Project Number: 430W3313

DISCLAIMER

This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to the Earthquake Commission (EQC), Accident Compensation Corporation (ACC) and Wellington City Council (WCC). Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of, or reliance on any contents of this Report by any person other than EQC, ACC and WCC and shall not be liable to any person other than EQC, ACC and WCC, on any ground, for any loss, damage or expense arising from such use or reliance.

The data presented in this Report are available to GNS Science for other use from June 2008.

BIBLIOGRAPHIC REFERENCE

Beavan, J. and Wallace, L. 2008. It’s Our Fault – Wellington geodetic studies: task completion report – Fault coupling results from inversion of new and reprocessed GPS campaign data from the Wellington region, GNS Science Consultancy Report 2008/172. 12 p.

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CONTENTS EXECUTIVE SUMMARY ......................................................................................................... II 1.0 INTRODUCTION .......................................................................................................... 1

2.0 METHOD USED TO INVERT GPS VELOCITIES FOR FAULT COUPLING AND BLOCK ROTATION ..................................................................................................... 2

3.0 RESULTS OF UPDATED INVERSION ........................................................................ 3

4.0 FUTURE WORK ........................................................................................................... 4

5.0 ACKNOWLEDGEMENTS ............................................................................................ 5

6.0 REFERENCES ............................................................................................................. 5

FIGURES

Figure 1 Observed (red, with error ellipses) and predicted (black) GPS velocities in mm/yr relative to

the Australian Plate, based on the best-fitting block rotation/fault coupling model (new model using 1996-2007 data). Thick gray lines are block boundaries used in the block modelling. (A) results for entire North Island; (B) results for lower North Island and northern South Island. ................................................................................................................................................ 7

Figure 2 Interseismic coupling results from inversion with (a) newly processed GPS data from 1996 to 2007, (b) previously processed data from 1992 to 2004. Interseismic coupling (phi) is a scalar coupling coefficient, where phi = 0 indicates steady aseismic creep (no elastic strain accumulation in the surrounding rock) and phi = 1 interseismic coupling at the full relative block rate. The coupling results appear quite different between the models for some faults that are largely offshore; however, these faults are poorly resolved by the available data, and the coupling estimates have very large uncertainties. Dashed black line shows approximate source area of Manawatu slow slip event. In (a), the contours labelled 10, 30, 40, & 60 show the depth to the subduction interface in km. ............................................................... 8

Figure 3 Arrows show the predicted relative motion across block boundaries in mm/yr based on our kinematic model, with 68% confidence uncertainty ellipses. The arrows show relative motion between adjacent blocks (one block moving relative to the other), with the base of the arrow situated inside the moving block. For example, the arrows along the Jordan Thrust (between NE end of Hope Fault and Te Rapa Fault) show about 15 mm/yr of mixed thrusting and dextral strike-slip movement. KMFS = Kapiti-Manawatu fault system; WLF = Wellington Fault; MCF = Masterton/Carterton/Mokonui faults; WRF = Wairarapa Fault; WF = Wairau Fault; AF = Awatere Fault; CF = Clarence Fault; HF = Hope Fault; TRF = Te Rapa Fault; BF = Boo Boo Fault. ................................................................................................................. 9

APPENDICES Appendix A GPS Data Analysis............................................................................................................................. 6

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EXECUTIVE SUMMARY

The seismic hazard due to future earthquakes on the Hikurangi subduction interface beneath the southern North Island has been one of the least-well characterised elements of the seismic hazard model for the Wellington region.

To improve this situation we re-measured in 2007 a GPS survey network across the southernmost North Island, analysed the data along with previous surveys, and used the derived velocities of ground survey marks to deduce the state of “coupling” (or “locking”) between the two sides of the subduction interface fault. This information has been used to infer possible scenarios for future subduction-interface earthquakes, which in turn have been used in other parts of the “It’s Our Fault” project to update seismic hazard estimates for the Wellington region.

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1.0 INTRODUCTION

The velocities of survey marks measured by GPS1 in the Wellington region provide insight into the state of interseismic coupling on the subduction interface, and the rate of fault slip on major crustal faults in the lower North Island. As well as being of scientific interest in their own right, these parameters will be used as input to the regional fault-interaction modelling that is one of the major aims of the It’s Our Fault (IOF) project.

Previously, Wallace et al. (2004) inverted campaign GPS site velocities, geological fault slip rates and earthquake slip vectors for tectonic block motions and interseismic coupling on major faults in the North Island, including the subduction interface. A slightly updated version of the 2004 analysis used GPS data from the Wellington region over the time period 1992 through 2002, with the bulk of data coming from campaigns in 1995, 1996, 1997, 1999 and 2002. It also used data from the northern South Island, principally from 1996, 2000 and 2004. We compare the results of this analysis (“old model”) with our new analysis (“new model”) later in this report.

For the new model described in this report, we utilise site velocities derived from Wellington region GPS data collected in 2007 for the IOF project, together with earlier campaign data collected in the southern North Island and northern South Island, to produce an updated version of the Wallace et al. (2004) model for subduction coupling and block rotation. This update, based on a superior data set, provides fresh insight into the contemporary state of interseismic coupling on the subduction interface and the potential for future subduction thrust events, as well as slip rate estimates on some of the major faults in the region (e.g., Wellington and Wairarapa faults and Kapiti-Manawatu fault system).

Over the past year or so we have spent considerable effort in reanalysing the entire New Zealand campaign GPS data set, using modern software and placing all the results into a consistent reference frame (Appendix A). This effort has largely been funded from projects other than IOF, but it has nevertheless been an essential step in producing the velocity field in the Wellington region that is required for modelling the state of coupling on the region’s faults. We have completed this reanalysis effort back to the start of 1996. This does not include the Wellington region GPS campaigns from 1992-1995, but it does mean that the northern South Island campaigns from 1996 onwards have for the first time been analysed with the southern North Island campaigns in a consistent manner. This may be important in better defining the southern edge of coupling on the Hikurangi subduction interface. The major data sets that contribute to the new GPS solution in the Wellington region are the 1996, 1997, 1999, 2002 and 2007 (IOF) Wellington campaigns, and the 1996, 2000 and 2004 “Top of the South Island” campaigns.

1 The Global Positioning System (GPS) is a network of about 30 satellites in ~12-hour orbits. Signals from the satellites can be picked up by ground-based GPS receivers and antennas, and used – with appropriate processing – to measure the positions of survey marks (“sites”) with an accuracy of a few millimetres. By taking occasional or continuous measurements at the same survey mark over a several-year period, the motion of that site can be measured with an accuracy of about 1 millimetre per year. If the motion of the point is fairly steady in time, the motion can be described as a velocity.

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2.0 METHOD USED TO INVERT GPS VELOCITIES FOR FAULT COUPLING AND BLOCK ROTATION

Clockwise tectonic rotation of the eastern North Island is evident in the GPS velocity field (Fig. 1). As these GPS velocities are derived from measurements taken during the interseismic period between large earthquakes, there is also a component of elastic strain due to coupling on faults, particularly the subduction zone. One way to separate the short-term, interseismic (elastic) and the long-term tectonic rotation signals in the North Island GPS velocity field is to simultaneously invert the GPS velocities for rotation poles of tectonic blocks and the degree of interseismic coupling on faults bounding the blocks (e.g., McCaffrey, 2002). This approach assumes that the plate boundary zone is composed of tectonic blocks on a sphere (spherical caps); these elastic blocks move independently of one another, and their motion can be described by an angular velocity. For the remainder of the report, we refer to a “pole of rotation” as the point where the angular velocity vector intersects the Earth’s surface. These blocks interact along their boundaries (faults; grey lines in Figure 1), and in most cases they cannot freely slip past each other along their bounding faults (due to friction), and they become coupled together during the interseismic period. As a consequence of this interseismic coupling, elastic strains are transmitted into the crust surrounding the fault. Under the elastic block assumption, the elastic strain in the crust surrounding the block boundaries will eventually be recovered in an earthquake when it becomes converted to fault slip across the block-bounding fault.

Using this method, we invert GPS velocities in the North Island for angular velocities of several forearc blocks, and the degree of interseismic coupling on the Hikurangi subduction thrust and other faults in the region (including the Wellington/Mohaka Fault and the Wairarapa Fault). In addition to GPS velocities, we also include geological fault slip rates from several faults in the North Island as data in the inversion to provide additional constraints on the relative kinematics of the blocks, and to ensure development of a kinematic model that is consistent with the available geological data. In addition to the updated GPS velocity field, we also incorporate new data on offshore fault motion in Cook Strait (Pondard et al., 2007; also pers. comm. from P. Barnes, N. Pondard and G. Lamarche at NIWA) to better constrain the transition from the subduction-dominated domain in the North Island to strike-slip faulting in the northern South Island. Many of the offshore data were compiled as part of the IOF project.

We use the NIWA offshore fault data to, we hope, improve the connection between the kinematic block models and faults in the North and South Islands. In previous block models (Wallace et al., 2004, 2007) we treated the South and North Islands independently, essentially putting a dividing line across Cook Strait. However, in the new model we include slip rate and slip vector estimates of the offshore active faults, as well as using the newly mapped faults as block boundaries, linking them up with the onshore structures. We also used paleomagnetic and structural geological evidence (Little and Roberts, 1997) for a fundamental change from rapid tectonic rotation of northeastern Marlborough to no rotation of central and southern Marlborough, to establish a boundary between northeastern Marlborough and central Marlborough.

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We perform the inversion (“new model”) using the new 1996-2007 velocity data set incorporating the IOF data, and compare it with the old model that used 1992-2004 velocities. We use exactly the same constraints on geological fault slip rates and fault coupling in the two inversions, so that any differences in the two solutions are entirely due to the differences in the input GPS velocity data (see next section).

It is important to note that the results we obtain for interseismic coupling and fault slip rates are averages over the duration of the data set. If these parameters change with time (and our observations of slow slip events means that we know the interseismic coupling may be changing with time), we will not see this variation using our current inversion method and GPS campaign data that are widely spaced in time. However, if we compare the average values of interseismic coupling over, say, the 1996-2002 and 1996-2007 intervals, we may see differences in these average values if a slow slip event has occurred somewhere in the 1996-2007 interval.

3.0 RESULTS OF UPDATED INVERSION

We obtain an excellent fit (χn2=1.6) to the GPS and geological data in the new inversion (Fig.

1). The new model (Fig. 2a) shows many similarities to the old one (Fig. 2b), but there are some differences, largely due to the new block configuration through Cook Strait. Inclusion of the new data produces only a small change in the distribution of interseismic coupling beneath the Manawatu region (Fig. 2). This is a surprising result, given the occurrence of the 2004-2005 Manawatu slow slip event, which involved up to 35 cm of slip on a ~100 km by 80 km patch of the subduction interface at 30-60 km depth (Wallace and Beavan, 2006). The 2004-2005 slow slip event reversed some of the accumulated slip deficit accumulated during the time period prior to the event, thus reducing the interseismic coupling in the source region of the event (Fig. 2). However, when averaged over an 11-year time interval, the change in coupling due to the slow slip event is not particularly evident. We do, though, see a slightly reduced interseismic coupling estimate to the south-southwest of the Manawatu event in the new inversion, raising the possibility that slow slip also occurred on that portion of the interface between 2002 and 2007, perhaps in an event similar to the Kapiti slow slip event of 2003-2004 (Beavan et al., 2007). (Until recently, there were no continuous GPS stations in place that could have unambiguously recorded such an event.)

In the inversion using the “new” velocities (Fig. 2a), the southern segment of the Wellington Fault has an interseismic coupling coefficient (phi) of ~0 indicating aseismic creep, compared to the inversion in Fig 2b. which has a phi of ~1 for that section of the fault. The uncertainties on interseismic coupling estimates on the Wellington Fault are very high, due to trade-offs between fault slip rate, Wellington Fault coupling and subduction interface coupling. Due to these trade-offs, coupling estimates for the Wellington Fault are not reliable, and we do not think that the difference in southern Wellington Fault coupling estimates between the two inversions is significant. The same is true for coupling estimates on the block-bounding fault just off the southeast Wairarapa coast, and the fault representing the Kapiti-Manawatu fault system. We know from observation of the lack of surface creep on the fault that the Wellington Fault is 100% coupled near the surface. However, we do not know how the coupling varies with depth, and our present observations are unable to give us information on this. It is hoped that the new GPS stations installed in the Wellington region as part of IOF will provide additional information, after these stations have been re-observed, as is proposed, several years from now as part of the IOF project.

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The region in central New Zealand where present-day subduction interface coupling is >50% measures about 360 km along strike (Fig. 2a). For the southern 120 km, coupling of >50% extends to about 20 km depth. For the next 120 km the 50% coupling contour reaches 40 km depth, and for the northern 120 km, the 50% contour trends back towards about 25 km depth. The up-dip extent of coupling, offshore towards the trench, is not known because the GPS data do not provide resolution far offshore. In the inversion, the coupling at the trench is forced to be 100% for technical reasons, but this is unlikely to be the case in reality. In using the coupling results from GPS for modelling future earthquake scenarios, it is important to recognise this and to run various cases with different up-dip extents of coupling.

Another result of the new inversion is the set of estimated long-term block-boundary slip rates, particularly those for the Cook Strait region (Figure 3). However, some of these rates are strongly constrained based on geological fault slip rates, as explained in Wallace et al. (2004). In general, all of the estimated slip rates are within the range of minimum and maximum geological slip rate estimates for the block-bounding faults. The major exception to this is the Wairarapa Fault, where we estimate ~8 mm/yr strike-slip in our kinematic model compared to the geological estimate of 10-12 mm/yr. This does not sound like a large discrepancy, but it has proved very difficult to obtain a satisfactory fit to the geodetic data using the higher rates. This inconsistency will be the subject of further study.

Interestingly, in our new inversion, the poles of rotation between the northeastern Marlborough blocks and the southern and central Marlborough blocks occur on the boundary between northeastern and central Marlborough. Such a kinematic scenario was predicted by the work of Little and Roberts (1997), who proposed that the boundary between the rotating and non-rotating domains of Marlborough was acting as a hinge point for this change in rate of block rotation.

4.0 FUTURE WORK

The new inversion in this report represents an initial attempt to model the new GPS velocity data set. More work is required in at least the following areas:

1. Update the geological slip rates used as input data on faults in the southern North Island where recent paleoseismic work has been undertaken as part of the IOF project;

2. Discuss with NIWA scientists whether we have made optimal choices in construction of block boundaries across Cook Strait, and confirm that the slip rates and slip vectors being used for those faults are the best available;

3. Investigate trade-offs in coupling estimates between the subduction interface and crustal faults;

4. Investigate the discrepancy between geodetic and geological estimates of slip rate on the Wairarapa Fault.

We plan to report further on these tasks after the next set of GPS data has been collected in the Wellington region.

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5.0 ACKNOWLEDGEMENTS

We thank the Foundation for Research, Science and Technology and the Earthquake Commission for funding the collection and analysis of the pre-2007 GPS data, and the IOF sponsors for funding the 2007 Wellington region GPS campaign and the analysis described in this report. We thank Russ Van Dissen and Russell Robinson for their reviews, which enabled us to improve this report.

6.0 REFERENCES

Altamimi, Z., Sillard, P., Boucher, C., 2002, ITRF2000: A new release of the International Terestrial Reference Frame for earth science applications. Journal of Geophysical Research 107(B10): 2214, doi:10.1029/2001JB0000561.

Beavan, J., Wallace, L., Douglas, A., Fletcher, H., 2007, Slow slip events on the Hikurangi subduction interface, New Zealand, Dynamic Planet, Monitoring and Understanding a Dynamic Planet with Geodetic and Oceanographic Tools, IAG Symposium, Cairns, Australia, 22-26 August, 2005, Series: International Association of Geodesy Symposia , Tregoning, Paul; Rizos, Chris (Eds.), Vol. 130, 438-444.

Bibby, H. M., 1982, Unbiased estimate of strain from triangulation data using the method of simultaneous reduction. Tectonophysics 82(1-2): 161-174.

Crook, C. N., 1992, ADJCOORD: a Fortran program for survey adjustment and deformation modelling, NZ Geol. Surv. Earth Def. Sec. Report, 138, Dept. Sci. Indust. Res., Lower Hutt, N. Z.: 22 pp.

Dach, R., Hugentobler, U., Fridez, P., Meindl, M., (eds.), 2007, Bernese GPS Software Version 5.0, Astronomical Institute, University of Berne, Berne, Switzerland, January 2007.

Little, T.A., Roberts, A.P., 1997, Distribution and mechanism of Neogene to present-day vertical axis rotations, Pacific-Australian plate boundary zone, South Island, New Zealand. Journal of Geophysical Research, Solid earth, 102(B9): 20,447-20,468.

McCaffrey, R., 2002, Crustal block rotations and plate coupling, in Plate Boundary Zones, edited by S. Stein and J. Freymueller, AGU Geodynamics Series vol. 30, 100-122.

Pondard, N., Barnes, P.M., Lamarche, G., Mountjoy, J., 2007, Active faulting and seismic hazard in Cook Strait using high-resolution seismic data (abstract). Geol. Soc. NZ Misc. Publ. 123A: p 132.

Wallace, L.M., Beavan, R.J., McCaffrey, R., Darby, D.J., 2004, Subduction zone coupling and tectonic block rotations in the North Island, New Zealand. Journal of Geophysical Research 109(B12): B12406, doi:10.1029/2004JB003241.

Wallace, L.M., Beavan, R.J., 2006, A large slow slip event on the central Hikurangi subduction interface beneath the Manawatu region, North Island, New Zealand. Geophysical Research Letters 33: doi:10.1029/2006GL026009.

Wallace, L.M., Beavan, R.J., McCaffrey, R., Berryman, K.R., Denys, P., 2007, Balancing the plate motion budget in the South Island, New Zealand using GPS, geological and seismological data. Geophysical Journal International 168(1): doi:10.1111/j.1365-246X.2006.03183.x

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APPENDIX A GPS DATA ANALYSIS

The GPS phase data from each daily observation session were processed in a network solution using the high-accuracy Bernese version 5.0 processing package (Dach et al., 2007), to determine daily estimates of relative coordinates and their covariance matrices. International GNSS Service (IGS) elevation-dependent antenna phase-centre models were used to account for the different antennas used at different time periods between 1996 and 2007. Ocean loading corrections have not been introduced into the analysis at this stage. Tropospheric delays were estimated hourly at each station in a piecewise continuous fashion, and the tropospheric gradient was estimated daily in a piecewise continuous fashion. The dry Niell model was used as the a priori model, with the wet Niell mapping function used to map slant-path delays to zenith.

During each day’s processing, IGS final orbits and associated polar motion parameters were held fixed, and a 3-parameter translation was applied to the coordinate results so as to best fit the ITRF2000 (IG00b realisation) coordinates of a set of regional IGS stations at the epoch of observation. The IGS stations used for reference frame realisation were: ALIC, AUCK, CEDU, CHAT, HOB2, KARR, MCM4, NOUM, TIDB, YAR1/YAR2. The daily coordinate results are therefore nominally in the IG00b realisation of the ITRF2000 reference frame at the epoch of measurement.

All daily coordinate-difference solutions and their covariances were input to the least squares adjustment software ADJCOORD (Crook, 1992; Bibby, 1982) to check for outliers and to obtain the appropriate χ2 factor for subsequent scaling of the covariance matrix. Station AUCK was held fixed to obtain a minimally-constrained solution. The covariance matrices require scaling because the temporal correlation of the GPS phase data is neglected in the estimation of the formal errors in the Bernese software, so that the formal uncertainties are underestimated compared to the scatter in daily coordinate results. The scaling factor depends on the noise properties of the data, and on the sample interval of the GPS phase data used to obtain final coordinates. We determined the factor by analysing the data in 3-month groups assuming a static solution (no site velocity estimation). A factor of 52 (=25) was typically determined for the 180-second samples we use in the final stages of our processing (30-second samples are used for data editing and cycle-slip fixing, then the data are decimated to 180 seconds for subsequent processing). This factor is consistent with what we have found in previous analyses. This procedure ensures that the relative coordinate uncertainties are consistent with the scatter of repeated observations.

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Figure 1 Observed (red, with error ellipses) and predicted (black) GPS velocities in mm/yr relative to the Australian Plate, based on the best-fitting block rotation/fault coupling model (new model using 1996-2007 data). Thick gray lines are block boundaries used in the block modelling. (A) results for entire North Island; (B) results for lower North Island and northern South Island.

A B

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Figure 2 Interseismic coupling results from inversion with (a) newly processed GPS data from 1996 to 2007, (b) previously processed data from 1992 to 2004. Interseismic coupling (phi) is a scalar coupling coefficient, where phi = 0 indicates steady aseismic creep (no elastic strain accumulation in the surrounding rock) and phi = 1 interseismic coupling at the full relative block rate. The coupling results appear quite different between the models for some faults that are largely offshore; however, these faults are poorly resolved by the available data, and the coupling estimates have very large uncertainties. Dashed black line shows approximate source area of Manawatu slow slip event. In (a), the contours labelled 10, 30, 40, & 60 show the depth to the subduction interface in km.

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Figure 3 Arrows show the predicted relative motion across block boundaries in mm/yr based on our kinematic model, with 68% confidence uncertainty ellipses. The arrows show relative motion between adjacent blocks (one block moving relative to the other), with the base of the arrow situated inside the moving block. For example, the arrows along the Jordan Thrust (between NE end of Hope Fault and Te Rapa Fault) show about 15 mm/yr of mixed thrusting and dextral strike-slip movement. KMFS = Kapiti-Manawatu fault system; WLF = Wellington Fault; MCF = Masterton/Carterton/Mokonui faults; WRF = Wairarapa Fault; WF = Wairau Fault; AF = Awatere Fault; CF = Clarence Fault; HF = Hope Fault; TRF = Te Rapa Fault; BF = Boo Boo Fault.