Physics of the Earth and Planetary Interiors 181 (2010) 141–154 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Frequency-dependent shear wave splitting beneath the Japan and Izu-Bonin subduction zones Erin Wirth ∗, Maureen D. Long Department of Geology and Geophysics, Yale University, New Haven, CT, United States article info abstract Article history: Despite its importance for our understanding of physical processes associated with subduction, the geom- Received 19 November 2009 etry of mantle flow in subduction zones remains poorly understood, particularly in the mantle wedge Received in revised form 18 May 2010 above subducting slabs. Constraints on mantle flow and deformation can be obtained by measurements of Accepted 30 May 2010 shear wave splitting, a valuable tool used to characterize the geometry and strength of seismic anisotropy. Edited by: G. Helffrich A complete characterization of shear wave splitting is particularly important for understanding the man- tle wedge beneath Japan, which overlies multiple subduction zones with complex slab morphologies; previous studies indicate that the upper mantle beneath Japan exhibits complicated anisotropy that Keywords: Shear wave splitting manifests itself in complex splitting patterns. To characterize better the geometry of mantle anisotropy Seismic anisotropy beneath Japan, we analyzed direct S waves from local earthquakes originating in the subducting slabs for Japan evidence of shear wave splitting using data from 54 broadband seismic stations in Japan’s F-net array. In Subduction dynamics addition, both local S and teleseismic SKS phases were examined using data from four F-net stations in the Mantle wedge Izu-Bonin arc. In order to characterize any frequency dependence of splitting parameters that may indi- cate the presence of complex anisotropy, we carried out our splitting analysis in two different frequency bands (0.02–0.125 Hz and 0.125–0.5 Hz). Our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath Japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency. © 2010 Elsevier B.V. All rights reserved. 1. Introduction and extent of the anisotropy, respectively. Since shear wave split- ting gives us information about anisotropy that can be related to Elastic anisotropy is a nearly ubiquitous property of the upper strain-induced LPO, we can ultimately learn about mantle flow and mantle and manifests itself in the seismic wavefield in a variety of deformation. ways, including both body waves and surface waves. Shear wave A complete characterization of upper mantle anisotropy is espe- splitting is a particularly valuable tool for characterizing seismic cially important in subduction zones. The presence of upper mantle anisotropy because it is an unambiguous indicator of anisotropic anisotropy in subduction zones has been well documented (e.g., structure and is generally unaffected by isotropic wavespeed het- Ando et al., 1983; Fouch and Fischer, 1996; Fischer et al., 1998; erogeneity (e.g., Silver, 1996; Savage, 1999; Long and Silver, 2009). Currie et al., 2004; Long and van der Hilst, 2005), but despite Studies have shown that anisotropy in the upper mantle appears to advances in our understanding of the structure of subducting slabs, be dominated by lattice preferred orientation (LPO) of mantle min- the character of anisotropy and the pattern of mantle flow in erals, primarily olivine (e.g., Mainprice, 2007; Karato et al., 2008, subduction zones remain poorly understood. Previous studies of and references therein), which results from deformation. Because subduction zone anisotropy have yielded a wide variety of shear of the causative link between dynamic processes in the upper man- wave splitting patterns, including fast directions that are parallel, tle and the resulting anisotropy, the characterization of anisotropy perpendicular, and (less often) oblique to the trench and a wide using tools such as shear wave splitting can be used to gain valu- range of observed delay times. The classically accepted flow model able information about the geometry of mantle flow. The shear for subduction zones, which is characterized by 2D corner flow wave splitting parameters (fast polarization direction and delay in the mantle wedge and entrained flow beneath the slab, fails to time ıt) give us information about the orientation of the olivine account for the diverse observations. Consequently, other models fast axis (related to the direction of mantle flow) and the strength have been proposed including trench-parallel flow in the man- tle wedge (e.g., Smith et al., 2001), trench-parallel flow beneath the subducting slab (e.g., Russo and Silver, 1994), and 2D corner ∗ flow coupled with trench-parallel 3D return flow induced by trench Corresponding author. Tel.: +1 203 432 9808; fax: +1 203 432 3134. E-mail address: [email protected] (E. Wirth). migration (e.g., Long and Silver, 2008). 0031-9201/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2010.05.006 142 E. Wirth, M.D. Long / Physics of the Earth and Planetary Interiors 181 (2010) 141–154 In addition to the variety of proposed flow models, differences in olivine LPO development in different parts of the subduction system have also been invoked to help explain the diversity of observed splitting patterns. Laboratory results have shown that in water-rich, high-stress environments such as those associated with subduction zones, the fast axes of olivine crystals will align per- pendicular to the direction of mantle flow (Jung and Karato, 2001). The presence of this B-type olivine fabric in the shallow corner of the mantle wedge, in conjunction with trench-perpendicular flow, could explain the trench-parallel fast directions observed in sub- duction zone settings under the necessary physical conditions. It has also been suggested that the presence of partial melt (Holtzman et al., 2003) or a pressure-induced transition to B-type olivine fab- ric (Jung et al., 2009) might affect the relationship between mantle flow and the resulting anisotropy above or below the subducting slab, respectively. A key difficulty in interpreting shear wave splitting measure- ments in subduction zones lies in isolating the effect of anisotropy in different parts of the subduction system (the overriding plate, the mantle wedge, the subducting slab, and the sub-slab mantle). Con- straints on anisotropy in the mantle wedge and overriding plate can be obtained by measuring the splitting of local S phases; constraints thus obtained can then be used to isolate the effect of anisotropy in the slab and sub-slab mantle on teleseismic phases (e.g., Long and Silver, 2008). However, extreme care must be taken in this approach, since local S phases typically have shorter character- istic periods than teleseismic phases, and local S measurements are often made at higher frequencies than teleseismic splitting measurements (e.g., Nakajima and Hasegawa, 2004). Shear wave Fig. 1. Map showing topography and tectonic setting of Japan. Black dashed splitting measurements have been shown to be dependent on lines mark the trench locations with slab contours plotted every 100 km in gray frequency in a variety of tectonic settings, including continental (Gudmundsson and Sambridge, 1998). Colored triangles show the location of the 54 F-net stations examined in this study and are broken down by color into geographic interiors (e.g., Clitheroe and van der Hilst, 1998; Fouch et al., 2004) region. Arrows indicate the direction of absolute plate motion for both the Pacific and subduction zones (e.g., Marson-Pidgeon and Savage, 1997; and Philippine Sea plates as given by the HS3-Nuvel1A model (Gripp and Gordon, Fouch and Fischer, 1998; Long and van der Hilst, 2006; Greve et 2002). al., 2008; Greve and Savage, 2009). From a finite-frequency point of view, waves with different periods will be sensitive to different The Japanese arc system is characterized by an extensive dis- volumes of mantle material (e.g., Alsina and Snieder, 1995; Favier tribution of active faults. In Hokkaido, the fault system is along and Chevrot, 2003; Long et al., 2008), so frequency-dependent shear the western side of the Hidaka Mountains, part of the thrust belt wave splitting is likely an indication of heterogeneous anisotropy resulting from the Kuril forearc collision and backarc extension. at depth. The majority of these faults, as well as those located in northern Honshu, trend north-south, implying east-west directed compres- sional stress. Northern Honshu is characterized by active volcanic 2. Tectonic history and setting chains with high mountain ranges to the south due to interaction between the Okhotsk and Amur microplates. In southwest Honshu In this study we focus on F-net stations located on the islands and part of Shikoku, faults run NW–SE and NE–SW; again indi- of Hokkaido, Honshu, Shikoku, and in the Izu-Bonin arc, regions cating east-west compressional stress. The seismicity of Japan is affected by the subduction of the Pacific and Philippine Sea plates directly related to the plate boundaries and active faults. Off the (Fig. 1). Here we briefly summarize the tectonic setting of our region eastern coast of Hokkaido, the Kuril trench marks the location of of focus, largely based on an overview by Taira (2001). The for- plate boundary earthquakes. On land, the thrust belt to the west of mation of the Japanese island arc system is primarily the result the Hidaka Mountains is also seismically active. The Japan trench of subduction of the Pacific plate, beginning sometime around the and Nankai trough off the coasts of northern Honshu and southwest Permian. Currently, in northern Honshu and Hokkaido, the Pacific Japan respectively, are known to generate frequent, high magni- plate is subducting beneath the Kuril and Japan trenches and the tude earthquakes. In addition, high magnitude events along the Izu-Bonin trough to the south at a rate of ∼10 cm/yr.