Introduction
A submarine earthquake is a seismic rupture that initiates within the seafloor of a body of water—most commonly an ocean—and is the principal source of tsunamis. The energy released by such events is quantified by moment magnitude, while observed effects on people and built systems are described using intensity scales such as the Mercalli scale. Instrumental records from seismometers, together with concepts like epicenter, hypocenter, epicentral distance, and the behavior of seismic body waves (P and S waves), form the basis for measurement and post‑event analysis.
These earthquakes arise from the slow motions of lithospheric plates (the lithosphere averaging roughly 80 km in thickness) as they move over the ductile asthenosphere and deeper mantle. Plate boundaries relevant to submarine seismicity include convergent margins, where subduction produces large interface and megathrust ruptures; transform faults characterized by lateral shear; and, less frequently, divergent spreading centers. At plate contacts, roughness and locked asperities permit elastic strain to accumulate; failure of these locked patches yields rapid slip on faults and rupture propagation along the seafloor. The rupture initiation point (hypocenter) and the surface projection directly above it (epicenter) typically coincide with the zone of maximum slip and local damage.
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Submarine seismic rupture differs from continental earthquakes in its dominant impacts. On land, shaking commonly causes fire, collapse and airborne debris; beneath water, the most consequential effect is alteration of the seabed and the impulsive displacement of overlying water that can generate tsunami waves whose amplitude and runup depend on rupture length, slip magnitude and seafloor geometry. Secondary consequences of undersea earthquakes include damage to submarine infrastructure—most notably the severance of fiber‑optic communications cables—which can disrupt Internet and international telephony. This vulnerability is acute where major cable routes traverse active seismic regions such as the Pacific Ring of Fire.
Seismologically, submarine events belong to a broad taxonomy—mainshocks, foreshocks and aftershocks; interplate and intraplate quakes; blind thrusts, doublets and earthquake swarms; slow and supershear ruptures; and tsunami‑generating or remotely triggered events. The principal causal mechanisms are fault slip (including subduction zone rupture), submarine volcanism and caldera collapse, and anthropogenic or induced seismicity that alters stress conditions. Ongoing prediction and forecasting efforts rely on coordinated observational networks and institutional initiatives aimed at probabilistic hazard assessment.
The study of submarine earthquakes intersects numerous applied and theoretical disciplines: seismic wave anisotropy and shear‑wave splitting inform stress and fabric in source regions; the Adams–Williamson relation constrains interior density and elastic structure; Flinn–Engdahl regionalization supports event cataloging; and disciplines such as earthquake engineering and sedimentary recognition of seismites translate geophysical observations into hazard mitigation and paleoseismic records. Collectively, these approaches underpin the seismological understanding needed to assess tsunami risk and to protect coastal infrastructure and communities.
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Tectonic plate boundaries
Tectonic plate boundaries are narrow zones where lithospheric plates meet and interact; the relative motion across these margins—convergence, divergence, or lateral (strike‑slip) movement—determines the dominant deformation style, stress regime, and seismic behavior at the boundary. Convergent margins commonly accommodate subduction and vertical displacement, divergent margins generate seafloor spreading and new crust, and transform boundaries are dominated by lateral slip; each produces characteristic patterns of faulting and earthquake rupture.
Submarine earthquakes occur where faults beneath the seafloor release accumulated elastic strain, typically in episodic stick‑slip events. When rupture includes appreciable vertical offset of the seabed, the displaced water column can generate tsunamis; thus the geometry and sense of fault motion beneath the ocean are critical controls on tsunami genesis.
The magnitude of submarine earthquakes and their tsunamigenic potential is governed by the frictional and mechanical properties of the fault zone: whether segments are locked or creeping, the roughness of fault surfaces, and the presence of sediments and pore fluids all influence rupture propagation and slip magnitude. Kinematics such as the angle and rate of convergence or slip further modulate stress accumulation and release.
Subduction zones at convergent margins are especially susceptible to very large, tsunami‑generating events because megathrust faults can rupture over vast lateral and downdip extents, producing large vertical seafloor offsets. However, transform and oblique boundaries that cut continental margins or submarine slopes can also produce significant undersea earthquakes; depending on fault geometry and attendant vertical motion, these events may trigger tsunamis directly or indirectly by inducing submarine landslides.
Regions exemplifying frequent, large tsunamigenic submarine earthquakes include the circum‑Pacific Ring of Fire and the Sumatran margin (including the Great Sumatran Fault system), both reflecting active plate interactions where seafloor‑adjacent ruptures have produced historically large tsunamis.
Effective tsunami hazard assessment therefore requires mapping plate boundary types and slip vectors and rates, characterizing fault‑zone frictional behavior (locked versus creeping, sediment and fluid conditions), and identifying areas where seafloor rupture or slope failure could produce large water‑column displacements.
Convergent plate boundary
Convergent margins are characterized by the downward transport of older, relatively dense oceanic lithosphere beneath an adjacent, lighter plate. As the descending slab penetrates the asthenosphere and upper mantle it is progressively compressed and heated, undergoing metamorphism and partial melting; the original oceanic crust is therefore consumed and its material returned to the mantle cycle.
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The mechanical bending of the subducting plate at the contact with the overriding plate creates a pronounced bathymetric low—an oceanic trench. Repeated episodes of subduction and continued flexure of the sinking plate maintain and often deepen this trench, which commonly has an arcuate planform that mirrors the plate boundary geometry.
Subduction zones concentrate tectonic strain and thermal contrasts, so they are sites of intense submarine seismicity driven by slab pull, frictional coupling and bending stresses within the lithosphere. Concurrently, fluids released from the metamorphosing slab lower the mantle wedge solidus, inducing partial melting; resultant magmas ascend to form volcanic arcs that run roughly parallel to the trench axis.
The behavior of any subduction system depends on contrasts in density and rheology between cold lithosphere and hot asthenosphere, the cooling influence of seawater on lithospheric thermal structure, and the relative motions of the interacting plates. Together these factors control slab dip, the depth and extent of slab-derived melting, spatial patterns of earthquakes, and trench morphology.
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Globally, the Pacific Ring of Fire illustrates these relationships, hosting numerous trenches, volcanic arcs and frequent earthquakes around the Pacific basin. Prominent examples include the Mariana Trench and the Puerto Rico Trench—deep bathymetric depressions produced by plate convergence—and the Sumatran volcanic arc adjacent to the Great Sumatran fault, where subduction-related magmatism and active faulting coexist.
Transform plate boundary
Transform-fault boundaries are loci where two lithospheric plates slide past one another along a near-vertical fault plane, producing predominantly strike-slip motion rather than the vertical displacement characteristic of convergent or divergent margins. The irregular, interlocking geometry of plate edges along these faults promotes heterogeneous slip behavior: segments may lock and accumulate elastic strain over time, then release that strain episodically in earthquakes. Unlike divergent boundaries, which generate new lithosphere, or convergent boundaries, which consume it by subduction, transform faults chiefly translate existing lithospheric material laterally without large-scale creation or destruction. The San Andreas system exemplifies this class of boundary: the Pacific Plate moves northwest at roughly 5 cm yr−1 relative to the North American Plate, which moves in the opposite sense, producing the observed right‑lateral displacement along the fault zone. Functionally, transform faults accommodate horizontal displacement between adjacent plates, transfer motion along plate margins, and organize linear zones of crustal deformation along those margins.
Divergent plate boundary
Divergent plate boundaries form where tectonic plates pull apart under the influence of upwelling mantle convection. As the lithosphere fractures, asthenospheric magma rises through the resulting fissures and, on the ocean floor, cools rapidly on contact with seawater to produce new igneous (basaltic) crust attached to the plate margins. The cumulative emplacement of this material along a linear zone builds an oceanic spreading ridge—a continuous submarine mountain chain produced by successive additions of newly formed crust.
Because divergence is recurrent, fissures commonly reopen, permitting repeated pulses of magma that accrete successive layers of lithosphere and progressively widen the ocean basin. If heat and pressure concentrate over extended intervals, much larger magma volumes may be released; such influxes can uplift plate edges and, upon cooling beneath the uplifted seafloor, form submarine volcanoes. Deformation at divergent boundaries may be steady, yielding continuous ridge accretion, or episodic, when sudden mechanical failure of a fissure generates seismic events. These processes are exemplified by the Mid-Atlantic Ridge, where magma-driven spreading and associated earthquake tremors are actively observed.
Major submarine earthquakes — summary
Major submarine earthquakes in the instrumental and historical record are concentrated along active plate margins and include some of the largest seismic events ever observed. The largest recorded event is the 22 May 1960 Valdivia (Great Chilean) earthquake (epicenter off south-central Chile), with an estimated moment magnitude of Mw 9.5. Other Mw ≳9 events include the 26 December 2004 Indian Ocean earthquake off northwestern Sumatra (Mw 9.2), which generated catastrophic tsunamis and an estimated 230,000 fatalities, the 11 March 2011 Tōhoku earthquake (epicenter ~130 km off the Oshika Peninsula, hypocenter depth ~32 km; Mw 9.1), and the 26 January 1700 Cascadia earthquake (offshore margin from Vancouver Island to northern California; Mw ≈ 9.0).
Several additional large submarine shocks (Mw 8.0–8.5) have occurred predominantly around the Japanese archipelago, reflecting intense subduction-zone activity: the 15 June 1896 Sanriku earthquake (Mw 8.5), the 20 December 1946 Nankai earthquake (Mw 8.1), and the 7 December 1944 Tōnankai earthquake (Mw 8.0). Other significant offshore events include the 18 November 1929 Grand Banks earthquake (off Newfoundland; Mw 7.2), the 26 December 2006 Hengchun earthquakes in the Luzon Strait (off southwest Taiwan; Mw 7.1), the 4 April 1771 earthquake near the Yaeyama Islands (Mw ≈ 7.4), and a 4 May 1998 submarine event that damaged Yonaguni Island (Ryukyu Islands; magnitude not specified in source material).
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Collectively, these events illustrate that the largest and most destructive submarine earthquakes are typically associated with convergent plate boundaries and can produce far-reaching tsunamis and coastal impacts; their spatial clustering around subduction zones underscores the geodynamic control on submarine seismicity.
Storm‑caused earthquakes
High‑resolution seismic records from the Transportable Array (USArray) have revealed a previously underappreciated causal link between intense ocean storms and small seismic events on the seafloor. A 2019 study using dense instrumentation demonstrated that the passage of large storms over particular offshore regions can impose transient loads or stress perturbations on the seabed that temporally coincide with, and are capable of triggering, measurable undersea earthquakes rather than representing random coincidence. Documented occurrences include Georges Bank and the Grand Banks of Newfoundland, and similar storm‑triggered seismicity has been observed along the Pacific Northwest margin, indicating that storm–seafloor coupling operates across diverse continental‑margin settings. These results underscore the value of dense, high‑resolution seismic arrays for detecting subtle geophysical signals and highlight the need to incorporate dynamic oceanographic forcing into models of seismic triggering and in the design and interpretation of coastal and offshore seismic monitoring.