Shallow Subduction and Earthquake
Convergent plate boundary causes highly various tectonic feature as a consequences of the plate interaction and lithospheric evolution (Marshack & Pluijm, 1997). Mountain building, volcanism, earthquake and even more things can be explain by this process. Cold and dense oceanic lithospheres will sink into the mantle they are now have negative bouyant. Exisiting weaknesses on the transform faults and within the contact of continent and oceanic lithospheres are the subject for the compression that will possibly leads to the subduction process. Oceanic slabs that now beneath the continental lithospheres are responsible for the subdution process to continue with its main forces such as slab suction and slab pull force (Conrad et al., 2004)
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High strain area represents within the active subduction process. Figure 1 (Bird et al., 2004) shows Comparison between strain rate in all different tectonic process in the world and it indicates that subduction process resulting in the much bigger strain rates than other tectonic process. Many factors are influence the subduction activity. There are three end member of convergent plate boundary divided by their characteristic response to the subduction process as suggest by Conrad et al. (2004) are:
· upper plate shortening (mountain bulding); deformation will take place further inland away from the trench, upper plate movement toward the trench and usually associated with seamount or oceanic plateau collision and in extreme case continental plate travels with the subducted oceanic slab which creates shallow subduction (Figure 2).
· upper plate extension (back arc spreading); overriding plate move away from trench, deformations are close to the trench.
Among those two end member there is one that put the obliquity of the plate interaction into account that will effect the permanent deformation in the upper crust. Lateral movement on the trench similar to the transform boundary can also occurs in the subduction zone[1].
These end members of convergent plate boundary can be inferred also with seismicity behaviour in the area. Greater seismicity are formed within the uncoupled slab pull stress as a result from bidirectional movement on the plate interaction. When overriding plate motion move toward the trench it creates back arc compression and great earthquakes as a result from seismic coupling. Compressional stresses exerted the slab pull forces and eventually weakened the slabs. Gravitational pull has been reduced by this process and makes the slab is poorly coupled and let the slab suction takes a bigger porton to bring the slab deeper. Reversely, when the overriding plates moves away from the trench seismic coupling will be smaller resulting in more extensional environment with smaller-moderate earthquakes because slabs can move smoothly into mantle because they are well coupled to subducting plate which allows transmission of the gravitational pull as a boundary forces (Ruff & Kanamori, 1980; Conrad et al.2004). These process will directly affected the style of deformation. Recent studies using geodetic measurement on active plate boundaries (Figure 3; Unavco, 2009) compared to the strain rate map (Figure 1) simply show us that higher strain zone corelates with the plate movement against each other; not necessarily perpendicular.
Variation is seismic coupling may also become the result from geometry and morphology of the subduction zone, subducted sediments carried by the underplating slab, frictional property of the underplating and magnitude of the earthquake. Bigger earthquake means stronger seismic coupling (Kanamori, 1977). Frictional coupling plays an important role determining what will happen during the earthquake building. Low friction will be close to the failure near the wedge zone close to the failure zone as the aseismic deformation can merge as natural outcome of the rate and state of friction. Increasing frictional coupling will increase the seismic coupling and locking possibility of the seismogenic during the energy build-up. Aleutian, Japan-Kuriles-Kamchatka and Chile are the subduction segment expected to produce great earthquake while Marianas–Izu-Bonin, Java–Bengal, and Tonga produce moderate sized earthquake (Ruff & Kanamori, 1980; Figure 4; Courtesy of USGS).
Mantle disturbance by the subducting plate can also decreases the dip of the subduction that further increasing seismic coupling by widening the seismogenic zone (Hager & O’Connell, 1978) as well as change in coherency in slab material and smoothness of the subducted sediments, oceanic plateau or seamounts (Scholz and Small, 1997; Larssen et al., 1995; Ruff, 1989). Slab coupling will weaken by these processes and it would lead to increasing slab suction force which trigerred additional locking and reduce the plate velocity (Conrad et al., 2004).
Studies from the Oaxaca and El Savador segment of the pacific plate subduction beneath the North America indicated different responses of deformation given by the overriding plate. Higher horizontal movement observed along Oaxaca segment which has steeper angle of subduction compared to El Savador segment. Crustal movement study by based on GPS data shows different crustal movement in the region that may also leads to different deformation style in particular region. El Savador segment moving toward the the NW-SE direction or parallel with the trench while Oaxaca segment moves almot perpendicular to the trench toward pacific plate (Figure 5; Correa Mora et al., 2008). Different strain rates between these region may also resulted by the different overriding plate movement and its response to the subduction that can be affected by the changing in force balance of the overriding plate (Richards & Lithgow-Bertelloni, 1996).
[1] www.geo.mtu.edu/EHaz/ConvergentPlatesClass/.../CD_subduction.ppt
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