Abstract
Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages, plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones (DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones, and can be described by the two parameters of delay time δt and fast wave polarization direction ϕ. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs. Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearly perpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time δt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ’s subduction angle α DSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively.
Highlights
Seismic anisotropy observations can provide useful information about the Earth’s interior, such as the material properties and movement patterns, which reflects its inner dynamic processes (Teng et al 2012)
With regard to seismic anisotropy in subduction zones, the geometric relationship between the fast wave polarization direction and the trench strike can be categorized into parallel, oblique, and perpendicular
There are several exceptions within the 39 subduction zones, for example, both NZ1 and NZ2 in southern America are classified as type I double seismic zones (DSZs), while HEB in Tonga is classified as type II DSZ, and EA2 cannot be classified as either type I or type II DSZ (Zhang and Wei 2012)
Summary
Seismic anisotropy observations can provide useful information about the Earth’s interior, such as the material properties and movement patterns, which reflects its inner dynamic processes (Teng et al 2012). With regard to seismic anisotropy in subduction zones, the geometric relationship between the fast wave polarization direction and the trench strike can be categorized into parallel, oblique, and perpendicular. The shear wave splitting characteristics of the mantle wedge are relatively complex, while the fast wave polarization directions of the subslab mantle are mostly parallel to the trench strike (e.g., Long 2013). Long and Silver (2009a, b) compiled shear wave splitting data of 15 subduction zones from previously published literatures and found that only trench migration rate correlates well with the strength of subslab anisotropy in a Pacific hotspot reference frame. A significant difference has been found in the stress characteristics of different DSZ types, and this phenomenon is attributed to the dehydration of minerals in subducting slabs (Peacock 2001; Hacker et al 2003). We attempted to find correlations between them and used Fresnel polarization ellipsoid to discuss their geodynamical implications and physical mechanism
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