Sunday, March 14, 2010
Subduction styles/EQ magnitude
Strong Earthquakes in Latin America can be attributed to a greater release of seismic energy associated with flat slab vs.steep slab segments. The geometry of these flat slab regions allow for more stress to build up between the plates (Gutscher, et al.,2000).
Flat Slab segments lack the arc volcanoes which are prominent in western America from subducting young oceanic lithosphere, as depicted in the image above of Normal slab subduction. Normal slab subduction is characterized by maintaining a distinct low viscosity and high temperature melt in the boundary between the slab and overriding plate. Flat subduction occurs when a distinct decrease in the dip angle of the slab results the slab tip flattening underneath the plate overriding it. The wedge corner is subject to high stresses as the slab-tip is pushed into the mantle thereby constricting the corner flow above the slab.
Flat slab segments are attributed to the high magnitude earthquakes in LA, and one possibility behind the differing slab geometry is the buoyancy between the two slabs with the younger buoyant slab resisting subduction. The subduction of young buoyant lithosphere can be modified when encountering denser older lithosphere changing the thermal structure at the margin and displacing the asthenosphere away from the trench. The cooler temperature at the forearc predicts a larger locked zone therefore increasing the risk of great interplate earthquakes.
While Earthquakes occur all along the subduction zone, those in the ‘Seismogenic zone are most destructive. This is because they occur near the surface. The earthquake cycle in the Seismogenic Zone can be said to occur in two stages, the Interseismic Period and the Coseismic Period as described below:
The Interseismic Period Time between earthquakes: (10’s to 100's of years) where plate convergence continues at approximately 8cm a year (between Nazca and S. American plates) with the two plates are locking over a portion of the subduction plate boundary. This results in not only uplift, but also a horizontal shortening of the overlying plate margin.
The Coseismic Period/Earthquake rupture occurs over a few minutes. Once the accumulating stress exceeds the strength of the fault, failure occurs in the locked zone and great earthquake occurs. During this episode stored elastic strain is released which in turn results in subsidence and horizontal extension where slow uplift and horizontal shortening had previously accumulated. In addition, the underwater displacements can cause tsunamis. Once the stress is relieved, the cycle resets and stress begins to build again.
In Latin America there is a combination of Steep slab/Flat slap segments, with the greater earthquakes occurring along the zones of flat slab subduction. (Fig. below shows a run down of where earthquakes of greater magnitude occur as opposed to those of lesser magnitude).
Note: I couldn't find a comprehensive enough image to suit my purpose for this blog post so I created my own from a variety of figures (upper two images of steep slab/flat slab), with the principle source being from Gutscher, 2002.
Gutscher, MA., 2002. Andean subduction styles and their effect on thermal structure and interplate coupling. Journal of South American Earth Sciences 15: 3