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Wedge Mechanics: Relation with Subduction Zone Earthquakes and Tsunamis

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Encyclopedia of Complexity and Systems Science

Definition of the Subject

Mechanics of wedge‐shaped geological bodies such as accretionary prisms at subduction zones and fold-and‐thrust belts at collision zones is of great scientific interest, mainly because it enables us to use the observed morphology and deformation of the wedge‐shaped bodies to constrain properties of the thrust faults underlying them. Davis et al. [12] drew analogy between these wedge‐shaped bodies and the sand wedge in front of a moving bulldozer and established a mathematical model. Their work triggered wide applications of wedge mechanics to geology. The fundamental process described in wedge mechanics is how gravitational force, in the presence of a sloping surface, is balanced by basal stress and internal stress. The internal state of stress depends on the rheology of the wedge. The most commonly assumed wedge rheology for geological problems is perfect Coulomb plasticity [12], and the model based on this rheology is referred to as the Coulomb wedge model.

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Abbreviations

Subduction zone earthquake cycle:

Megathrust fault, the interface between the two converging lithospheric plates at a subduction zone, moves in a stick-slip fashion. In the “stick” phase, the fault is locked or slips very slowly, allowing elastic strain energy to be accumulated in both plates around the fault. Every few decades or centuries, the fault breaks as high-rate “slip” to release the strain energy, causing a large or great earthquake, usually accompanied with a tsunami. An interseismic period and the ensuing earthquake together is called a subduction zone earthquake cycle. The word cycle by no means implies periodicity. Neighboring segments of the same subduction zone may exhibit different temporal patterns of earthquake cycles.

Accretionary wedge (prism) :

At some subduction zones, as one plate subducts beneath the other, some sediment is scraped off the incoming plate and accreted to the leading edge of the upper plate. Because of its wedge shape, the accreted sedimentary body is called the accretionary wedge (or accretionary prism). If all the sediment on the incoming plate is subducted, there is still a sedimentary wedge in the frontal part of the upper plate, but it is usually very small and consists of sediments derived from the upper plate by surface erosion.

Coulomb plasticity :

Coulomb plasticity is a macroscopic, continuum description of the most common type of permanent deformation of Earth materials such as sand, soil, and rock at relatively low temperature and pressure and is widely used in civil engineering and Earth science. In detail, the deformation mechanism is actually brittle shear failure, with or without emitting elastic wave energy. The macroscopic yield criterion is the Coulomb's law, in which shear strength increases linearly with confining pressure. If the strength does not change with permanent deformation, the material is said to be perfectly plastic. Note that in Earth science the word plasticity is also used to indicate thermally activated creep, but it is very different from the meaning used herein.

Velocity‐weakening and strengthening:

These are macroscopic descriptions of dynamic frictional behavior of contact surfaces. Velocity‐weakening , featuring a net decrease in frictional strength with increasing slip rate, is the necessary condition for a fault to produce earthquakes. It differs from slip‐weakening in that a velocity‐weakened fault will regain its strength when the slip slows down or stops. Velocity‐strengthening is the opposite of velocity‐weakening and is the necessary condition for a fault to resist earthquake rupture. Detailed physical processes on the contact surfaces or within the fault zones controlling their frictional behavior are still being investigated.

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Acknowledgments

We thank EE Davis, N Kukowski, SE Lallemand, and K Satake for reviewing the article and providing valuablecomments. This work is Geological Survey of Canada contribution 20070221.

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Authors and Affiliations

  1. Pacific Geoscience Centre, Geological Survey of Canada, Sidney, Canada

    Kelin Wang & Jiangheng He

  2. School of Earth and Ocean Sciences, University of Victoria, Victoria, Canada

    Kelin Wang & Yan Hu

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  1. Kelin Wang
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  2. Yan Hu
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  1. RAMTECH LIMITED, 122 Escalle Lane, Larkspur, CA, 94939, USA

    Robert A. Meyers Ph. D. (Editor-in-Chief) (Editor-in-Chief)

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Wang, K., Hu, Y., He, J. (2009). Wedge Mechanics: Relation with Subduction Zone Earthquakes and Tsunamis . In: Meyers, R. (eds) Encyclopedia of Complexity and Systems Science. Springer, New York, NY. https://doi.org/10.1007/978-0-387-30440-3_590

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