The recognition of slow earthquakes in geodetic and seismological data has transformed the understanding of how plate motions are accommodated at major plate boundaries. Slow earthquakes, which slip more slowly than regular earthquakes but faster than plate motion velocities, occur in a range of tectonic and metamorphic settings. They exhibit spatiotemporal associations with large seismic events that indicate a causal relation between modes of slip at different slip rates. Defining the physical controls on slow earthquakes is, therefore, critical for understanding fault and shear zone mechanics. In this Review, we synthesize geological observations of a suite of ancient structures that were active in tectonic settings comparable to where slow earthquakes are observed today. At inferred slow earthquake regions, a range of grain-scale deformation mechanisms accommodated slip at low effective stresses. Material heterogeneity and the geometric complexity of structures that formed at different inferred strain rates are common to faults and shear zones in multiple tectonic environments, and might represent key limiting factors of slow earthquake slip rates. Further geological work is needed to resolve how the spectrum of slow earthquake slip rates can arise from different grain-scale deformation mechanisms and whether there is one universal rate-limiting mechanism that defines slow earthquake slip.
The global distribution and pressure–temperature range of seismologically observed slow earthquake hypocentres implies that no single mineral phase, lithology or metamorphic reaction controls slow earthquake slip.
There is no universal deformation structure or deformation mechanism that is currently a clear indicator of slow earthquakes in the rock record. Multiple different mechanisms or combinations of mechanisms can produce the same macroscopic behaviours.
Geological evidence from slow earthquake source regions suggests that material heterogeneity, geometric complexity and deformation at low differential stress are common to slow earthquake sources.
A seismologically observed low-frequency earthquake source could consist of multiple anastomosing faults, shear bands and/or vein networks (potentially including synchronous slip across multiple subparallel surfaces), rather than a single planar fault surface.
Geodetically observed slow slip events can be accommodated by ductile shear zones, which are commonly identified in many exhumed fault zones.
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We thank Yujin Kitamura and Alissa Kotowski for providing data for Fig. 3b and Noah Phillips and Alissa Kotowski for providing photomicrographs in Fig. 4a and Fig. 4c, respectively. Thanks also go to Alissa Kotowski, Christie Rowe and Randy Williams for discussions and feedback on an early draft of the manuscript. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Discovery Grant RGPIN-2016-04677 (J.D.K.), the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, Starting Grant agreement 715836 (A.F.) and the Earthquake Hazards Program of the U.S. Geological Survey (D.R.S.).
The authors declare no competing interests.
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- Accretionary wedge
The accumulated rock scraped off the oceanic plate and transferred to the upper plate at subduction margins, forming a wedge shape in cross section.
- Tectonic tremor
Low-amplitude seismic signals defined by non-impulsive arrivals, similar to noise but distinguished by coherence over large geographic areas.
The point on a fault where an earthquake rupture starts.
- Frictional sliding
Displacement between two surfaces in contact, which is resisted by a shear stress proportional to the normal stress resolved on the surface.
- Diffusion creep
A grain-scale deformation mechanism in which grains accommodate strain by the diffusion of matter through or around grains.
- Crystal–plastic deformation
The intragranular deformation mechanisms that cause individual grains to change shape by dislocation-based mechanisms.
- Double-couple source mechanisms
Model that uses couples of point forces to describe the seismic wave radiation pattern for an earthquake whose displacement is within the plane of the fault.
- Critically stressed
When the shear stress resolved on a fault is just below the frictional strength of the fault, so that small perturbations to the stress field can cause failure.
- Dislocation motion
Used here to refer to deformation mechanisms that involve movement of dislocations, linear imperfections in the crystal lattice of grains, to accommodate strain.
- Dissolution–precipitation creep
A deformation mechanism by which grains change shape through dissolution at high-stress sites, accompanied by fluid-assisted diffusive mass transfer towards, and reprecipitation at, low-stress sites.
- Finite strain
The total strain, or change in shape, that has affected a rock.
- Cataclastic flow
A brittle process in which a volume of rock deforms by frictional sliding and grain rolling combined with fracture, causing an overall change in shape.
A geometry in which surfaces or strands diverge and rejoin; braided.
A rock fabric that can be approximated as a plane, often defined by the preferred orientation of mineral grains and/or by compositional banding.
Very-fine-grained fault rock that deformed predominantly by plastic mechanisms.
- S-C-C′ composite fabrics
Composite fabrics that form inside shear zones (namely, by plastic deformation) — the S-foliation represents local shortening, while C and C′ foliations are small-scale shear bands.
- Composite fabrics
Foliation that is defined by more than one set of oriented fabrics in the rock, which form discrete sets.
Process by which relatively competent layers split apart into smaller sections when stretched, and the surrounding, relatively incompetent, material deforms to accommodate the change in shape of the competent layer.
- Buckle folding
Folding that is inferred to form by layer-parallel shortening when relatively competent, or viscous, layers or features are surrounded by less competent rock.
Mixtures of rock types that are characterized by a block in matrix fabric, used here to refer to rock units that formed and deformed due to tectonic shearing.
- Pelitic rocks
Rocks that have a high clay content, or their metamorphic equivalents.
Process by which rotation of layers during folding or shearing causes the original orientation, angular relationships and distinct features of the layers in the rock to be almost completely obliterated.
Process of thrust faulting that causes multiple, approximately parallel, slices of rock to be thrust on top of one another.
- Prograde deformation
Deformation that occurs while the rocks experience an increase in temperature and/or pressure, typically during burial (including subduction-related burial).
The pre-deformation or pre-metamorphic equivalent of a deformed or metamorphosed rock.
- Dislocation creep
Intracrystalline deformation mechanism in which strain is accommodated by migration of dislocations, linear imperfections in the crystal lattice of grains, accompanied by dislocation climb.
- Cataclastic bands
Layers of fault rock in which the grain size is reduced owing to cataclastic processes when the layer accommodated shear displacement.
The quenched remnants of a molten rock that formed by frictional heating on a fault surface during earthquake slip.
Minerals (including clays and micas) that are made up of stacks of parallel sheets of silicate tetrahedra, which are weakly bonded together.
- Extensional hydrofractures
Opening-mode cracks, formed when the pore fluid pressure exceeds the minimum compressive principal stress and the differential stress is less than twice the cohesion of the rock.
- En echelon
Describes the geometry of parallel or subparallel overlapping structures (usually opening-mode veins or faults) that are offset from one another in the direction perpendicular to their long axes, and are oblique to the overall structural trend.
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Kirkpatrick, J.D., Fagereng, Å. & Shelly, D.R. Geological constraints on the mechanisms of slow earthquakes. Nat Rev Earth Environ 2, 285–301 (2021). https://doi.org/10.1038/s43017-021-00148-w