JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B9, PAGES 20,277-20,286, SEPTEMBER 10, 1999

Seismic anisotropy beneath the lower half of the North Island, New Zealand

Katrina Marson-Pidgeon 1 and Martha Kane Savage School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

Ken Gledhill

Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

Graham Smart

School of Earth Sciences, University of Leeds, Leeds, England, United Kingdom

Abstract. Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6 + 0.1 s. This lack of.variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions.

1. Introduction

Shear wave splitting of teleseismic phases such as ScS and SKS is now a common method used to investigate mantle anisotropy, which can in turn be related to deformation of the upper mantle [e.g., Silver, 1996]. This phenomenon occurs when a shear wave enters an anisotropic medium, upon which it is split into two orthogonally polarized waves which travel with different velocities [e.g., Crampin, 1981 ]. Shear wave splitting measurements can be characterized by two parameters; the polarization direction of the fast shear wave, q•, can be related to the symmetry of the anisotropic system, and the time separation between the two waves, fit, can be related to the strength of anisotropy and the path length through the anisotropic material. Thus, if the cause of the anisotropy is known, the splitting parameters can be related to the deformation and tectonic structure of a region. For example, mantle anisotropy is usually assumed to be due to strain- induced lattice-preferred orientation of olivine. The polarization direction of the fast shear wave is then assumed to align parallel to the mantle flow direction, if it is in the form of

•Now at Research School of Earth Sciences, Australian National University, Canberra, Australia.

Copyright 1999 by the American Geophysical Union.

Paper number 1999JB900212. 0148-0227/99/1999JB900212509.00

progressive simple shear [Ribe, 1989], or to the direction of maximum finite extension [Ribe, 1992].

This study expands on that of Marson-Pidgeon and Savage [1997] in which teleseismic S, ScS, and SKS splitting results obtained at the permanent station SNZO, situated in South Karori, New Zealand (Figure 1), are reported. The ScS delay times were found to be significantly smaller than the S and SKS delay times, which the authors suggest is due to differences in dominant frequency. A near-linear increase in delay time with period is observed, which can explain the small ScS delay times, and suggests that frequency-dependent anisotropy exists beneath station SNZO. The cause of this is attributed to the existence of small-scale oriented

heterogeneities. In this paper we report splitting results obtained from teleseismic ScS and SKS events recorded on

nine temporary broadband seismograph stations covering the lower half of the North Island of New Zealand (POMS II array) (Figure 1). These results can then be compared to those obtained at SNZO by Marson-Pidgeon and Savage [ 1997].

The study area lies above the Hikurangi subduction zone (Figure 1), which is the southernmost extension of the Tonga- Kermadec subduction system. The subduction of the Pacific plate beneath the Australian plate is oblique to the direction of relative plate motion (Figure l a). The major geologic features, such as the mountain ranges and active faults, tend to follow the NE-SW trend of the trench. The subduction

terminates in the northern South Island, where it gives way to the Alpine fault system. The top of the subducted plate is present at a depth of only around 20 km beneath the southeasternmost stations, increasing to a depth of around

20,277

20,278 MARSON-PIDGEON ET AL.' ANISOTROPY BENEATH LOWER NORTH ISLAND

171 ø 172 ø 173 ø 174 ø 175 ø 176 ø 177 ø 178 ø

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(a) 171 ø

Australian Plate

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SKS SKS SKS ScS SKS

I

100 200 300 400

Distance (km)

-- 50

-- lOO

- 150

-- 200

-- 25O

-- 300

- 350

Figure 1. (a) A map of the lower North Island of New Zealand, showing the location of the POMS II broadband stations, along with the major faults. The location of station SNZO is also shown, along with the stations in the Tararua array (small triangles). The stations LMAU and LKIR are in the same location as two of the stations in the Tararua array. The dashed lines show the approximate position of the subducting Pacific plate at depth (200 and 150 km contours from Adams and Ware [1977] and 60, 40, and 20 km contours from Ansell and Bannister [ 1996]). The dotted line shows the approximate position of the Hikurangi trough. Inset shows the tectonic setting of New Zealand, the arrows indicate the direction of relative plate motion. (b) Schematic cross section along the line A-A' indicated in Figure la showing the positions of the stations and various ray paths in relation to the subducted slab. Ray paths were calculated using Snell's law, assuming that the slab has an 8% higher velocity than the surrounding mantle.

MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND 20,279

150 km beneath the northwesternmost station (Figure 1). Thus, by comparing splitting measurements obtained at each station we are able to investigate anisotropy in both the mantle above the slab, referred to as the mantle wedge, and the mantle below the slab, referred to as the subslab mantle. This is the first time such a study has been done in New Zealand, as previous studies have only used stations which lie above the forearc region of the subduction zone. A study of shear wave splitting using local earthquakes recorded at SNZO has determined shear wave velocity anisotropy of 4.4% _+ 0.9% with a fast polarization direction of 29 ø _+ 38 ø for the subducting slab, from 20 to 70 km depth, beneath SNZO [Matcham et al., 1999]. This has been combined with other studies in the same region to determine a depth distribution of anisotropy beneath SNZO. Splitting measurements obtained from events recorded on the Tararua array (Figure 1 a) give q): 28 ø + 11ø, 1St: 1.5 + 0.4 s for SKS [Gledhil! and Gubbins, 1996] and q): 41 o + 15 ø with a clear increase in delay time with depth for intermediate-depth events [Gledhill and Stuart, 1996]. Two of the Tararua stations are in the same location as two of the stations in the POMS II array (LMAU and LKIR). Observations of long-period quasi-Love wave anomalies recorded at station SNZO by Yu and Park [1994] give a NNE-SSW fast direction in the upper mantle near the Tonga-Kermadec trench, NE of New Zealand. Travel times from events recorded on stations in the southwest Pacific were

studied by Galea [1993], revealing a maximum P velocity direction of N62øE in the mantle lid.

2. Data

A network of nine broadband seismometers was operated by Leeds University jointly with the Institute of Geological and Nuclear Sciences (IGNS) and Victoria University of Wellington (VUW) from July 1993 to June 1994. The stations covered the lower half of the North Island, with a spacing of around 100 km (Figure 1). The instruments recorded continuously, at a sample rate of 20 Hz, and the data were separated into earthquake archives using the preliminary determination of epicenters (PDE) origin times. Only events with magnitudes >5.5 were archived, giving a total of 3 13 possible events.

One advantage of using teleseismic shear phases such as ScS and SKS is that owing to their near-vertical incidence, anisotropy beneath the stations is well resolved laterally. Another advantage is that the longer periods are less affected by scattering than short-period local studies. Each event was visually inspected, and only those with isolated impulsive arrivals which fit the following criteria were subsequently analyzed. The ScS events had to occur at distances <35 ø to ensure that there is no phase change on reflection from the core-mantle boundary (CMB) and at depths >400 km to limit source-side crustal/upper mantle anisotropy. The SKS events had to occur at distances >85ø to ensure that the waveform i s

not contaminated by interference with other phases. It does not matter at which depth the SKS events occur because SKS phases are only affected by anisotropy on the receiver side of the path due to the P-to-S conversion at the CMB. No direct S or SKKS phases were used. Two ScS and 11 SKS events that fit the criteria and produced clear arrivals were used. The ScS events occurred at depths of 555 and 434 km. A list of the

events used in this study is given in Table 3 of Marson [1997] (available as an electronic supplementl).

A high level of microseismic noise was observed owing to the island nature of New Zealand, and each trace had to be filtered before analysis. A two-pole Butterworth filter was applied to the SoS traces with corners at 0.1 and 0.3 Hz and to the SKS traces with corners at 0.04 and 0.1 Hz.

3. Method

The splitting parameters q) and •St are estimated using the methe'd of Silver and Chan [ 1991 ]. This method searches all the possible splitting parameters to find the pair that most successfully corrects for the effects of anisotropy. For the case of ScS this is obtained by minimizing the smallest eigenvalue of the corrected covariance matrix, giving an estimate of the incoming polarization also. In a laterally homogeneous isotropic Earth, SKS should be radially polarized; thus the presence of significant energy on the transverse component may be an indication of anisotropy. In this case, the splitting parameters are estimated by minimizing the energy on the corrected transverse component.

In order to assess how well the estimated splitting parameters (qb, •St) correct for anisotropy a number of diagnostic plots are produced and inspected. These plots are also important to determine whether the shear wave splitting characteristics are caused by other phenomena, such as scattering. An example of the diagnostic plots produced for the case of SKS is given in Figure 2. Each event was given a rating of "good" or "marginal" in order to give an indication of how reliable the measurement was. Measurements which produced similar fast and slow pulse shapes and linear particle motion after correction, along with fairly small contours, were given the rating "good." Measurements which did not fit these criteria were given the rating "marginal."

4. Results

The results of the individual ScS and SKS splitting measurements made at each station are given in Table 4 of Marson [1997] (available as an electronic supplement). The SKS splitting measurements are shown in Figure 3, and the average SKS splitting values at each station are given in Table 1. These were formed from weighted averages of the individual good measurements, placing lower allowable limits on the standard deviations of • and b! of 5 ø and 0.2 s, respectively. The ScS measurements could not be included in the station

averages as these give consistently smaller delay times compared to SKS. This may be due to frequency effects, as was found at station SNZO [Marson-Pidgeon and Savage, 1997]. Testing for frequency dependence using data from the POMS II array will be discussed later in this section.

The splitting values are remarkably consistent for all of the stations (Figure 3), despite the ray paths sampling different

•Thesis tables 3, 4, and 6 are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username - anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800-966-2481; $15.00. Payment must accompany order.

20,280 MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND

(a) ß

Radial

Transverse

Vertical

ß . .

20 40 60 Seconds

(b)

•••F Cørrected Radial

••••F Cørrected Transverse i i • i i i i i

0 20 40 60 Seconds

40 60 40 60 =

<-5O

(c) (d) Lag (s)

Figure 2. (a) Original (filtered) radial, transverse, and vertical component seismograms, displaying the time interval (between A and F) used to make the measurement. (b) Corrected radial and transverse components. Note that the corrected transverse component has been minimized. (c) (top) Superposition of fast (solid) and slow (dashed) components, displayed (left) uncorrected and (right) corrected. Note that similar pulse shapes are observed in the fast-slow coordinate frame. (bottom) Corresponding particle motion diagrams. Note that the elliptical particle motion becomes linear when corrected. (d) Contour plot of the energy on the corrected transverse component, with minimum energy shown as a star along with the 95% confidence interval (thick contour) and multiples of that contour interval.

parts of the mantle wedge and subducted plate. The average fast polarizations at each station range between 37 ø + 7 ø and 52 ø + 4 ø (approximately NE-SW), and the average delay times range between 1.5 + 0.1 s and 1.8 + 0.2 s (Table 1). Any variation between station averages is probably not significant, taking into account the fairly small number of events recorded at each station and the uncertainties

associated with the splitting measurements. Thus an average q• and fit can be calculated using all the good SKS measurements obtained at each station as q• = 46 ø + 2 ø and/St = 1.6 + 0.1 s. As there is only limited back azimuthal coverage, we cannot tell if there is any azimuthal variation in the splitting parameters.

Because a number of stations have been used in this study, it is enlightening to plot the waveforms recorded at each station for the same event on one plot in order to compare them. An example is given in Figure 4 for event 93246. The original radial and transverse components for each station are displayed on the left, along with the same components rotated into average fast (46 ø ) and slow (136 ø ) components, displayed on the right. Similar waveforms are recorded at each station, and also similar delay times, given by the difference between the fast and slow components, are obtained at each station (except SNZO).

Measurements where little or no splitting is observed are termed null measurements and are characterized by error

ellipses that are narrow in the q• direction and highly elongated in the fit direction. Null measurements are consistent with either an isotropic material or an incoming polarization direction (q•p)which is close to the fast or slow direction of anisotropy; thus there is no corresponding orthogonal component to split. Fourteen SKS null measurements were obtained in the following way. Any measurements yielding q• within 15 ø of q•p or q•p + 90 ø were considered nulls. The diagnostic plots of such events were subsequently analyzed as a double check. In all cases, there was little energy on the original transverse component, and error ellipses like those described above were found.

Inconsistent splitting measurements are obtained at station LBLU, and the presence of high energy on the vertical component which is correlated with energy on the transverse component is observed at this station and at LKIR for event 94119. This suggests that scattered energy may be affecting these measurements, and we do not include them in our

averages.

In the case of ScS phases the original shear wave polarization will depend on the focal mechanism of the earthquake. This fact can be used to check the measured splitting parameters. The original polarizations calculated by correcting for the observed shear wave splitting were compared to those predicted fi'om focal mechanisms [Marson,

MARSON-PIDGEON ET AL.' ANISOTROPY BENEATH LOWER NORTH ISLAND 20,281

174 ø 175 ø 176 ø 177 ø

_39 ø

_40 ø

_41 ø

_42 ø

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ß ß

ß .

1s

1.5 s•

2s

SCALE

I I

50 km

174 ø 175 ø 176 ø 177 ø

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_41 ø

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Figure 3. Map of SKS splitting measurements made at each station in the POMS II array, represented by a line oriented parallel to q), with length proportional to fit as given by the scale. Good SKS measurements are shown as solid lines; marginal SKS measurements are shown as dotted lines. The average SKS measurement at SNZO is also shown [Marson-Pidgeon and Savage, 1997], along with the stacked SKS measurement for the Tararua array [Gledhill and Gubbins, 1996].

1997, Table 6] (available as an electronic supplement). The focal mechanisms for the two ScS events used in this study were obtained from the Harvard centroid moment tensor

solutions [Dziewonski et al., 1994]. An average difference of around 15 ø was calculated, indicating that correcting for the observed splitting produces polarizations that are close to those expected from the focal mechanisms.

The ScS measurements give consistently smaller delay times compared to SKS, as was found for SNZO [Marson-Pidgeon and Savage, 1997], suggesting that this may be due to frequency-dependent anisotropy. Testing for frequency dependence was performed as described by Marson-Pidgeon and Savage [1997]. Events were filtered using overlapping, constant width frequency limits, measuring the period of the signal and making splitting measurements in each case. However, the results were inconclusive due to lack of sufficient broadband energy.

5. Discussion

5.1. Comparison With Other Studies in the New Zealand Region

Gledhill and Gubbins [1996] obtained SKS splitting parameters of q•- 28 ø +_ 11 o, •St- 1.5 + 0.4 s by stacking all stations in the Tararea array, which are in reasonable agreement with the average station values at LKIR (q• - 44 ø +_ 3 ø, •St- 1.5 +_ 0.1 s) and LMAU (q• - 48 ø +_ 3 ø, •St - 1.5 +_ 0.1 s) and indeed with all the average station values (Table 1). The

fast polarization obtained by Gledhill and Stuart [1996] using local intermediate-depth earthquakes (q) = 41 ø + 15 ø) is in better agreement with our station average fast polarizations than the average SKS fast polarization obtained by Gledhill and Gubbins [1996]. Perhaps any differences are due to underestimation of our uncertainties; the heavy filtering that we have to apply has probably made our error estimates artificially low.

Table 1. Average SKS Station Values

Station Latitude, øN Longitude, øE q• 8t

LASH -40.31 175.81 44 + 5 1.8 + 0.2 LBRU -41.32 175.38 37 + 7 1.7 + 0.3 LKER -39.64 176.38 44 + 6 1.6 + 0.2 LKIR -40.81 175.56 44 + 3 1.5 + 0.1 LMAU -40.97 175.02 48 + 3 1.5 + 0.1 LMOA -39.60 175.88 38 + 5 1.5 + 0.1 LSTR -39.27 174.60 48 _+ 4 1.7 _+ 0.2 LWAL -40.19 176.52 52+ 4 1.7 +0.2 SNZO -41.31 174.70 56 + 3 2.6 _+ 0.1

Only the good SKS measurements are used to calculate the averages, except for stations LBRU and LSTR. For these two stations, only one good measurement was made, so the good and marginal measurements were included in the averages. No station average is shown for station LBLU due to the possibility that the measurements are contaminated by scattering. Also shown is the SKS average for station SNZO.

20,282 MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND

LSTR /%

_

0 50 O0

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i

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LWAL

LMOA

LKIR

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LMAU

LSTR

SNZO

i i i

_

_

_

_

_

_

_

_

_

_

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F LKER

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I 1 0 50 100

fast is solid, slow is dotted

Figure 4. (left) Original SKS radial and (middle) transverse components along with (right) the same components rotated into average fast (solid) and slow (dotted) components, for event 93246. Station SNZO is also shown for comparison. The events are lined up by distance from the earthquake and are plotted on the same absolute amplitude scale.

In olivine the fast axis for P waves is the same as the fast

axis for S waves and is along the olivine a axis direction [e.g., Babuska and Cara, 1991]. The average fast polarization calculated using all the good SKS measurements at each station (qb - 46 ø + 2 ø) is in reasonable agreement with the orientation of the fast P velocity axis (NNE-SSW) obtained by Yu and Park [1994] for the upper mantle NE of New Zealand. This average fast polarization also agrees reasonably well with the direction of maximum P velocity (N62øE) inferred by Galea [1993] for the mantle lid in the southwest Pacific.

The splitting parameters obtained fi'om the POMS II array can be compared to those obtained by Marson-Pidgeon and Savage [1997] for the permanent station SNZO, situated in South Karori. The average SKS splitting parameters at SNZO are qb - 56 ø + 3 ø, fit: 2.6 + 0.1 s, compared to those obtained from the POMS II array, which are qb: 46 ø + 2 ø, fit - 1.6 + 0.1 s. The difference in fast polarization is probably not significant; however, there does appear to be a significant difference in SKS delay time of N1 s. The average ScS splitting parameters at SNZOareqb=37 ø +6 ø ,1St- 1.1 + 0.1 s, compared to those obtained from the POMS II array, which are qb: 40 ø + 5 ø, fit: 0.8 + 0.1 s. The ScS fast polarizations are consistent, and the difference in ScS delay time of 0.3 + 0.2 s is barely above the error estimate. The difference in SKS delay times between POMS II and SNZO does not appear to be due to a frequency effect, as the measurements were made at similar frequencies. A fairly shallow, localized region of strong anisotropy beneath SNZO, such as that determined by Matcham et al. [1999] for the subducting slab, could explain the difference in SKS delay times. The ScS delay times are around half the $KS delay times,

as was found for SNZO, suggesting that the POMS II data may be frequency-dependent. However, there may be a variation in frequency dependence between SNZO and the POMS II array. The following sections will concentrate on explaining the lack of variation between stations in the POMS II array.

5.2. Localization of Anisotropy as a Function of Depth

One possible origin of the anisotropy is in the source-side mantle for the ScS events. Source-side anisotropy can be ruled out for the SKS phase, as it travels as a P wave through the outer core [e.g., Silver and Chan, 1991]. However, the ScS splitting appears to be dominantly due to receiver-side anisotropy for the following reasons. The two ScS events used in the study occurred at depths of 434 and 555 km, restricting their source-side paths to the transition zone and lower mantle. The lower mantle is thought to be effectively isotropic [e.g., Meade et al., 1995], but anisotropy in the upper transition zone (410-520 km) is thought to occur intermittently in some regions [Fouch and Fischer, 1996], which may have affected the 434-km-deep ScS event. However, results from the two ScS events (which occurred at slightly different locations and had different initial polarizations) are consistent with each other and also give consistent fast polarizations with the SKS events. A localized patch of anisotropy in the D" region could possibly explain the delay time discrepancy between ScS and SKS. The ScS waves would travel through this anisotropic patch twice, whereas the SK$ waves would only travel through it once. Although this explanation cannot be ruled out, our investigations show that

MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND 20,283

frequency-dependent anisotropy is a more likely explanation for the delay time discrepancy.

The similarity in splitting measurements obtained at each station could be due to a wide region of similar anisotropy beneath the stations. At shallow depths the Fresnel zones of the rays do not overlap; therefore this anisotropic region would need to be at least 200 km wide, since the greatest distance between stations is around 200 krn. Another

possibility is that the rays are all sampling the same narrow anisotropic region at depth. This requires the Fresnel zones of the rays to overlap; therefore the depth of such a region can be constrained using Fresnel zone calculations [Sheriff, 1980, equation (4)]. The greatest distance between stations is around 200 km, and at this distance the Fresnel zones overlap by 50% at a depth of around 1600 kin. Thus, if the same deep anisotropic region is responsible for producing the splitting at each station, then this region has to lie in the lower mantle, which seems highly unlikely for a number of reasons. First, the lower mantle is thought to be isotropic, as mentioned above. Second, and more importantly, there is a constraint on the depth extent of the anisotropy in the vicinity of stations LMAU and LKIR from the study of local earthquakes recorded on the Tararua array by Gledhill and Stuart [1996]. An increase in delay time with depth was found for earthquakes occurring down to depths of around 260 km, translating into shear velocity anisotropy of around 1.4%. Thus it appears that anisotropy exists in at least the upper 260 krn in this region, ruling out a single source of anisotropy deeper than 1600 kin.

Contributions to the anisotropy observed in subduction zones can come from the crust, mantle wedge, slab, and subslab mantle. Figure lb shows various ray paths in relation to the different contributions. Rays arriving at the northwestern stations travel through the subslab mantle, slab, mantle wedge, and crust, whereas rays arriving at the southeastern stations travel only through the subslab mantle, slab, and crust. However, Gledhill and Stuart [1996] show that anisotropy exists throughout the upper 260 krn, and possibly deeper, in the Tararua region. This rules out the possibility of the anisotropy being located exclusively in the crust and/or slab in this region.

Gledhill and Stuart [ 1996] found a maximum delay time of 0.2 s for shallow events, which translates to a pervasive crustal shear wave velocity anisotropy of-•4%. We will assume that this same crustal anisotropy exists beneath all our stations. Assuming that the same depth dependence of delay time that Gledhill and Stuart [1996] found exists beneath all the stations, then the simplest interpretation of the lack of variation in splitting results is that similar fast polarizations and shear wave velocity anisotropy exist in the mantle wedge, slab, and subslab mantle. There are, of course, other possibilities such as an isotropic mantle wedge, which would require a higher shear velocity anisotropy in the subslab mantle beneath the northwestern stations compared to the southeastern stations in order to have similar delay times. Various mechanisms which could produce the observed seismic anisotropy are discussed in the following sections.

5.3. Slab Anisotropy

Rays travel varying path lengths in the different regions, yet uniform results are obtained at all the stations. For example, rays arriving at the northwestern stations have significantly longer path lengths in the slab than rays arriving

at the southeastern stations. The possibility that the greater dip of the slab beneath the northwestern stations could trade off against the longer path length was investigated using the ANISEIS processing code [Taylor, 1995]. The slab was modeled using a transversely anisotropic medium (hexagonal symmetry), consisting of 1/3 olivine and 2/3 isotropic material, with a fast axis oriented 46 ø from north (being the average $KS fast polarization direction). The slab beneath the southeastern stations was modeled using a thickness of 50 krn (corresponding to the path length through the slab) with a 15 ø downward dip of the fast axis. The slab beneath the northwestern stations was modeled using a thickness of 125 km (corresponding to path length) with a 50 ø downward dip. We determine that the northwestern model contributes >3

times the delay time of the southeastern model, ruling out the possibility that the anisotropy exists exclusively in the slab.

One possible source of anisotropy in the subducted slab is the presence of stress-aligned fluid-filled cracks. This possibility was considered by Gledhill and Stuart [1996], who suggested that the anisotropy in the slab could be caused by aligned cracks because compression along the strike of the subduction zone within the slab in the lower seismic zone

was consistent with crack-induced NE-SW fast polarizations. However, they also note that there is some doubt whether cracks could remain open at such depths.

Slab anisotropy may also result from fossil mineral alignment in the subducting oceanic plate. Anisotropy in oceanic plates is thought to be caused by the alignment of olivine crystals due to the mechanism of seafloor spreading, with the fast axis parallel to the spreading direction at the time of formation [e.g., Hess, 1964]. Unfortunately, this portion of the Pacific plate was formed during the Cretaceous long normal polarity, so there are no magnetic lineations to check the fossil spreading direction. However, other estimates of anisotropy in the nearby plate are consistent with these results and may indicate fossil spreading directions. For example, Shearer and Orcutt [1985] infer a fossil spreading direction of N30øE using P wave travel time data from a seismic refraction experiment in the southwest Pacific. Galea [1993] found a maximum P velocity direction of N62øE in the southwest Pacific, which may indicate a fossil spreading direction along N62øE. ¾u and Park [1994] also suggested that the lateral variations in anisotropy they observed near the Tonga-Kermadec trench may be due to variations in the fossil spreading direction.

5.4. Fast Polarization Directions

The principal results of interest are the direction of the fast polarization with respect to the subduction zone and local geological features and that similar fast polarizations are found in the mantle wedge and subslab mantle. The average NE-SW fast polarization direction is subparallel to the strike of the trench and the geological structures in the lower North Island, such as the mountain ranges, active faults, and major shear systems, and also agrees with the fast P velocity axis found in the southwest Pacific by Galea [1993] and ¾u and Park [1994].

Conventional models of mantle flow in subduction zones

suggest that this flow is predominantly two-dimensional and is entrained by slab motion. In this case, flow of material in the mantle wedge is controlled by the relative plate motion (RPM) between the subducting and overriding plates, producing an

20,284 MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND

olivine a axis roughly parallel to the downdip slab direction [e.g., Ribe, 1989]. Entrained mantle flow beneath the slab should be parallel to the direction of absolute plate motion (APM) of the subducting plate. However, the average fast polarization direction obtained in this study is closer to the strike of the subduction zone (approximately NE-SW) rather than the RPM (approximately east-west) or the APM (approximately NW-SE) directions. Thus the conventional model of two-dimensional entrained mantle flow is not

consistent with the data. Instead, other models of flow in the mantle wedge and subslab mantle need to be explored.

5.5. Lithospheric Mantle Wedge Anisotropy

Station LSTR sits ---150 km above the top of the subducted plate (Figure 1), so the portion of the mantle wedge sampled is likely a combination of upper plate lithosphere and asthenosphere. First, we will consider two main causes of lithospheric mantle wedge anisotropy; aligned melt-filled cracks and lattice-preferred orientation (LPO) of olivine due to strain.

Some studies suggest that anisotropy in the mantle wedge may be caused by melt-filled cracks [e.g., Ando et al., 1983; Blackman and Kendall, 1997]. In this case the cracks and the fast polarization direction should be parallel to the maximtun principal stress direction. Composite focal mechanisms for earthquakes in the overlying Australian plate indicate east- west compression [Robinson, 1986], in agreement with the direction of plate convergence. Assuming that this same stress field exists in the mantle wedge, then crack-induced anisotropy would produce an east-west fast polarization which is inconsistent with the observed fast polarizations. Instead, the observed NE-SW fast polarizations would require a maximum principal stress direction parallel to the trench, which is unlikely.

Thus the most likely source of mantle wedge anisotropy in this case is LPO ofolivine. Olivine is a major constituent of the upper mantle, is anisotropic, and tends to develop preferred mineral orientation. It is generally accepted that deformation in the mantle causes the observed LPO of olivine.

Large strain by progressive simple shear is expected to orient olivine a axes parallel to the flow direction [e.g., Ribe, 1989]. Finite strain is thought to align the a, b, and c axes with the extension, compression, and intermediate axes of the strain ellipse, respectively [e.g., Ribe, 1992]. Once a mechanism relating deformation to anisotropy is assumed, we must determine how tectonic processes cause the deformation.

One explanation for fast polarizations parallel to the strike of the trench was given by Yang et al. [1995], who studied mantle wedge anisotropy beneath the Shumagin Islands segment of the Aleutian-Alaska subduction zone. They note that fast polarizations roughly parallel to the plate boundary were also found by Shih et al. [1991] in northeastern Colombia and Fischer and Yang [1994] in the Kamchatka Peninsula. In all three regions, convergence is roughly trench- normal, and they suggest that LPO of olivine in these three regions may be produced by arc-orthogonal compression that is accompanied by arc-parallel or vertical shearing or extension. These three regions also have smaller delay times than are found in New Zealand.

In New Zealand, however, the plate boundary is characterized by oblique convergence rather than trench- normal convergence. In regions of oblique convergence the dominant motion in the upper plate may be strike-

slip/transcurrent rather than thrusting/compression [Vauchez and Nicolas, 1991]. This is supported by predominantly strike-slip movement in the lower North Island [e.g., Beanland, 1995]. Compressional deformation with a large transcurrent component should align the foliation plane vertically, with a possibly nonhorizontal lineation, producing a fast polarization parallel to the strike-slip deformation, that is, parallel to the plate boundary and the trend of the mountain belts. This form of deformation provides the largest splitting [e.g., Mainprice and Silver, 1993] and may explain the splitting values obtained here, which are among the highest in the world.

The fast polarization directions in the lithospheric portion of the mantle wedge are parallel to the geological structures in the lower North Island, such as the active faults and mountain ranges. This supports the hypothesis of vertically coherent deformation (VCD), in which the crust and mantle lithosphere deform coherently [e.g., Silver, 1996]. In active tectonic regions, such as New Zealand, the crustal geology is an indicator of present deformation, which in turn produces the observed anisotropy.

5.6. Asthenospheric Flow Above and Below the Slab

A portion of the mantle wedge anisotropy is likely due to LPO of olivine as a result of asthenospheric flow, as is most of the subslab anisotropy. The a axes ofolivine are expected to become aligned with the flow direction when the flow is in the form of progressive simple shear, as is expected to occur above subducted slabs [Ribe, 1989]. The comer flow model of Ribe [1989] predicts flow parallel to the downdip slab direction both above and below the slab (Figure 5a). However, in this case, the NE-SW fast polarization suggests that the flow is in a trench-parallel direction (Figure 5b).

Evidence for trench-parallel flow in the mantle beneath the subducting Nazca plate was documented by Russo and Silver [1994]. They suggest that this implies that the subducted lithosphere must be largely decoupled •om underlying asthenosphere. They also suggest that the primary influence of mantle flow is due to slab motions normal to the slab surface.

The Nazca trench is moving away •om the overriding plate, and this is referred to as retrograde motion. They suggest that retrograde motion, along with a barrier to flow at depth, would force flow laterally in the trench-parallel direction.

An alternative model is presented by Bock e! al. [1998] in which the effect of retrograde motion is neglected. Instead, the slab plays only a passive role in the form of a static barrier to mantle flow that is present through other causes. Thus the mantle may be deflected horizontally into a trench-parallel direction. Our results seem to support this model for the New Zealand region because if the slab is acting as a passive rigid barrier to flow, then the mantle both above and below the slab

should flow in the same trench-parallel direction, as has been observed in this study. However, we note that preliminary evidence suggests similar q), with perhaps larger 1St, occurs in the South Island where no subduction is occurring, and we have not found a location where return flow around the slab is

causing alternate polarization. Thus flow may be contributing less than shearing.

5.7. Similar q) in Mantle Wedge and Subslab Mantle

In active tectonic regions, such as New Zealand, the observed anisotropy is produced by present deformation. In

MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND 20,285

Conventional model Preferred model

Mantle wedge (lithosphere• •

, Ma. ntle we. dge, •asthenospnere)

Subslab mantle (asthenosphere)

(a)

Figure 5. Schematic diagram comparing the conventional model of mantle flow in subduction zones with the preferred model obtained from this study. (a) Conventional model of two-dimensional, entrained mantle flow in subduction zones. This model predicts mantle flow (and olivine a axes) parallel to the downdip slab direction both above and below the slab [Ribe, 1989]. (b) Results ikom this study indicate trench-parallel fast polarizations in the mantle wedge and subslab mantle. This suggests trench-parallel flow in the asthenosphere both above and below the slab. Trench-parallel fast polarizations in the lithospheric portion of the mantle wedge are thought to be caused by the shear deformation associated with oblique convergence. Anisotropy in the slab is probably due to fossil mineral alignment.

this case the main tectonic process is oblique subduction, and the geology of the overlying plate is an indicator of this, with the mountains and active faults trending subparallel to the strike of subduction. The process of oblique subduction tends to produce trench-parallel fast polarizations in the lithosphere due to the associated shear deformation, as well as trench- parallel fast polarizations in the asthenosphere both above and below the slab due to the part played by the slab in diverting the mantle flow into a trench-parallel direction. The same tectonic process appears to be producing the anisotropy in the mantle wedge and subslab mantle, so it is not surprising that similar fast polarizations are found.

5.8. Delay Times

Because on.ly teleseismic phases have been used in this study, the delay times do not give direct information about the vertical extent of the anisotropy. This is because there is a trade-off between the strength of anisotropy and the thickness of the anisotropic material. Results fi'om other studies are required to constrain the depth extent of anisotropy. Gledhill and Stuart [1996] found a shear wave velocity anisotropy of 1.4% in the subslab mantle beneath the Tararua array, and we will assume this value in the following calculation. The average SKS delay time of 1.6 s suggests that the anisotropy extends down to a depth of at least 500 km, which includes the upper transition zone. However, this calculation does not take into account frequency dependence or the possibility of varying anisotropy with depth.

6. Conclusions

Measurements of shear wave splitting have revealed information about the seismic anisotropy beneath the lower half of the North Island, New Zealand. Teleseismic ScS and SKS events recorded at nine broadband seismometers have

been used to investigate any variations in anisotropy across the subduction zone. The splitting parameters are remarkably consistent for all of the stations, with the average SKS fast polarization ranging between 37 ø + 7øand 52 ø + 4 ø and the average SKS delay time ranging between 1.5 + 0.1 s and 1.8 + 0.2 s. This lack of variation enabled average SKS splitting parameters to be calculated using all the stations, giving • = 46 ø + 2 ø, •it = 1.6 + 0.1 s.

The ScS measurements give consistently smaller delay times compared to SKS, as was found for the permanent station SNZO [Marson-Pidgeon and Savage, 1997]. This suggests that the POMS II data may be frequency-dependent; however, tests were inconclusive. We are unable to tell if there is any real difference between SNZO and the POMS II array, as differences in delay time may be due to a frequency effect.

Contributions to the observed anisotropy probably come fi'om the crust, mantle wedge, slab, and subslab mantle. A study of local earthquakes that included two of the southeastern stations suggested shear wave velocity anisotropy of 1.4% in the subslab mantle [Gledhill and Stuart, 1996]. Rays arriving at the northwestern stations travel through both the mantle wedge and subslab mantle, yet give similar results to rays arriving at the southeastern stations, which travel through the subslab mantle but not the mantle wedge. This suggests that similar fast polarizations are found in the mantle wedge and subslab mantle. The average NE-SW fast polarization direction is subparallel to the major geological features and to the strike of the trench, which cannot be explained by conventional models of two- dimensional entrained mantle flow. The anisotropy in the mantle wedge is likely a combination of lithospheric and asthenospheric anisotropy. Assuming that the lithospheric anisotropy is caused by LPO of olivine due to strain, it appears that the dominant shear deformation associated with oblique convergence is responsible for producing fast polarizations parallel to the plate boundary and the trend of the mountain belts. Any anisotropy in the slab is likely due to fossil mineral alignment in the subducting oceanic plate. Assuming that anisotropy in the asthenosphere both above and below the slab is due to LPO of olivine as a result of

asthenospheric flow and that this flow is in the form of progressive simple shear, then the NE-SW fast polarization suggests that this flow is in a trench-parallel direction. This supports models in which the slab is acting as a passive rigid barrier to mantle flow, as this predicts trench-parallel flow directions both above and below the slab. However, it also suggests that stations to the south should show evidence of return flow, which has not yet been found.

The anisotropy in both the mantle wedge and subslab mantle appear to be produced by current deformation. The main tectonic process occurring is oblique subduction, and the

20,286 MARSON-PIDGEON ET AL.: ANISOTROPY BENEATH LOWER NORTH ISLAND

geology of the overlying plate is an indicator of this, so it is not surprising that similar fast polarizations subparallel to the trench and geological features are found in the mantle wedge and subslab mantle. In order to be able to isolate the

contributions to the observed anisotropy from the crust, mantle wedge, slab, and subslab mantle a study using local subduction earthquakes would need to be undertaken.

Acknowledgments. Funding for this research was provided by a VUW postgraduate scholarship. Data collection was carried out by D. Francis of Leeds University and P. McGinty of VUW using instruments supplied by Leeds University. The deployment of the broadbands was funded by NERC grant GR3/7699 on the POMS project. The New Zealand Marsden Fund supported K. Gledhill and M. Savage.

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K. Gledhill, Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt, New Zealand. (K.Gledhill•gns.cri.nz)

K. Marson-Pidgeon, Research School of Earth Sciences, Australian National University, ACT 0200, Australia. (katrina•rses.anu.edu.au)

M. K. Savage, School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. (martha•geo. vuw.ac.nz)

G. Stuart, School of Earth Sciences, University of Leeds, Leeds LS2 9J2, England, U.K. (G.Stuart•earth.leeds.ac.uk)

( Received June 1, 1998; revised May 5, 1999; accepted June 10, 1999.)