SKS splitting and the seismic anisotropy of the mantle beneath the Hikurangi subduction zone, New Zealand

PHYSICS O F T H E EARTH AND PLANETARY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 95 (1996) 227-236

SKS splitting and the seismic anisotropy of the mantle beneath the Hikurangi subduction zone, New Zealand Ken Gledhill *'19 David Gubbins Department of Earth Sciences, University of Leeds, Leeds, UK

Received 29 September 1994; accepted 31 August 1995

Abstract

The analysis of 12 SKS phases recorded on broadband stations above the Hikurangi subduction zone in New Zealand shows clear evidence of mantle anisotropy, with the fast direction (28 ° + 5°) almost parallel to the strike of subduction and the dominant geology of the region. The slow shear-wave delay times show a systematic change with the azimuth of the arrivals which, if hexagonal symmetry is assumed, indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal, or that more than one anisotropic layer is present. The magnitude of the delays (1.5 + 0.4s) suggests that the anisotropy is most probably confined to the top 300kin of the mantle.

1. I n t r o d u c t i o n The study of the anisotropic structure of subduction zones has the potential to provide useful information about the strain patterns and dynamics of such regions. Theory predicts that when mantle minerals such as olivine are subjected to strain, crystallographic alignments occur which show characteristic relationships to the principal axes of finite strain (e.g. McKenzie, 1979, Ribe and Yu, 1991). Previous observational studies of subduction zones have identified seismic anisotropy, and found that the mantle in such regions is more complex than elsewhere (Ando et al., 1983; Bowman and Ando, 1987; Shih et al., 1991; Silver and Chan, 1991; Fisher and Yang, 1994; Iidaka and Obara, 1994, Tsukada et al., 1994, * Corresponding author. IOn leave from: Seismological Observatory, Institute of Geo-

logical and Nuclear Sciences, Wellington, New Zealand.

Kaneshima and Silver, 1996). These studies have focused on the subduction zones of Japan, Tonga, Alaska, and South America, and have used various shear phases such as direct S, ScS, and SKS to identify the shear-wave splitting which is characteristic of seismic anisotropy. However, most of these studies have dealt with the mantle wedge above the subducted slab, rather than the mantle beneath the slab. The lower North Island of New Zealand lies above the fore-arc region of the Hikurangi subduction zone and it is therefore possible to study the anisotropic structure of the mantle beneath the subducted slab using land-based seismographs. In many other subduction zones this region lies under water and is therefore less easy to study. The Hikurangi subduction zone is the southernmost extension of the Tonga-Kermadec subduction system. Beneath the North Island of New Zealand the Pacific plate is being subducted beneath the Australian plate, and

0031-9201/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0031-9201(95)03118-9

228

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227 236

this is characterized by a band of deep earthquakes (Adams and Ware, 1977) which shallow to the south. The deep earthquakes terminate in the northern South Island, where the subduction gives way to the Alpine fault system. The subduction is oblique to the direction of relative plate motion; the strike of the subduction is close to NE-SW, whereas the relative plate motion direction is close to east-west (Fig. l(a)). The rate of plate convergence is approximately 5 cm year-t in the lower North Island. Beneath the lower North Island the subduction deepens from perhaps 15km at the east coast, to about 25 km at the west coast. Off the west coast the angle of subduction steepens to about 50 ° (Adams and Ware, 1977), reaching a depth of over 250km before the intermediate-depth earthquakes die out. However, under Mount Taranaki to the north there are isolated earthquakes at 600 km depth which may represent either the extension of the slab at depth or a detached piece of slab material which is still cold enough to generate earthquakes. The dominant leatures of the surface geology in the lower North Island are aligned sub-parallel to the strike of subduction. The Tararua ranges, a part of the axial mountain range system which extends to East Cape in the north-east of the island, is almost parallel to both coasts in the lower North Island. The active faults also follow this trend, striking NE-SW. In this paper, we report results of the analysis of SKS phases from teleseismic earthquakes recorded on a network of nine broad-band stations operated in New Zealand by Leeds University in 1991 and 1992,

3s.s (a)

/,~

,o.s

,0-s

M

A broad-band network of nine three-component stations was operated in the North Island of New Zealand (Fig. l(b)) by Leeds University from January 1991 to September 1992 (this is referred to as the Leeds Tararua array; Stuart et al., 1995). Each station comprised a three-component Guralp CMG3T broad-band velocity seismometer (Guralp System, Reeding, UK) and a Ref-Tek R72A-02 data acquisition system (Reflection Technology, Dallas, TX) recording on 360 mb hard disk. The timing was derived from the OMEGA radio time signal, giving an accuracy of +0.01 s, and the network could

180"E

,Ts-E (hi

175'E

41-s

176"E

41-s

175"E

2. Data and analysis

180"E --35-s

175"E

176"E

Fig. 1. The study region. (a) Tectonic setting of the North Island of New Zealand. The dashed lines show the approximate position of the subducting Pacific plate at depth (Adams and Ware, 1977), and the continuous line indicates where the upper surface of the plate would reach the surface if it continued at the same angle as at depth. The asterisks mark the position of the active volcanoes. The relative plate motion (RPM) between the Australian and Pacific plates is indicated, as well as the absolute plate motion (APM) direction of the Pacific plate. The box shown is the area covered by (b). (b) The location of the Leeds Tararua broad-band

stations.

therefore be treated as an array for most purposes. The stations operated at a sampling rate of 25 Hz for part of 1991, but used the higher rate of 50Hz during 1992. The resolution of the instruments was 16-bit

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

with a broad response ( - 3 dB) from 0.0333 Hz to 10Hz (25 Hz sampling) or 20Hz (50 Hz sampling). The instruments recorded continuously, and the data were later separated into earthquake archives using either the origin times of local earthquakes supplied by the New Zealand Seismological Observatory (Institute of Geological and Nuclear Sciences), or the PDE origin times in the case of teleseismic events. A data set of over 100 possible SKS arrivals was extracted from the PDE archive, and the theoretical arrival times of various teleseismic phases were added using IASPEI standard travel times. The SKS arrivals of events larger than magnitude 5.5, and greater than 85 ° distant, were chosen. The horizontal traces for all events were rotated into radial and transverse components based on the calculated back-azimuths, On visual inspection it was found that only a small number of these events showed clear SKS arrivals. A high level of microseism was noted, possibly because of the island nature of New Zealand. However,

229

~S on IndlvklualStollons[R, T Components]for [vent 073

Stack of All S~o,s

_ ~',,

"-'0

: ~~

5

10

15

~

20 25 lime [~]

~0

35

Fig. 2. Example of the stacking of an SKS arrival. The top diagram shows the radial and transverse component pairs for the nine stations, and the bottom diagram shows the results of the stacking process (radial and transverse components). The d.c. offset is removed before stacking, and the stacked traces are low-pass filtered (fourth-order Butterworth) at 2s period before analysisto remove high-frequency noise.

this microseism w a s n o t well correlated between stations (Stuart et al., 1995). In the hope of improving the signal-to-noise ratio of the data set, the traces from all stations which recorded an event were stacked using the theoretical SKS arrival times (Fig. 2). This improved the signal-to-noise ratio greatly, and resulted in a final SKS data set of 12 events (Table 1). A further five SKS arrivals were judged to have too poor a signal-to-noise ratio to be used in the analysis, although the SKS phase could be identified

on the stacked traces. We experimented with other methods of estimating the best stacking delays to use, but found little improvement over the use of the theoretical arrival times. Once stacked, the traces were low-pass filtered at 2 s period (fourth-order Butterworth) to remove high-frequency noise. The SKS phase should show purely radial polar-

Table l Earthquakes used in the SKS study Event

Year

Month

Day

h

min

s

Latitude (°N)

Longitude (°E)

Depth (kln)

Magnitude

194 281 323 330 353 062 073 116 117 155 180 211

1991 1991 1991 1991 1991 1992 1992 1992 1992 1992 1992 1992

07 10 11 11 12 03 03 04 04 06 06 07

13 08 19 26 19 02 13 25 26 03 28 29

02 03 22 19 01 12 16 18 11 06 11 04

50 31 28 40 33 29 01 06 18 10 57 30

14.6 15.6 51.0 48.5 40.4 39.5 04.4 04.2 25.7 54.3 34.1 47.7

42.182 45.587 04.554 42.051 45.253 52.915 52.451 40.368 40.378 51.130 34.201 39.495

- 125.641 149.049 - 77.442 142.523 151.176 159.886 - 178.945 - 124.316 - 124.575 178.743 - 116.436 143.501

011.0 146.0 021.0 056.0 027.0 039.0 197.0 015.0 022.0 022.0 001.0 016.0

6.2 6.0 6.4 6.1 6.0 6.5 5.9 6.3 6.5 5.9 6.2 5.9

Columns are the event identification (based on the day number), the origin time (year, month, day, hour, minute, and second), the latitude, longitude and depth of the earthquake, and the body-wave magnitude.

230

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

ization in an isotropic, spherically symmetric Earth. Any energy on the transverse component is therefore an indication of either anisotropy or heterogeneity. Almost all of the SKS phases3).dOTheShOwenergy ondia_the transverse component (Fig. uppermost

~.,

Uncorrected SKS

Corrected SKS

' ' ....... ' " " . . . . = R '

'= . . . . . . . . . . . . . . . . .

~

'

R~

7 /

(=) ~

Uncorrected SKS [R, T Components] for Event 281 ~t-' ' ' I ' ' ' I ' ' '-~

.g '=' ~

-~ -~-

....

; ..........

~. . . . . . . . . . . tee

~

r 0

10

20

30

RotatKI SKS [r ast, Slow] for [vent 281 t

i

i

0

i

i

10 i

i

i

20

i

i

i

Uncorrected SKS .......

=0

T

",

. . . . . N-

Corrected SKS ' ,

.....

,

i

30

SKS for Event 281

CO~ ~

i

~=

•

=...~,

-u'

i

-ii

-log(

T

0

10

20

30

Time [see]

I~)

,.~

Un¢orm-hKI SKS [R, T Componlmts] for Event 353 ~ ' ' '' ' ' ' ' ' '

--.~~ - ~ x

'

0

10

~

~ 20

30

Rolet0dSKS[Fad, SI0w]for Event353 -~ _~ ~__'~ ' ' z ~. . . . . . ~-+"g ~ , , , ~ ~. = I " 0 ' ' 10 ~ 20 Corred~ StS for [ ~ ~ =

__.~ a . ,.i. ~. o ,,

_~...~k'

~

30

' ~ - ~

I

0

, , 10 I , , , 20 I , , , --4 30 Time [~]

Fig. 3. Two examples of SKS arrivals. The top diagrams are the SKS arrivals rotated into radial and transverse components. (Note the energy on the transverse components.) The middle diagrams show the SKS arrivals rotated into fast and slow components, and the lower diagrams show the SKS arrivals (radial and transverse components) after correction for the anisotropy. (Note the absence of energy on the transverse components.) (a) An SKS arrival recorded on Day 281, 1991. (b) An SKS arrival recorded on Day

353, 1991.

Fig. 4. Particle motion plots for the SKS arrivals shown in Fig. 4. The left diagrams are the uncorrected SKS arrivals, and the right diagrams are the corrected arrivals. (Note that correcting for the anisotropy gives an almost linear particle motion in the radial direction.) (a) An SKS arrival recorded on Day 281, 1991. (b) An

SKS arrival recorded on Day 353,1991.

grams in Fig. 3(a) and Fig. 3(b) show clear evidence of energy on the transverse components, and characteristic waveforms of split SKS phases (Silver and Chan, 1991). If the time delay is small compared with the period of the SKS arrival, then the radial component is only slightly broadened and distorted, whereas the transverse component is approximately proportional to the derivative of the radial component. The corresponding particle motion diagrams are shown in Fig. 4(a) and Fig. 4(b) (left-hand diagrams). Assuming that this transverse energy was due to anisotropy, the fast shear-wave polarization (~b) and the slow direction delay (~t) between the two shearw a v e arrivals were derived using the following process. We calculated the root m e a n square (r.m.s.) radial to t r a n s v e r s e amplitude ratios for a data wind e w including the SKS phase for a range of assumed fast directions (~b) and splitting times (~t), and displayed the results as a c o n t o u r plot of amplitude ratio against ~b and ~t (Fig. 5). The peak in this plot

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

is0 17o 16o 150 • 140la0 lao~. 11010o o)

'

'

'

'

'

'

'

231

4. For comparison, the traces of the two best recorded SKS arrivals at single stations are displayed in Fig. 6. The parameters used to produce these plots are the same as those measured for the stacked traces for the respective events. Using the single station arrivals gave similar results to the stacked traces, but the measurements had greater errors, and often only a few stations for each event provided stable measure-

90-

i~_o 70m 60-

ta) •

2L- '

'

'

I P--~'

'

I

'

'~-L-3

4030 -

0

20J

100

0.0

10 20 30 Rololed SKS [Fod, Slow] for Event 55,1 .

~

.

.

.

.

~

,

015

110

115 210 215 Delay (see)

310

3.5

4,0

Fig. 5. Example contour plot of ~b vs. 5t for an SKS arrival recorded on Day 353, 1991. The contours are the relative amplitudes on the radial and transverse components for a window

><

_

0

~i

--

gives the most likely values of the SKS splitting parameters, and the value and shape of the peak can be taken as a measure o f reliability of the measurement. It should be noted that in these contour plots ~b is given with respect to the radial direction. This

'o

slow directions calculated by the above process. The bottom diagrams show the corrected SKS arrivals,

and the particle motion diagrams for both the u n c o r rected and corrected SKS arrivals are shown in Fig.

30

~

containing the SKS arrival for the range of ~b and 8t shown.

method o f deriving the SKS splitting parameters is similar to that used by other workers (e.g. Silver and Chan, 1991), the only difference being that here we have used the amplitude ratio on the two horizontal components, instead of just the amplitude on the transverse component. This was thought to give a good indication o f the reliability of the measurement in our relatively high microseismenvironment. If the calculated values of ~b and 8 t are used to correct for the anisotropy, then the resulting corrected traces show almost purely radial polarization. This is demonstrated for two SKS arrivals in Figs. 3 and 4. The top traces in Fig. 3(a) and Fig. 3(b) show the original SKS arrivals rotated into horizontal radial and transverse components, and the middle diagrams show the SKS arrivals rotated into the fast and

10 20 C o r r ~ SKS for Even| 353

~ s

s

t

1o

i

20

30

rime [sec]

fb~ I~

Unoorred~SKS [R, l] fori E~! 062 i ion LIN2 i

.a ~- ÷ ,<_ 0 t0 20 30 Rofoted SKS [Fad, Slow] for Event 062 '~ 2L--, , , ~ , ' L.,--L ' ' ' - ~ ~ o .0~._.---..._~....xj~ ~ ~ ~ ~ - ~ - , , , i , , , i , , , .4 0

10 20 CorreckKI SKS for Event 062

~ 2E-, , , I '...-k ' .~ _o .~.___.-....._..-- ~ ~= ~ ~ _ o

lO

9)

t ' ' '-:1 ......./...~

20

3o

lime [soo] Fig. 6. Two examples of unstacked SKS arrivals. The format is the same as in Fig. 3, and the parameters used to rotate and correct for the anisotropy are those derived by the stacked traces for the respective events. (a) An SKS arrival recorded at Station LTN3 on Day 353, 1991. (b) An SKS arrival recorded at Station LTN2 on Day 062, 1992.

K. Gledhill. D. Gubhins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

232 Table 2

Observations

NORTH

Results of the SKS splitting measurements Event

Back-azimuth (deg)

4, (deg)

~t (s)

//~----"~...

194

040

40 ! 22 a

_

281 323 330 353 062 073 116 117

342 098 336 343 351 003 042 042

28 ! 05 98(08) + 07 a 32± 13 27 5:07 33 5:04 33 5:05 425:05 a 18 5:06

1.80 4- 0.20 _ 1.96+0.50 1.76 + 0.25 1.64 5:0.20 1.04 + 0.25 _ 0.88 5:0.50

Model, 90 deg

Model, 80 deg

155

002

26 5:09

1.36 5:0.40

NORTH

NORTH

211 Average (8) Average (12)

336

245:05 28.0 5:04.8 28.8 _+ 10.8

1.84_+ 0.25 1.5 5:0.40

WEST

EAST

SOUTH

WEST

1

FAST WEST

EAST

Columns are the event number, event back-azimuth, the polariza-

SOUTH' =~

SOUTH1,~

tion (~b), and the delay time (~t). a These measurements of ~b show no splitting, and the polarization is based on the back-azimuth of the event. These events do

Model, 70 dog

Model, 60 dog

NORTH

NORTH

The 'Average (8)' values are the weighted averages for the eight events which have both polarizations and delays marked, whereas the 'Average (12)' value is the weighted average of all 12 polarization measurements.

WEST

EAST WEST

sou1.H

:35"S

~TS'E

,eO'E35"S

40'S

40'S

1 eee

EAST

SOUTH lr,ee

Fig. 8. The delay times of the split SKS phases. The upper diagram shows the measured delay times between the split SKS arrivals plotted against the back-azimuths of the events. The lower four diagrams show how a non-horizontal hexagonal symmetry axis will affect the delay times as a function of azimuth. These are plotted for the expected SKS incident angle of about 100 from vertical, and in all diagrams the arrow indicates the measured fast shear-wave direction. The diagram labelled 'Model, 90 dcg' shows the case for a horizontal symmetry axis (90° from vertical), and the other diagrams show the changes as the symmetry axis is varied from horizontal. The model diagrams were calculated using the elastic constants published by Mainpriee and Silver (1993), and the arrows indicate the average fast shear-wave direction. (Note that a symmetry axis inclined at 60-70 ° gives a result which shows qualitative agreement with the measurements.)

ments. For this reason, it is not possible with this data set to check if there is any variation in the shear-wave splitting parameters across the array.

175"E

I~'E

Fig. 7. The fast shear-wave polarizations derived from SKS arrivals shown as an equal-area rose diagram and superimposed on a map of the study region. Symbols are as in Fig. l(a).

3. Results

are

The results of the 12 SKS splitting m e a s u r e m e n t s given in Table 2 and Figs. 7-9. The few single-

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

.

.

.

.

.

.

.

233

then the result is 28.8 ° with a standard deviation of

00

00

so ._~ 60

v

~-6 4o _

_ ~

a. 20

O.;o-~o

~ 20 4~ ~ ~ 1~o

The measured delays range from 0.88 s to 1.96 s with a weighted average of 1.5s and a standard deviation of 0.4 s. There is evidence of a variation in delay time with back-azimuth (Fig. 7), although the azimuth range covered only a little over 90 ° (Figs. 8 and 9).

Back Azimuth (deg) 2.5

.' ['

'

i

. . . .

4. Discussion and conclusions

2.0 ~

"6" 1.5 ~. . 10 0.so.o -;o -~o b ~ 40 eb eb loo Back Azimuth {dog) Fig. 9. The fast shear-wave polarizations and slow shear-wave delay times plotted against the event back-azimuths. The delays show a systematic change with back-azimuth. The continuous lines show the calculated theoretical polarizations and delays for a two-layer model with the lower layer having a ~b of 10° and a delay of 1.6s, and the upper layer having a ~b of 50° and a delay

of0.6s,

station SKS splitting measurements available are similar to the stacked results (Fig. 6), Eight of the 12 measurements gave stable values for the polarization and delay of the split SKS phase, whereas the other four show no obvious splitting, although two of these have low signal-to-noise ratios (2.5 and three). Of these four, three (Events 194, 116, and 180) have back-azimuths near the direction which is identified as the fast direction by the eight events which do show splitting. These can therefore be given fast shear-wave azimuths based on the back-azimuths, The remaining event (323) which shows no obvious splitting has a back-azimuth of 98 °, and a reasonable signal-to-noise ratio of 8.4. If this back-azimuth is assumed to be near the slow direction of the anisotropy, this implies a fast direction of 8 °. Using only the eight events which show clear evidence of splitting gives a weighted average fast polarization azimuth of 28.0° (measured clockwise from north) with a standard deviation of 4.8 °. If all 12 events are averaged, under the assumptions discussed above,

The SKS splitting measurements reported here can be directly related to anisotropy in and below the subducting slab. The maximum contribution of the overlying Australian crust has been measured to be approximately 0.2s (Gledhill, 1991, 1993a,b; Gledhill and Stuart, 1996), which is much less than the values measured using SKS. We can therefore concentrate on the possible causes of the seismic anisotropy within the subducting slab, and in the mantle below the slab. The two most likely causes of the observed seismic anisotropy are fossil anisotropy within the subducting slab relating to earlier deformation, or flow-induced anisotropy in the mantle below the slab. The principal results of interest from the above SKS splitting measurements are the direction of the fast shear-wave polarization azimuth with respect to the subduction zone and local geological structure, and the apparent variation of the delays with the azimuth of the SKS arrival. For convergent margins the fast shear-wave direction should be parallel to the relative plate motion vector between the downgoing and overlying plate, or alternatively, in the direction of absolute plate motion (Silver and Chan, 1991). Silver and Chan (1991) pointed out that most theories of mantle dynamics would predict flow and consequent crystallographic alignments at convergent margins to be parallel to the relative or absolute plate motion. However, in the present case, the fast polarization azimuth is closer to the strike of the subduction (NE-SW), rather than the relative (eastwest), or the absolute (SW-NE) plate motion directions. Vinnik et al. (1992) found similar SKS splitting parameters (4' = 20°, ~t-- 1.8 s) using a single event recorded at the long-period Station SNZO near Wellington in the southern North Island of New

234

K. Gledhill, D. Gubbins / Physics of the Earth and Planetary Interiors 95 (1996) 227-236

Zealand, and the results from three ScS2 bounce points further north in the South Fiji Basin also gave similar parameters(45 °, Is; FarraandVinnik, 1994). The geological structures in the lower North Island follow a similar trend to the strike of subduction, with both the Tararua mountain ranges and the active faults in the region all striking NE-SW. This thus supports the internal coherent deformation (ICD) hypothesis of Silver and Chan (1991), in which the mantle anisotropy is dominated by the last significant episode of internal coherent deformation by processes such as orogenies, rifting episodes, and strike-slip deformation. In regions which are currently tectonically stable the fast shear-wave polarization will reflect the last episode of deformation, but in active tectonic regions such as the study region, the fast shear-wave polarization will be related to the present deformation of which the geology of the overlying plate is an indicator, Similar behaviour is seen in other subduction zones in Alaska (e.g. Silver and Chan, 1991) and South America (e.g. Shih et al., 1991; Kaneshima and Silver, 1996). However, there are also cases where the fast polarization is that predicted by plate convergence. In the Kurile-Kamchatka subduction zone, measurements which have extracted the parameters of source-side seismic anisotropy from S, sS, and SKS phases (Kaneshima and Silver, 1992; Fisher and Yang, 1994) show a fast shear-wave azimuth sub-parallel to the convergence direction, However, most of these measurements of seismic anisotropy in subduction zones have sampled the region above the subducting slab rather than below the slab, which is the region of interest in this study, The only exceptions appear to be the studies by Kaneshima and Silver (1992, 1996) and Russo and Silver (1994) dealing with the mantle anisotropy beneath Central Peru, and Vinnik and Kind (1993), who presented measurements for the KurileKamchatka subduction zone. Using various shearwave phases, Kaneshima and Silver (1992), Kaneshima and Silver (1996) found that there was less than 0.5% anisotropy in the upper mantle above the Nazca plate, with most of the anisotropy below the descending Nazca plate above 400km. The fast shear-wave polarization directions identified by their studies were sub-parallel to the strike of subduction, although some of the observations were from near a

bend in the subduction system. Vinnik and Kind (1993) found evidence that the fast shear-wave direction was parallel to the strike of the KurileKamchatka subduction system. There are two possible causes for the variation of the SKS delays with azimuth. It is often assumed that the observed anisotropy can be modelled by a transversely isotropic layer with a horizontal symmetry axis even when the possible cause is crystalline alignment which has orthorhombic symmetry in the single crystal case (e.g. Savage and Silver, 1993). The work of Mainprice and Silver (1993) suggests that this is a reasonable approximation. The overall anisotropy was close to hexagonal in the rocks they studied, even though the major constituent was orthorhombic olivine. Such a model of the anisotropy will show very little variation of either the polarization direction or delay times with azimuth. However, if the symmetry axis is not horizontal but tilted at a angle about the fast polarization direction, variations in the delay times with back-azimuth may be observed even when no variations in the polarization direction are noted. In this case, the variation in the delay times with back-azimuth will have a 2zr dependence (Savage and Silver, 1993). The second possible cause for a variation of the delays with azimuth is the presence of more than one anisotropic layer. If both layers are transversely isotropic with horizontal symmetry axes, but the symmetry axes have different azimuths, then a variation in the delays and polarizations with back-azimuth will be seen as long as the delays are short compared with the wavelength (Savage and Silver, 1993). In this case, however, the variation will have a zr/2 dependence. It should therefore be possible to distinguish these two possible causes of the variation of the delays with back-azimuth. The delays do show a variation with back-azimuth which has a ~r/2 dependence (Fig. 9), if three of the four events which do not show splitting are ignored. The continuous line in Fig. 9 is the best-fitting two-layer model (thl = 10 °, ~tl = 1.6s; 4~2--50 °, ~t2 = 0.6 s) which ignores the null splitting results. If the null splitting results are included, then no stable two-layer model can be found. However, as can be seen from Fig. 9, the two-layer model does fit much of the data reasonably well. A similar situation is found for the model with a dipping hexagonal

K. Gledhill, D. Gubbins / Physics o f the Earth and Planetary Interiors 95 (1996) 227-236

symmetry axis. For example, a transversely isotropic layer with the symmetry axis rotated up 30-40 ° from horizontal in a plane at right angles to the strike of the subduction zone would qualitatively produce the variation in delays observed (Fig. 8). The modelled diagrams shown in Fig. 8 were produced using a purely transversely isotropic layer with the elastic constants based on those published by Mainprice and Silver (1993) for mantle rocks in the depth range 170-200km. However, such a solution is not unique because relatively small changes in the elastic constunts cause large changes in the pattern of delays. Thus, it is not possible with the current data set to decide between the two altematives, although the null splitting results tend to favour the non-horizontal symmetry axis model, If our observation of seismic anisotropy in the mantle beneath the Hikurangi subduction zone is caused by mantle flow, then the fast shear-wave direction almost parallel to the strike of subduction suggests that the mantle flow is in the trench-parallel direction, and not directly related to the plate motion vectors. A similar situation below the Nazca plate in South America has been explained by Russo and Silver (1994) as being due to trench-parallel flow beneath the Nazca plate along much of the Andean subduction zone. They suggested the cause to be the retrograde motion of the slab, the decoupling of the slab and the underlying mantle, and a partial barrier to mantle flow at depth. Several factors suggest that a similar explanation may be possible in the current case. The subduction beneath the North Island of New Zealand is oblique to the direction of relative plate motion (see Fig. l(a)) and terminates only about 2° south of the study area. The shape of the subduction zone, as revealed by the seismicity, changes substantially between East Cape in the north-east of the North Island, and Marlborough at the top of the South Island just north of where the subduction terminates (Anderson and Webb, 1994). The slab steepens to the south so that the amount of mantle material beneath the slab is reduced, suggesting that mantle material may be being squeezed out to the north. The opening of the Central Volcanic Region in the centre of the North Island (Darby and Meertens, 1995) suggests that the slab may be in retrograde motion, and this could provide another driving force for trench-parallel mantle flow.

235

The SKS splitting delays do not give direct information about the vertical extent of the anisotropy because the near-vertical paths allow a trade-off between the strength of anisotropy and the depth extent. Other evidence is therefore required to estimate which part of the travel path between the core and the surface is responsible for the measured SKS delays. Other studies in New Zealand (Gledhill, 1991, 1993a,b; Gledhill and Stuart, 1996) suggest that the crust contributes a maximum of about 0.2s to the measured delay times, much less than the measured SKS value of 1.5 _+ 0.4 s. Mainprice and Silver (1993) used petrofabric measurements of five South African kimberlite nodules to estimate the elastic constants of the rocks at mantle depth, and produced curves of the expected SKS delay times with the depth extent of the anisotropy. If these curves are used, then the maximum measured delay time of just under 2s suggests that the anisotropy may extend 250-300km below the subducted slab. The SKS splitting parameters reported in this paper indicate a fast shear-wave direction in the mantle beneath the Hikurangi subduction zone subparallel to the strike of subduction (NE-SW) and the dominant geological structures in the region, and more than 60 ° away from the absolute plate motion or relative plate motion directions. If the anisotropy is caused by strain-induced lattice preferred orientation (LPO) owing to mantle flow (e.g. Silver and Chan, 1991), this suggests trench-parallel flow below the subducted slab. The variation of the measured SKS delay times with azimuth suggests the possibility that the symmetry axis of the anisotropic system involved is not horizontal, but rotated up by 30-40 ° in a plane at fight angles to the subducted slab, or that more than one anisotropic layer is present. The size of the measured slow shear-wave delays suggests that the anisotropy is likely to be confined to approximately the top 300 km of the upper mantle. The need to stack the SKS arrivals to improve the signal-to-noise ratio resulted in only a single measurement of the anisotropy beneath the network for each recorded earthquake, and the data set shows a lack of azimuth coverage. However, it is hoped that the results reported in this paper will soon be strengthened by the use of other broad-band stations now operating with a wider spacing in the North Island of New Zealand, and the utilization of other

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K. Gledhill. D. Gubbins / Physics of the Earth arm Planetary Interiors 95 (1996) 227-236

s h e a r - w a v e p h a s e s s u c h as d i r e c t S f r o m s u b d u c t i o n z o n e e a r t h q u a k e s , a n d ScS.

Acknowledgements We thank G. Helffrich, who originally suggested this project. T h i s w o r k w a s u n d e r t a k e n w h i l e o n e o f US ( K . G . ) w a s o n s t u d y l e a v e f r o m the S e i s m o l o g i c a l

Observatory, Institute of Geological and Nuclear Sciences, W e l l i n g t o n , N e w Z e a l a n d , a n d w o r k i n g as a

Research Fellow at Leeds University. Financial support f r o m b o t h o r g a n i z a t i o n s is gratefully a c k n o w l e d g e d . T h i s is Institute o f G e o l o g i c a l a n d N u c l e a r

Sciences Contribution 610.

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