Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear-wave splitting

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Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from

shear-wave splitting

J.O.S. Hammond, J. Wookey, S. Kaneshima, H. Inoue, T. Yamashina, P.

Harjadi

To cite this version:

J.O.S. Hammond, J. Wookey, S. Kaneshima, H. Inoue, T. Yamashina, et al.. Systematic varia- tion in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear- wave splitting. Physics of the Earth and Planetary Interiors, Elsevier, 2010, 178 (3-4), pp.189.

�10.1016/j.pepi.2009.10.003�. �hal-00610669�

Accepted Manuscript

Title: Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from

shear-wave splitting

Authors: J.O.S. Hammond, J. Wookey, S. Kaneshima, H.

Inoue, T. Yamashina, P. Harjadi

PII: S0031-9201(09)00213-1

DOI: doi:10.1016/j.pepi.2009.10.003

Reference: PEPI 5211

To appear in: Physics of the Earth and Planetary Interiors Received date: 6-7-2009

Revised date: 3-10-2009 Accepted date: 13-10-2009

Please cite this article as: Hammond, J.O.S., Wookey, J., Kaneshima, S., Inoue, H., Yamashina, T., Harjadi, P., Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear-wave splitting, Physics of the Earth and Planetary Interiors (2008), doi:10.1016/j.pepi.2009.10.003

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Accepted Manuscript

Systematic variation in anisotropy beneath the mantle

1

wedge in the Java-Sumatra subduction system from

2

shear-wave splitting

3

J. O. S. Hammond ^∗,a,b , J. Wookey ^a,c , S. Kaneshima ^b , H. Inoue ^d , T.

4

Yamashina ^d , P. Harjadi ^e

5

a Department of Earth Sciences, University of Bristol, Bristol, UK.

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b Department of Earth and Planetary Sciences, Kyushu University, Hakozaki 6-10-1,

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Fukuoka 812-8581, Japan

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c Department of Earth Sciences, University College London, Gower Street, London, UK

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d Badan Meteorologi, Klimatologi dan Geofisika, Jakarta, Indonesia

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e National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan

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Abstract

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The tectonic context of south-east Asia is dominated by subduction. One such major convergent boundary is the Java-Sunda trench, where the Australian- Indian plates are being subducted beneath the Eurasian plate. We measure shear-wave splitting in local and teleseismic data from 12 broadband sta- tions across Sumatra and Java to study the anisotropic characteristics of this subduction system, which can provide important constraints on dynam- ical processes involved. Splitting in S-waves from local earthquakes between 75-300 km deep show roughly trench parallel fast directions, and with time- lags 0.1-1.3 s (92% ≤0.6 s). Splitting from deeper local events and SKS, however, shows larger time-lags (0.8-2.0 s) and significant variation in fast direction. In order to infer patterns of deformation in the slab we apply a hybrid modelling scheme. We raytrace through an isotropic subduction zone velocity model, obtaining event to station raypaths in the upper man-

*Manuscript

Accepted Manuscript

tle. We then apply appropriately rotated olivine elastic constants to various parts of the subduction zone, and predict the shear-wave splitting accrued along the raypath. Finally, we perform grid searches for orientation of de- formation, and attempt to minimise the misfit between predicted and ob- served shear-wave splitting. Splitting from the shallow local events is best explained by anisotropy confined to a 40 km over-riding plate with horizon- tal, trench parallel deformation. However, in order to explain the larger lag times from SKS and deeper events, we must consider an additional region of seismic anisotropy in or around the slab. The slab geometry in the model is constrained by seismicity and regional tomography models, and many SKS raypaths travel large distances within the slab. Models placing anisotropy in the slab produce smaller misfits than outside for most stations. There is a strong indication that inferred flow directions are different for sub-Sumatran stations than for sub-Javanese, with >60 ^◦ change over ∼375 km. The former appear aligned with the subduction plate motion, whereas the latter are closer to perpendicular, parallel to the trench direction. There are significant differences between the slab being subducted beneath Sumatra, and that beneath Java: age of seafloor, maximum depth of seismicity, relative strength of the bulk sound and shear-wave velocity anomaly and location of volcanic front all vary along the trench. We speculate, therefore, that the anisotropy may be a fossilised signature rather than due to contemporary dynamics.

Key words: Seismic anisotropy; Subduction zones; Indonesia; S-wave

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splitting

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1. Introduction

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At the Java-Sunda trench the Indian-Australian plates are being sub-

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ducted beneath the Eurasian plate. This is one of the most seismically ac-

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tive regions in the world, with earthquakes occurring from the surface to the

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base of the transition zone. This makes the region an ideal natural labora-

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tory to study subduction dynamics using seismic imaging techniques. Here,

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we present a study of seismic anisotropy (the variation of seismic wavespeed

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with direction) beneath the Java-Sunda subduction system, providing some

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insights into the deformation history and active processes in a subduction

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setting.

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As a seismic shear-wave propagates through an anisotropic medium it is

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split into two quasi shear-waves, one with particle motion in the direction of

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fast seismic wavespeed, and another perpendicular to this. This results in

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the two shear-waves travelling at different speeds and arriving at a seismic

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station separately; a phenomenon called shear-wave splitting (see Crampin

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and Booth, 1985; Silver and Chan, 1991). At a 3-component seismic station

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it is possible to measure these separate shear-waves and determine the fast

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direction of wave polarisation and the time between the shear-waves (see

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Silver, 1996; Savage, 1999; Kendall, 2000, for reviews).

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In many cases anisotropy in the upper mantle is assumed to be caused

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by the lattice preferred orientation (LPO) of olivine. Typically, the olivine

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fast axis (a-axis) aligns in the direction of upper-mantle flow (e.g., Babuska

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and Cara, 1991; Mainprice et al., 2000; Mainprice, 2007). As a result shear-

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wave splitting can provide direct information about dynamic processes, such

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as mantle flow, and accumulated strain due to previous deformation events

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Accepted Manuscript

which have ‘frozen’ a source of anisotropy into the lithosphere beneath a

40

seismic station.

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This technique has been applied to many subduction zone settings around

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the world, both using local slab related events, and teleseismic events (e.g.,

43

Fouch and Fischer, 1996; Long and Silver, 2008). The results are far from sim-

44

ple with fast directions appearing trench parallel, perpendicular and oblique,

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often at the same subduction zone [e.g., New Zealand, (Morley et al., 2006),

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Japan (Nakajima and Hasegawa, 2004), South America (Helffrich et al., 2002)

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and the Carribean (Pinero-Felicangeli and Kendall, 2008)]. This suggests a

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complex region of anisotropy and many different mechanisms have been in-

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voked to explain these results.

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Deformation in the overriding plate and return flow in the mantle wedge

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are discussed as causing trench parallel and perpendicular fast directions

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for New Zealand (Morley et al., 2006). The presence of water has been

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suggested to change the alignment mechanism of olivine, thus producing

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fast directions perpendicular to the direction of maximum stress (Jung and

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Karato, 2001) and this has been invoked to explain trench parallel splitting

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beneath Japan (Nakajima and Hasegawa, 2004), although the authors also

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acknowledge that trench parallel flow, caused by along strike dip variations

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might also explain these results. Evidence for deeper sources of anisotropy

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has also been described to explain splitting seen in teleseismic shear-wave ar-

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rivals, for example slab anisotropy in New Zealand (Brisbourne et al., 1999),

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sub-slab anisotropy due to toroidal flow beneath the slab in the Carribean

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(Pinero-Felicangeli and Kendall, 2008) and basal drag beneath the slab in

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South America (Helffrich et al., 2002).

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Some attempts have been made to explain subduction zone shear-wave

65

splitting observations with uniform global models. Long and Silver (2008)

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compiled global observations of shear-wave splitting at subduction

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zones. They postulated that the largely trench parallel splitting

68

observations which arise from sub-mantle wedge regions (i.e. slab

69

or sub-slab) are dominated by lateral flow in the sub-slab region.

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This is induced by return flow due to trench migration. Long and

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Silver (2008) explain mantle wedge anisotropy (above the slab) as

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being due to the interaction of lateral flow due to trench migration

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and corner flow driven by viscous coupling between the slab and

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the wedge. Such corner flow would induce a trench perpendicular fast

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direction in the splitting. The authors argue that the observed supra-slab

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trench parallel or trench perpendicular splitting results observed globally are

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due to the dominance of one or the other of these mechanisms.

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Another model invoked to explain global observations of subduction zone

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anisotropy suggests preferentially-oriented hydrated faults in the subducting

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slab (Faccenda et al., 2008). During the subduction process, faulting in the

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downgoing plate allows water to migrate into the crust and mantle. This

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results in the formation of sheets of silicates in the top few kms of the slab,

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inducing a shape preferred orientation, resulting in high degrees of anisotropy.

84

Superimposed on this, an alignment of serpentinized rocks will cause an

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additional crystallographic preferred orientation, thus giving the potential

86

for large degrees of shear-wave splitting from a thin layer at the top of the

87

slab (Faccenda et al., 2008).

88

The variability in shear-wave splitting results, and the wide variety of

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Accepted Manuscript

mechanisms described to explain them shows that subduction zone settings

90

are very complicated. Possible regions of anisotropy exist in the crust, litho-

91

sphere and mantle wedge, and in or below the slab. In this study we try to

92

understand the problem in more detail by forward modelling our shear-wave

93

splitting results with realistic subduction zone models.

94

2. Data and methodology

95

We use data from 9 stations of the JISNET (Japan Indonesia Seismic

96

Network) array (Ohtaki et al., 2000) to determine the anisotropic character-

97

istics beneath Sumatra and Java. The stations were deployed in 1998 and

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some stations are still recording. We use data up until the end of

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2003. We also use 2 stations from the GEOFON array (UGM, 9 years, GSI,

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3 years) and 1 from the Ocean hemisphere network project (PSI, 13 years),

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a station used by Long and Silver (2008) to investigate subduction zone

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shear-wave splitting. Station locations are shown in Figure 1.

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We employ earthquakes with distances between 85 ^◦ -140 ^◦ , events ideal for

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SKS/SKKS-wave splitting analysis. A total of 47 earthquake-station paths

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provided SKS/SKKS-phases of sufficient quality to enable splitting to be

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estimated (2% of data used). All SKS/SKKS data were filtered

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between 0.05-0.3 Hz. We also measure splitting in S-waves from local

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earthquakes where the incidence angle is less than 45 ^◦ [i.e., within the shear-

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wave window (Evans, 1984)]. Earthquake locations from the catalogue of

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Engdahl et al. (1998) are used where possible, others are taken from the

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USGS catalogue, which could introduce some significant mislocation

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errors into our results. This results in 127 measurements of splitting

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Accepted Manuscript

from local earthquakes (49% of data used). Individual results are shown

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in Tables S1-S4. All local earthquake data were filtered between 0.1-

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1 Hz. The overlap between SKS/SKKS- and local S-wave filter bands helps

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to minimise frequency dependent effects between the two data sets.

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We estimate shear-wave splitting in both the core phases and S-phases

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from local earthquakes using the method of Teanby et al. (2004), which is

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based on the methodology of Silver and Chan (1991). It uses a grid search

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over fast direction (φ) and time-lag (δt), minimising the second eigenvalue

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of the covariance matrix for the particle motion around a given time win-

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dow. The advantage of the Teanby et al. (2004) method is that it automates

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window selection, making 100 measurements around the relevant phase, and

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applying cluster analysis to identify the most stable result. Figure 2 displays

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a typical result of a local S- and teleseismic SKS-wave splitting result at PSI.

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3. Results

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From our initial analysis we reject results with 1σ errors greater than

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0.4 s in δt and 30 ^◦ in φ. Furthermore, the polarisation of the shear-wave af-

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ter the anisotropy correction is applied (source polarisation, hereafter spol)

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is compared with the back azimuth for SKS arrivals. For radially polarised

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phases (i.e a P-S conversion at the core/mantle boundary) these should be

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similar, thus we reject results with differences >30 ^◦ . Local earthquakes are

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not radially polarised as they have not been converted to a P-wave at any

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point along their ray path. However some earthquakes are large enough to

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have a previously determined focal mechanism (we use the Global CMT

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and USGS catalogue), from which the initial polarisation can be calculated.

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Accepted Manuscript

For these earthquakes the spol measurement and initial polarisation are com-

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pared to provide an extra quality assessment (10% of local splitting mea-

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surements included this check). Again spol and the initial polarisation

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must agree within 30 ^◦ to be included. Those results coming from earthquakes

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without an available focal mechanism are assumed to be correct. Results are

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shown in Figure 3 and Tables S1-S4. We discuss the results for Sumatra and

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Java separately in the following sections.

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3.1. Sumatra

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3.1.1. Local S-wave splitting

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Seismic stations BSI, PSI, PPI, KSI and KOTA are located approximately

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above the 150 km contour of the subducting Indian plate (based on the slab

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contours of Gudmundsson and Sambridge (1998)). As a result splitting is

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estimated in shear-waves generated from earthquakes in the ∼100 km-200 km

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depth range. One station, GSI, is located closer to the trench, and thus

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provides estimates of splitting from earthquakes in the depth range ∼30-

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80 km. All these splitting results show little variation with depth (Figure 4).

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Time-lags for all stations, including GSI, are generally between 0.2-0.4 s, and

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fast directions roughly trench, parallel, with some scatter, and parallel

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to the strike of the Sumatran fault (Figures 3 and 4). This indicates that

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the source of this anisotropy is in the over-riding plate and that the mantle

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wedge above the slab is largely isotropic.

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3.1.2. SKS/SKKS-wave splitting

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The stations located above the 150 km slab contour (BSI, PSI, PPI, KSI

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and KOTA) show ∼1 s of splitting, with a general north-south trend. How-

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ever some variation in the fast direction is evident, particularly at PSI (Figure

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3). The station close to the trench, GSI, is markedly different with a large

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time-lag of 1.8 s and a trench perpendicular fast direction of 52.5 ^◦ . The ob-

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vious discrepancy between the time-lags for local and teleseismic splitting

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suggests that a deeper source of anisotropy, within or beneath the slab, must

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be present.

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3.2. Java

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3.2.1. Local S-wave splitting

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Local earthquakes, within the shear-wave window, have been recorded

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down to depths of 700 km at most stations in Java. Splitting results from

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these events display considerable variation in both fast direction and time lag

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on individual stations as well as from different stations (Figure 3). Most sta-

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tions show both trench parallel and trench perpendicular fast directions. The

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trench perpendicular results are in general from intermediate depth earth-

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quakes (250-350 km), with both shallower and deeper earthquakes having

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more trench parallel fast directions. Some deep events produce large delay

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times and fast directions oblique to the strike of the trench (UGM, BRB).

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These seem isolated to one region of the subduction zone, with adjacent

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splitting measurements from nearby earthquakes showing much smaller delay

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times, albeit with similar fast directions. Splitting at BMI is very compli-

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cated, possibly because it lies above a part of the slab which bends beneath

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Sulawesi.

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3.2.2. SKS/SKKS-wave splitting

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In general SKS-wave results have an east-west fast direction, with ∼1.6 s

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of splitting (Figure 3). There does seem to be some subtle variation in the

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splitting results with the estimated fast directions occurring in two groups,

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∼75 ^◦ and ∼105 ^◦ . The magnitude of splitting is again larger than the splitting

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seen in local events, except for a few deep earthquakes at UGM, and BRB.

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Again, similarly to the results at Sumatra, this suggests a slab or sub-slab

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source of anisotropy.

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4. Modelling

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It is evident that more than one region of anisotropy exists beneath Suma-

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tra and Java. Shallow earthquakes (<300 km deep) beneath Sumatra and

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Java show little variation in time-lag with depth, and show consistent trench

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parallel fast directions. This suggests that the mantle wedge is isotropic, and

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that the splitting observed from these earthquakes is accrued in the over-

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riding plate. However larger time-lags observed in SKS/SKKS splitting at

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Sumatra and Java, and splitting from deeper (>300 km deep) earthquakes

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beneath Java suggest that anisotropy must exist in the slab, or sub-slab as

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well.

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To understand the anisotropy in more detail we have developed a tech-

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nique based on ray-tracing through an inhomogeneous subduction zone model

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(Guest and Kendall, 1993). We build a model consisting of four regions:

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layer 1, the over-riding plate and mantle wedge; layer 2, a thin

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10 km layer above the slab; layer 3, the subducting slab; and layer

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4, the sub-slab mantle (Figure 5). The surface of the slab is calculated

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using the regionalized upper mantle (RUM) seismic model slab contours

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(Gudmundsson and Sambridge, 1998) which is consistent with a relocated

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local seismicity study (Fauzi et al., 1996) and deep events from the USGS

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earthquake catalogue. The model is also consistent with regional tomogra-

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phy models for Indonesia (Gorbatov and Kennett, 2003). Uncertainty in

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these slab models could introduce error into these results, how-

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ever recently published slab models of Syracuse and Abers (2006)

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show only minor changes (∼20 km shift east) in Sumatra-Java sub-

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duction zone to those calculated by Gudmundsson and Sambridge

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(1998).

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We use ATRAK, a ray-tracer capable of tracking seismic rays in multi-

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layered 3-D media with general anisotropy (Guest and Kendall, 1993). This

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technique has been utilised to investigate anisotropy in other subduction

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zones [e.g., Kurils (Kendall and Thomson, 1992) and New Zealand

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(Brisbourne et al., 1999)]. Our model differs from previous studies by the

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fact that we trace rays through an isotropic model, assuming that anisotropy

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only minimally distorts the raypath (a reasonable assumption for relatively

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weakly anisotropic media). We trace rays for each station separately (except

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UGM and SWH which are very close together, and so are incorporated into

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one model). We appeal to reciprocity and shoot rays from the station, satu-

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rating the model (>12,000 rays per station, Figure 5). We then match each

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event to its nearest ray, thus defining the raypath taken from the earthquake

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to the station. For local earthquakes we use the latitude, longitude and

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depth to match the ray, and for SKS/SKKS phases we calculate the piercing

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point of the phase at a depth of 800 km beneath the station [through the

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Accepted Manuscript

AK135 1-D velocity model (Kennett et al., 1995)], and use this information

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as the event location (Figure 5). This ray-path for each station-event pair

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is then broken down into a length, time and angle spent in each part of

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the model. With this information we can then impose an anisotropy on a

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specified region of the model (layers 1-4), represented by an appropriately

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rotated elastic tensor (we use olivine elastic constants from Abramson et al.

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(1997)). Anisotropy is confined to the olivine stability field (i.e. <410 km),

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and can be restricted to a particular depth range within a layer.

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For each contiguous section of the raypath we can use the Christoffel equa-

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tion ( ˇ Cerven` y, 2001) to calculate the fast and slow shear-wave velocities and

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particle motion, thus, along with the distance in the layer, we can calculate

243

a time-lag and fast direction through the layer.

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We then perform a grid search over orientation (γ, ∆γ=5 ^◦ ), and

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anisotropic strength (by Voigt-Reuss-Hill dilution of the elastic

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tensor; denoted volume fraction aligned - VFA, ∆VFA=0.01–0.05).

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The angle γ is the orientation of the olivine a-axis in a plane which

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is either horizontal (shallow models) or inclined at the average dip

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of the subducting slab (deep models). The b-axis is held normal to

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this plane. For each node in the grid we calculate δt and φ for each

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event-station raypath in the data. The model is then evaluated

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as the summed misfit between these and the real measurements

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(for δt and φ separately, and for a combination). Typically, VFA

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is poorly constrained, so is not interpreted in great detail, instead

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we primarily analyse the best fitting orientation γ, illustrated by

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the density rotorgram in Figure 6.

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This can be performed for each layer in the model. Four models

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are run in total, the first constraining the layer 1 anisotropy, and

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the last three incorporate this result with layer 2, 3 and 4 respec-

260

tively. Multiple layers of anisotropy can be included, though only

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one may vary, as a result we can not formely asses trade-off between

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these layers. Where two layers of anisotropy are included we com-

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bine the splitting operators using the n-layer splitting equations of

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Silver and Savage (1994), and the initial source polarisation from

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the splitting measurements. An example result is shown in Figure

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6 for station GSI with anisotropy in layer 1 only.

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The locations of stations and earthquakes used in this study are ideal to

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image the various parts of the subduction system. We image paths in the

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sub-slab, slab, supra-slab, mantle wedge and over-riding plate. This enables

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us to go one step further than typical shear-wave splitting studies and place

271

constraints on the location of the anisotropy, and thus better understand the

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dynamics of Java-Sunda subduction.

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The modelling scheme outlined here allows us to extract information on

274

which part of the subduction system each event is sampling. Therefore, we

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are able to investigate further the observation made in section 3, that the

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mantle wedge appears largely isotropic. Figure 7 shows the variation in delay

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time as a function of path length through various parts of the model. It is

278

evident that there is no clear relationship between splitting delay times and

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path length in the mantle wedge. In fact, those data with paths almost

280

exclusively in the wedge display the smallest delay times (<0.4 s, except two

281

SKS events at PSI which, in our model, never sample the region below the

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wedge). An estimate can be placed on the amount of anisotropy in a 40 km

283

thick over-riding plate and the mantle wedge below. Figure 7a shows a linear

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fit to the data with paths exclusively in the wedge (removing the two SKS-

285

wave arrivals at PSI). The line has equation δt = 0.0007l + 0.1306, where l

286

is the path length in the mantle wedge and over-riding plate. Assuming an

287

average S-wave velocity of 4.5 km/s, this equates to an anisotropy of 1.8%

288

in a 40 km thick plate, and 0.3% anisotropy in the mantle wedge below this

289

plate. This shows that the anisotropy in the over-riding plate is dominating

290

the shear-wave splitting. In comparison, some correlation is evident between

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splitting delay time and path length spent in the region beneath the mantle

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wedge, but above 410 km (layers 2-4 in our model) (Figure 7b). The splitting

293

results can be explained by a model which has 20% aligned olivine crystals

294

along this path length.

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This observation supports the idea that the mantle wedge is largely

296

isotropic, and that anisotropy is largely confined to the over-riding plate,

297

slab and sub-slab region. To further constrain the location of the anisotropy

298

we split the modelling into two steps, shallow and deep anisotropy. To con-

299

strain the characteristics of the shallow anisotropy we use events with depths

300

<300 km, and to model deeper anisotropy we use events with depths >300 km

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and SKS-events.

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4.1. Shallow anisotropy

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We impose anisotropy in the uppermost 40 km of layer 1 of the

304

model, simulating the overriding plate lithosphere. The choice of this

305

thickness is fairly arbitrary (i.e. a layer half as thick with twice the anisotropy

306

strength will fit the data just as well), but we base our model on the limit

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Accepted Manuscript

of our splitting results (the shallowest events at GSI are ∼30 km). In these

308

cases the olivine a-axis is horizontal and the misfit is calculated

309

only using events shallower than 300 km.

310

Table 1 and Figure 8 show that the best fitting anisotropy orientations at

311

all stations are well constrained and show trench parallel orientations. BRB

312

is not modelled as we have too few splitting measurements. Errors in depth

313

locations could have an effect on our estimated VFA, however we

314

expect this effect to be minimal due to the fact that we impose

315

a 40 km thick layer of anisotropy at the top of the model. Events

316

which are mislocated from below 40 km to above 40 km, and events

317

which are mislocated to below 40 km from above 40 km will cause

318

the VFA to be over and underestimated respectively. This is only

319

a problem at GSI, where event depths range from 30-80 km. At

320

other stations only one event occurs shallower than 70 km (SWH,

321

33 km).

322

4.2. Deep anisotropy

323

To fit local slab events with depths >300 km, and SKS/SKKS phases

324

we test 3 classes of model where anisotropy is confined to a thin

325

layer above the slab (layer 2), the slab (layer 3) or the sub-slab (layer

326

4). We impose an olivine anisotropy with its a-axis oriented (γ)

327

in a plane inclined at the average dip of the subduction zone (b-

328

axis is normal to the plane). In all deep models we also impose

329

the best fitting shallow anisotropy for each station (determined

330

above). Thus, each model has two anisotropic regions, one fixed in

331

orientation and strength (shallow; layer 1) and one varying (deep;

332

Accepted Manuscript

layers 2–4).

333

Results are shown in Figures 8 and 9 and Table 2, and it is evident

334

that it is hard to confidently discriminate between the slab and sub-slab

335

models (layer 3 and 4), with both giving compatible anisotropy orientations.

336

Anisotropy in a thin layer at the top of the slab (layer 2) consistently has the

337

largest misfit, and requires a very high degree of alignment, suggesting this is

338

a less likely cause of our observations (Figure 9 and Table 2). For both Java

339

and Sumatra the misfit is considerably lower for a model where anisotropy is

340

confined to the slab (assuming a thickness of 100 km). A robust feature in all

341

models (with the exception of GSI) is that the deeper anisotropy orientation

342

beneath Sumatra is more north-south, and rapidly changes at the Sunda

343

strait becoming more east-west beneath Java (Figures 8 and 9). GSI shows

344

a trench perpendicular anisotropy orientation, suggesting that a different

345

mechanism is causing anisotropy at the trench compared to that beneath the

346

volcanic/back-arc region (Figure 8). GSI has the longest path in the

347

sub-slab region. The anomolous trench perpendicular anisotropy

348

orientation, and large time delays at this station may suggest that

349

sub-slab anisotropy is dominant in the forearc. BRB and KHK do

350

not have enough observations to model. Mislocated events for deep and

351

SKS/SKKS-wave events will have minimal effects on their raypaths.

352

5. Discussion

353

From the modelling it is evident that anisotropy in the over-riding plate

354

is required to explain the splitting observations from events <300 km deep

355

at Java and Sumatra. The modelled anisotropy orientation is near paral-

356

Accepted Manuscript

lel to the strike of the trench. The simplest explanation for this anisotropy

357

is vertically aligned cracks associated with deformation in the over-riding

358

Eurasian plate, with a largely isotropic mantle wedge below. This observa-

359

tion is supported by a recent local earthquake tomography study for central

360

Java, where high P-wave anisotropy (7-10%) is observed in the crust and

361

lithospheric mantle (Koulakov et al., 2009). Similar observations of over-

362

riding plate anisotropy and isotropic mantle wedges have been seen at New

363

Zealand (Morley et al., 2006), the Carribean (Pinero-Felicangeli and Kendall,

364

2008) and South America (Polet. et al., 2000). The fact that the wedge is

365

isotropic suggests that either 2D corner flow caused by viscous coupling of

366

the slab and wedge is of similar magnitude as lateral flow from trench mi-

367

gration, or that they are both weak in this region, thus meaning no coherent

368

LPO can be developed (Long and Silver, 2008). This observation is in drastic

369

contrast to other subduction zones such as Japan, where Hiramatsu et al.

370

(1998) observe two 75-175 km deep anisotropic zones that are caused by melt

371

filled cracks, Tonga where Smith et al. (2001) suggest that along strike flow

372

in the mantle related to slab rollback and the Samoan plume is inducing an

373

anisotropy and the Aleutians where Long and Silver (2008) suggest that 2D

374

corner flow is causing a large anisotropy. In all these regions they observe

375

large δt ∼1-2 s, much larger than we observe beneath Java and Sumatra.

376

Models of deeper anisotropy, constrained by deep subduction events, and

377

SKS/SKKS show that a slab source of anisotropy is most likely, although a

378

sub-slab source can not be ruled out and gives similar best fitting anisotropy

379

orientations. However, in all these models a sharp change in anisotropic

380

characteristics is observed between Sumatra and Java (Figure 8). This change

381

Accepted Manuscript

is also evident in the raw data (Figure 10). SKS fast direction estimates for

382

the same event recorded at KOTA, the southernmost Sumatran station, and

383

LEM the westernmost Javan station vary by ∼60 ^◦ over ∼375km (Figure 10).

384

This change in anisotropic characteristics is mirrored by changes in other

385

geophysical parameters close to the Sunda Strait. The most obvious change

386

near this location is in the age of the downgoing slab (Figure 1). Magnetic

387

anomalies show a change in age from ∼60Ma to ∼100Ma from southern

388

Sumatra to Western Java (Sdrolias and Muller, 2006). The sharpness, and

389

exact location of this boundary is unclear, especially at depth in the sub-

390

ducted slab. Syracuse and Abers (2006) observed that this age boundary

391

co-incides with a change in volcanic characteristics. They found that the

392

volcanic front in Sumatra is located above the 90 km slab contour, with an

393

abrupt change close to the Sunda Strait where volcanism is present above

394

the 150 km slab contour. This change occurs over <150 km, similarly to the

395

abrupt change in anisotropy, and also correlates with the abrupt reduction

396

in deep (>500 km) seismicity beneath Sumatra (e.g., Engdahl et al., 1998),

397

although no change in the shape of the slab is observed. Syracuse and Abers

398

(2006) also correlate this change in the location of the volcanic front with

399

variation in the ratio of potassium to silica (Wheller et al., 1987), but the

400

reasons for this are unclear (Syracuse and Abers, 2006). Regional tomogra-

401

phy profiles of Gorbatov and Kennett (2003) show an abrupt change in the

402

bulk sound speed in the slab close to the Sunda Strait further suggesting a

403

sharp change in the slab characteristics in this region. The possible causes of

404

the observed rapid change in anisotropy, with reference to the other changes

405

in geophysical characteristics are discussed further below.

406

Accepted Manuscript

5.1. Change in geometry

407

The Indian and Australian plates are subducting northwards beneath the

408

Java-Sunda arc. As a result subduction moves from normal to the trench

409

beneath Java to oblique subduction beneath Sumatra (most obviously char-

410

acterised by the Great Sumatran transform fault). This change in the geom-

411

etry of subduction across the region could potentially explain the change in

412

anisotropy observed in the splitting results.

413

The alignment of the deep anisotropy orientations at all stations beneath

414

Sumatra, except GSI, roughly matches with the APM of the subducting

415

slab and oblique to the orientation of the trench. This would suggest that

416

in this case shear associated with the subducting plate moving through the

417

mantle is aligning sub-slab olivine, and causing APM aligned anisotropy.

418

However, beneath Java, the opposite is true. The deep anisotropy orientation

419

is perpendicular to the APM of the subducting slab. The slab between

420

Sumatra and Java appears continuous (Syracuse and Abers, 2006), so it

421

seems unlikely that basal drag would create an anisotropy beneath southern

422

Sumatra, and not western Java <375 km away. Thus, basal drag does not

423

seem a likely cause of anisotropy.

424

Long and Silver (2008) suggest that lateral flow beneath the slab, pro-

425

duced by trench migration, can induce a trench parallel anisotropy orienta-

426

tion, and this is observed beneath Java. However, this would predict trench

427

parallel anisotropy orientations beneath Sumatra, and we observe trench

428

oblique orientations. Again, this change from lateral flow beneath western

429

Java, to an oblique flow beneath southern Sumatra seems unlikely over such

430

a short distance.

431

Accepted Manuscript

5.2. Water in the mantle

432

Jung and Karato (2001) have suggested that the presence of water can

433

make the b-axis of olivine align with the flow direction, manifesting in a

434

anisotropy orientation perpendicular to the direction of flow. Thus a change

435

in the amount of water beneath Java and Sumatra could induce a change

436

from APM parallel anisotropy orientations to APM normal anisotropy ori-

437

entations. The most likely region of water rich mantle is above the slab, in

438

the mantle wedge. We observe that the mantle wedge in this region is largely

439

isotropic, with little variation in time-lags with depth, suggesting that this

440

is an unlikely scenario to cause the sharp change in anisotropy.

441

5.3. Fossil anisotropy

442

Figure 5 shows that the deep and teleseismic events travel some distance

443

in the subducting slab, and thus anisotropy in the slab will manifest itself in

444

the observed splitting results. Interestingly, the sharp change in anisotropic

445

characteristics between Java and Sumatra co-incides with major changes in

446

slab characteristics.

447

The age of the subducting plate beneath Java and Sumatra varies between

448

100 Ma in the north to 43 Ma at 2 ^◦ S and up to 160 Ma in the east (Sdrolias

449

and Muller, 2006). This transition in ages occurs near the region where we see

450

an abrupt change in anisotropy with the plate subducting beneath Sumatra

451

40-80 Ma and beneath Java 80-140 Ma. Other features of the slab are also

452

different between these regions. Seismicity beneath Sumatra only extends to

453

a depth of ∼300 km and as the slab bends beneath Java it extends to the base

454

of the transition zone and this change in seismicity is located approximately

455

at the same location as an abrupt change in the bulk sound speed (Gorbatov

456

Accepted Manuscript

and Kennett, 2003) and change in the location of the volcanic front (Syracuse

457

and Abers, 2006). The abrupt changes in age and seismic properties suggests

458

that differences between the Javan and Sumatran slabs may be the cause for

459

the abrupt change in anisotropic characteristics.

460

Faccenda et al. (2008) show that preferentially oriented hydrated faults in

461

the uppermost part of the slab can produce a high degree of shear-wave split-

462

ting. A change in age between the slab beneath western Java and southern

463

Sumatra may effect the efficacy of fracture generation in the slab, and thus

464

hydrated faults may change in style or quantity across this region. However,

465

our models indicate that a thin highly anisotropic layer at the top of the slab

466

poorly fits our results, mainly due to the fact that rays spend such a small

467

amount of time in this region. If this layer behaves as a strong waveguide, a

468

feature not modelled by our simple subduction models, then this may explain

469

the observed shear-wave splitting.

470

Another possible location for a fossil anisotropy is in the slab lithosphere.

471

As oceanic lithosphere is formed and moves over the asthenosphere it cre-

472

ates a large anisotropic signature (e.g. at the East Pacific Rise Wolfe and

473

Solomon, 1998; Harmon et al., 2004), in the direction of plate motion. These

474

anisotropic characteristics can be frozen into the lithosphere, and may re-

475

main present in the subducted slab beneath Java and Sumatra. This mech-

476

anism has been suggested to be responsible for at least part of the observed

477

shear-wave splitting seen at the Marianas (Fouch and Fischer, 1998). Also,

478

Brisbourne et al. (1999) show that significant anisotropy, with trench par-

479

allel anisotropy orientations, exists in the Hikurangi subduction zone, New

480

Zealand. The Pacific plate in this region has a trench parallel age progres-

481

Accepted Manuscript

sion [i.e. the age of the Pacific oceanic lithosphere changes in a direction

482

parallel to the trench (Sdrolias and Muller, 2006)]. This is similar to that

483

seen beneath Java, suggesting that a fossilised anisotropic signature, frozen

484

in as the plate formed, could explain anisotropy in this region. The oceanic

485

lithosphere subducting beneath Java is 80-140 Ma, and Lithgow-Bertelloni

486

and Richards (1998) show that plate motions for the Australian plate be-

487

tween 74-119Ma are generally east-west. The oceanic lithosphere subducting

488

beneath Sumatra is 40-80 Ma, and Lithgow-Bertelloni and Richards (1998)

489

show that the plate motions for the Indian plate at this time were north-

490

south. The shear-wave splitting change observed between Java and Sumatra

491

could, therefore, be an effect of sampling these differing frozen anisotropic

492

signatures.

493

6. Conclusion

494

Shear-wave splitting has been measured in SKS-wave arrivals and S-waves

495

from local subduction zone earthquakes beneath the Java-Sunda subduc-

496

tion zone. Results show that the mantle wedge is mainly isotropic, with

497

anisotropy largely confined to the over-riding plate. This has been fur-

498

ther emphasised with modelling, which shows a best-fitting trench-parallel

499

anisotropy orientation, suggesting anisotropy seen in splitting from local

500

earthquakes is derived from anisotropy caused by vertically aligned cracks as-

501

sociated with deformation in the over-riding Eurasian plate. Splitting from

502

deeper earthquakes, and SKS-wave arrivals have much larger delay times,

503

suggesting a deeper source of anisotropy is present.

504

Modelling constrains the location of this anisotropy, with the most likely

505

Accepted Manuscript

anisotropic region being the slab. The best fitting anisotropy orientations

506

in the slab were predominantly north-south beneath Sumatra (except GSI

507

which lies above the trench, and shows trench perpendicular anisotropy ori-

508

entationss, suggesting a different mechanism to that seen at the volcanic

509

arc/back arc). This change in orientation occurs over less than 375 km and

510

at the same location as changes in other geophysical characteristics such as

511

slab age (Sdrolias and Muller, 2006), bulk sound speed (Gorbatov and Ken-

512

nett, 2003) and location of the volcanic front (Syracuse and Abers, 2006),

513

suggesting that a change in the slab characteristics is the reason for the

514

change in anisotropy.

515

We speculate that the anisotropy has been frozen into the subducting

516

lithosphere as it was formed, or moved over the asthenosphere. The subduct-

517

ing lithosphere beneath Java is older, and would have formed with eastward

518

plate motions (Lithgow-Bertelloni and Richards, 1998), resulting in east-west

519

anisotropy orientations, compared with the younger lithosphere subducting

520

beneath Sumatra which formed when the Indian-Australian plate was mov-

521

ing north (Lithgow-Bertelloni and Richards, 1998), resulting in north-south

522

anisotropy orientations.

523

7. Acknowledgments

524

We would like to thank Mike Kendall and George Helffrich for valuable

525

discussion and help improving this paper. Also, we would like to thank Alexei

526

Gorbatov for the use of his tomographic models. All the data used in this

527

paper was provided by the Japan Indonesian Seismic Network (JISNET),

528

IRIS, GEOFON and the Ocean Hemisphere Network Project, courtesy of

529

Accepted Manuscript

the Earthquake Research Institute, University of Tokyo. This research was

530

funded by a Japan Society for the Promotion of Science (JSPS) postdoctoral

531

fellowship (short term), JSPS/FF1/367

532

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