earthquake-only” story).
Reality calibrator — Lituya Bay (1958): mega-wave ≠ global tsunami
Why it matters: landslide geometry + confinement can produce terrifying local water heights without becoming a globe-crossing tsunami.
Volcanic Island Flank Collapse (Canary Islands & Beyond)
Volcanic islands can build steep, unstable flanks. Over time, parts of the edifice may fail into the sea as:
- sector collapses
- debris avalanches
- giant submarine landslides
These events are real — but the internet often inflates them into immediate “megatsunami tomorrow” scenarios.
What to do with Canary Islands fearbait:
- Keep it evidence-based and time-scale aware
- Separate “can happen geologically” from “imminent this week” claims
- Anchor readers in monitoring signals and official hazard assessments
Related context: Canary Islands — Volcanoes & Ocean Hazards.
Warning Signs & Monitoring (What Agencies Actually Watch)
Submarine landslides are hard to monitor directly because they occur underwater. But agencies and researchers track:
- Earthquakes (especially near margins and deltas)
- Tsunami gauges and tide stations (wave arrival patterns)
- Seafloor pressure sensors (where deployed)
- Repeat bathymetry (mapping changes over time)
- Subsea cable breaks (often caused by turbidity currents)
Practical reality check: coastal hazard planning focuses on tsunami readiness rather than predicting the exact date of an underwater slide. Near-source landslide tsunamis can arrive quickly — preparedness beats prophecy.
Myth vs Reality (Fearmongering Decoder)
Myth: “A mystery tsunami means a secret megaquake.”
Reality: landslide tsunamis can be triggered by smaller quakes, storms, or slope failure without a major rupture signature. Also consider meteotsunamis, storm surge, and seiche effects (see comparison above).
Myth: “Methane hydrates will explode the ocean.”
Reality: hydrate dynamics are complex and usually slow. The main concerns are seepage, slope stability, and long-term climate interactions — not instant detonation. Full myth-busting: Methane Seeps & Hydrates Explained.
Myth: “Canary Islands flank collapse = inevitable global megatsunami soon.”
Reality: flank failures exist, but “soon” claims usually lack evidence. Treat doom timelines as content strategy, not geology.
Event Index — Submarine Landslides, Seafloor Collapse & “Mystery Tsunami” Reports (301 Sink)
This is the permanent archive zone. Redirect short-lived “underwater collapse,” “mystery tsunami,” “seafloor crater,” and “submarine landslide” posts here (301), then preserve each as a dated entry with one strong source link.
How to use this section (editor notes)
- Entry format: date — location — suspected mechanism — wave impact — best source.
- Keep entries ~50–120 words (evergreen and scannable).
- If a year exceeds ~40 entries, split into “Submarine Landslide Events by Year” and link it here.
- Keep exactly one best source per entry (agency / peer-reviewed / database).
2023
- 2023-10 — Japan: Unusual tsunami waves recorded without a major earthquake. Initially framed online as “mystery tsunami,” later evidence points to rapid submarine caldera deformation — a mechanism related to submarine landslides and volcanic collapse rather than megathrust rupture. Full case study.
Older years (archive)
1998
- 1998-07-17 — Papua New Guinea (Offshore slump / complex source): Near-source tsunami impacts along the north coast triggered deep investigation into whether an offshore slump contributed significantly to wave heights. Why it belongs here: a “not-just-earthquake” interpretation became central. Source: Peer-reviewed paper documenting the 17 July 1998 Papua New Guinea tsunami and the submarine slump mechanism. Download PDF source: Papua New Guinea Tsunami (17 July 1998) — Tappin et al. (2008)
1929
- 1929-11-18 — Grand Banks, Newfoundland (Quake-triggered submarine landslide / turbidity current): A major earthquake triggered submarine slope failure; the resulting turbidity current was inferred from sequential undersea cable breaks and remains a cornerstone example of underwater mass movement behavior. Source: NOAA.
Prehistoric / geological timescale
- ~8150 years BP — Storegga Slide (Norwegian margin): One of the largest known submarine slide complexes; tsunami deposits make it a deep-time reference point for what giant underwater failures can do. Source: Nature
Sources & Further Reading (for the skeptical and the doom-resistant)
- NOAA explainer (meteotsunamis): What is a meteotsunami?
- NOAA explainer (tsunamis): The science behind tsunamis
- General background (submarine landslides): Submarine landslide overview (use as a basic starting point, not as your final authority)
Frequently Asked Questions
Can underwater landslides cause tsunamis?
Yes. A fast, large submarine landslide can displace water and generate a tsunami, and landslides can also amplify earthquake-generated tsunamis.
Why do “mystery tsunamis” happen without big earthquakes?
Because a submarine landslide can be triggered by a moderate quake, storm-wave loading, or preconditioned slope failure — producing waves that don’t match the typical earthquake tsunami pattern. Also consider meteotsunamis, storm surge, and seiche effects.
How big does a submarine landslide need to be to generate a tsunami?
It depends on speed, water depth, geometry, directionality, and distance to coast. Fast near-shore failures can produce severe local waves, while slower or deeper failures may produce little surface impact.
Are methane hydrates a major tsunami trigger?
Hydrates can influence sediment stability in some settings, but the popular “hydrate bomb” story is often exaggerated. Slope stability depends on many factors, including weak layers, fluid overpressure, and sediment loading. Full context: Methane Seeps & Hydrates Explained.
What’s the difference between a submarine landslide and a turbidity current?
A submarine landslide is the failure and movement of sediment/rock downslope. A turbidity current is a fast, sediment-laden flow that can travel long distances and carve channels; it may be triggered by a slide or occur as part of the same event sequence.
What’s the difference between a landslide tsunami and a meteotsunami?
Landslide tsunamis are generated by rapid mass movement that displaces water. Meteotsunamis are driven by fast-moving weather/pressure disturbances and are often amplified in bays, harbors, and shallow shelves.
Should coastal areas worry about volcanic island flank collapse?
Volcanic flank collapses are real geological processes, but “imminent megatsunami” claims are usually unsupported. Coastal resilience planning focuses on general tsunami readiness rather than predicting a specific collapse date.
