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A coupled phenomenon in which transient slow fault slip occurs with low-frequency seismic tremor, often near the downdip transition from locked to steadily creeping parts of a megathrust. Geodetic displacement and seismic signals provide complementary observations. Along-strike differences in recurrence, migration and cumulative slip help characterize subduction zone segmentation; tremor counts alone do not uniquely give fault slip or coupling.

Suppression of relative slip on part of a megathrust during an interval of plate convergence, producing a slip deficit and elastic deformation. GNSS, leveling and offshore geodesy constrain it through a fault-geometry and deformation model. A locking fraction is therefore an inferred kinematic quantity; it is not a direct measurement of friction or the magnitude of the next earthquake.

Subduction zone segmentation is the observation that slab structure, fault slip and upper-plate behavior change along a margin; their boundaries need not coincide exactly. In Cascadia, it is documented by several independent measurements.

Onshore GNSS velocities, coastal leveling and tide gauges constrain along-strike changes in megathrust locking and interseismic uplift. Episodic tremor and slip has distinct recurrence and migration patterns, and its occurrence is lower in parts of central Oregon than farther north and south. Coastal geological records constrain differences in coseismic subsidence and the extents of past ruptures. Those observations sample different depths and time windows, so a present-day locking boundary should not automatically be declared a permanent earthquake rupture barrier. A 2014 analysis found persistent central-margin partial creep compatible with reduced uplift, reduced paleoseismic subsidence and lower cumulative slow slip.^[ref]

A physically specific hypothesis links these variations to subslab mantle buoyancy. An onshore–offshore seismic tomography study imaged low-velocity asthenosphere beneath northern and southern Cascadia, spatially associated with stronger plate locking and more tremor, with different central-margin structure. Warmer or partly molten asthenosphere can be less dense. Its buoyant traction can push upward against the slab and change contact stresses on the megathrust. Hotspot-influenced material is one proposed northern source; three-dimensional deformation near the Gorda region offers a southern upwelling mechanism.^[ref]

There are additional controls. The Siletzia basaltic basement beneath the overriding plate varies in extent and thickness. Its permeability and stiffness can affect fluid escape and fault stresses: poorly drained material can preserve high pore pressure, lower effective normal stress and promote creep. Along-strike changes in incoming plate age, slab dip, fracture-zone hydration, sediment supply and plate-edge stresses can also modify the thermal and frictional environment. Central-margin creep has specifically been associated with the distribution of Siletzia and inferred fluid trapping.^[ref]

How do we distinguish these causes? Offshore and onshore seismic arrays test the geometry of the anomalies; resolution and synthetic-recovery tests determine whether apparent segmentation could result from uneven station coverage. Seismic attenuation, anisotropy, magnetotelluric sounding, rock physics and heat-flow data help separate temperature, melt, water and composition. Gravity anomalies and seismic interfaces constrain basement and slab geometry; geodetic inversions test whether the inferred locking pattern persists across plausible fault geometries. Forward models then ask whether the measured density or pore-pressure contrasts produce stresses and slip patterns of the required size and sign.

Segmentation is observed robustly; a unique cause is not established by spatial correlation alone. Seismic velocity is not a direct density measurement, central creep and deep tremor are not the same phenomenon, and multiple coupled structures can reinforce one another. The buoyant-mantle and upper-plate fluid hypotheses are testable explanations rather than mutually exclusive certainties.

Three earthquake sources matter: the offshore Cascadia megathrust, deep events inside the Juan de Fuca Plate, and shallow upper-plate faults, including the Seattle Fault. Shaking can damage buildings, bridges, ports and utility networks, interrupting transport, electricity and water supply. Soft basin sediments can amplify or prolong shaking; saturated loose fill can undergo soil liquefaction and lateral spreading. Steep slopes can fail as landslides. The same regional event can therefore have very different consequences on bedrock and on artificial fill.^[ref]

Large Cascadia tsunamis chiefly threaten the outer Pacific coast. Seattle's inland waterways also have local tsunami hazards from crustal faults and underwater landslides; one should not equate a Seattle waterfront site with an open-ocean coastal site. Cascade eruptions can affect the region through volcanic ash and transport disruption. Lahars are strongly channelled by river valleys, so their exposure is location dependent rather than uniform across Seattle. These geological differences influence infrastructure design, land-use decisions and emergency planning.^[ref][ref]

An accreted basaltic terrane forming important parts of the forearc basement in the Pacific Northwest. Spatial variations in its thickness, stiffness and permeability can affect the interpretation of gravity anomalies and the fluid/stress environment above the megathrust. A proposed relation to central Cascadia creep uses gravity, geodetic and paleoseismic observations, rather than lithology alone.^[ref]