Evidence for a role of the downgoing slab in earthquake slip partitioning at oblique subduction zones

Abstract

A new model, incorporating shear deformation within a subducting slab, is proposed to explain slip partitioning for oblique plate motion at subduction zones. On the basis of investigation of 450 interplate earthquakes at 24 subduction zone segments we find that the degree of slip partitioning is laxgely correlated with the calculated slab pull force. Such correlation suggests that other than the upper plate deformation, the slab pull force plays an important role in controlling oblique subduction. Our model proposes that the force balance between the slab pull force, the interplate coupling resistance, and the viscous mantle drag (the latter two are passive forces to the former one) produces a lateral shear within the slab, which causes the slab to deform and change its motion direction gradually toward trench normal as it subduers. The amount of direction change, which would be observed as the slip partitioning during earthquakes, therefore is closely related to the major plate driving force at the subduction zone, which is the slab pull force in out model.

Bearing of plate geometry and rheology on shallow-focus mega-thrust seismicity with special reference to 26 December 2004 Sumatra event

“…3 and Table 2). Though no direct information connecting obliquity and slab geometry is available, high-resolution reconstruction of depth-dip angle profiles across the Sumatra margin (Khan and Chakraborty, 2005) show minimum dip and attainment of early (around 25 km depth) dip-angle saturation in Benioff zone trajectory at the subduction segment (i.e., in sector II of present study) where slip vector residual assumes the minimum value (nearly zero, Liu et al, 1995). In the north (sector I of present study and further north) with increase in slip vector residual Benioff zone trajectories become steeper and attain dip saturation at much greater depth (around 40 km).…”

Cenozoic geodynamic evolution of the Andaman–Sumatra subduction margin: Current understanding

“…Plate obliquity (φ) along the subduction margin is defined as the angle between trench normal and the converging plate velocity vector. This is illustrated in Figure 2 based on the work of Liu et al. (1995).…”

“… Diagram (after Liu et al. 1995) illustrating formation of sliver plate, b. Incoming oceanic plate and overriding plate are a and c, respectively.…”

Role of unbalanced slab resistive force in the 2004 off Sumatra mega-earthquake (M w > 9.0) event

“…Besides, fore bearers of asperity subduction hypothesis (Ruff and Kanamori 1983;Cloos and Shreve 1996) noticed coincidence of distinct bend in Benioff zone configuration with the most active zone of seismic slip accumulation in many of the subduction margins around 30-40 km depth. The high-resolution reconstruction of depth-dip angle profiles across the Sumatra margin (Khan and Chakraborty 2005) showed minimum dip and attainment of early (around 25 km depth) dip angle saturation in Benioff zone trajectory in sector III, and the slip/stress vector residual was noted to be minimum (nearly zero, Liu et al 1995;Khan 2007). It is therefore may be proposed that the occasional incidences of great earthquakes around Sumatra plate margin were somehow associated with the spatio-temporal changes of the plate obliquity, subduction rate, and depth of plate flexing (Khan and Chakraborty 2005).…”

“…Further, the flexing zone of the lithosphere beneath the trench during subduction suffers instability by the net effect of slab pull and slab resistive forces (Marotta and Mongelli 1998), and *60% of the energy associated with the subducting slab is normally dissipated through the bending segment of the slab (Conrad and Hager 1999). Invariably recorded shallowest dip, lowest flexing depth (Widiyantoro and Van der Hilst 1997;Khan and Chakraborty 2005), youngest plate age (*47 Ma in northwest Sumatra, increases to *152 Ma towards east near Java and *67 Ma towards north near North-Andaman, Muller et al 1997) of the subducting lithosphere, minimum slip vector residual (Liu et al 1995, Khan 2007, intermittent occurrence of megathrust earthquake (1797 M *8.4, 1833 M *9.0, 1861 M *8.5, Lay et al 2005; M w [ 9.0, 2005 M w 8.6, Ammon et al 2005, 9.3;Stein and Okal 2005), and recently proposed strain-hardened model for mega-earthquakes (Khan and Chakraborty 2009) allowed us for understanding the role of major plate-driving forces (Forsyth and Uyeda 1975) for the occurrence of 2004 off Sumatra event.…”