PhD. Research Proposal

Enrolled: 1/2/2015 / Expected date of completion: 1/2/2018

Strain and Structure of the Hauraki Rift, North Island, New Zealand

Robert C. Pickle (293140622)

Supervisors:

Dr. Jennifer Eccles

Associate Professor Julie Rowland

Dr. Sigrun Hreinsdottir

Word count: 7664

Table of Contents 1. Research Aims and Objectives

2. Background

2.1 Geological Evolution of Northern New Zealand

2.1.1. Gondwana Origins and Inherited Basement Fabrics

2.1.2. Onset and Evolution of the Modern Plate Boundary

2.1.3 North Island Volcanism

2.2 The Hauraki Rift Zone

2.2.1. Initial Activity in the Hauraki Rift

2.2.2 Current Activity in the Hauraki Rift

2.3 Satellite Geodesy in NZ

3. Methodology

3.1 Geodetic Station Planning

3.2 Geodetic Station Deployment

3.3 Derivation of Crustal Strain Rate

3.4 Derivation of a General 2-D Crustal Strain Model

3.5 The Problem with Slow Deformation

3.6 Seismic Station Planning and Deployment

3.7 Analysis of Gravity and Magnetic Anomalies

4. Facilities

5. Budget

6. Schedule

7. References

1. Research Aims and Objectives Relative to the rest of New Zealand, little is known about the Hauraki Rift, an active continental rift east of the Auckland and Hamilton metropolitan areas that encompasses the Hauraki Gulf, Firth of Thames and Hauraki Plains. The region consists of several active and presumed-dormant faults parallel to its rift axis and is still widening and subsiding at a slow, but poorly constrained velocity today. Current knowledge of the activity is based on continuous GPS station data (Hreinsdottir, personal communication), the presence of active normal faulting (Lensen, 1981; Hochstein et al., 1986; Hochstein and Nixon, 1991) and a history of vertical offsets in the Hauraki Lowlands (De Lange and Lowe, 1990; Chick et al., 2001). While the region has been the subject of geophysical studies in the past (e.g. Hochstein & Nixon, 1979; Ferguson et al., 1980; Hodder, 1984; Hochstein et al., 1986; De Lange & Lowe, 1990; Hochstein & Ballance, 1993; Chick et al., 2001; Wise et al.; 2003; Kirkby, 2008) little of the area has been geodetically surveyed and a focused seismicity study targeting the axial faults only captured a total of 4 weeks data in the late 1970s (Hochstein et al., 1986).

The aim of this doctoral thesis is to increase the understanding of the active geodynamics and strain rates in the Hauraki Rift region and resolve the tectonic structure and faults of the rift zone.

The objectives of my research are to answer the following questions for the Hauraki Rift:

Which structures are currently active and what are the strain rates?

To answer this I intend to:

A) Deploy a three-year, 35 station GPS/GNSS geodetic campaign throughout the larger Hauraki Rift region to measure the relative strain rates of the crust.

B) Deploy a one-year, 2 seismometer array on the southern end of the Firth of Thames, which in conjunction with use of the current GeoNet network will better capture and locate the microseismicity of the Hauraki Rift faults.

C) Examine pre-existing satellite gravity, aeromagnetic and topographic data to infer the crustal structure of the Hauraki Rift.

How has rifting evolved?

To answer this I intend to:

A) Utilise the available satellite Free Air Gravity Anomaly alongside topographic data to derive the Bouguer Anomaly and predict regions of isostatic compensation.

B) Use the Bouguer and magnetic anomalies across the Hauraki Rift to better constrain and model the structure of the faults therein.

C) Take lateral gradients of the Bouguer and Magnetic Anomalies and to infer variations in density and composition within the subsurface which may determine regions where geologic structure has changed or been offset.

Upon completion of this research I will have attained high quality crustal velocity measurements that can be used to produce a 2-D strain map throughout the broader Hauraki Rift region as well as provide a valuable dataset for all future geodetic campaigns in the region. I will also be able to provide a framework for conducting geodetic GPS surveys at similarly slow moving rifts ( < 2 mm/yr ) based on the quality of the measurements acquired as well as any shortcomings given my deployment technique for this particular region.

I will also have acquired a year's worth of seismic data explicitly focused on detecting events at and around the Kerepehi Fault which prior to this work were too small to be detected by the established seismic network. Additionally I will have reviewed older seismic data to identify uncatalogued events and potentially re-locate past events in the Hauraki Rift to a higher degree of accuracy, which may point to activity in faults currently hidden or considered inactive. Being aware of any and all active faults is vital to fully comprehend the seismic risks associated within the greater Auckland region.

Finally I will have built upon older geophysical studies of the Hauraki Rift which were done prior to the availability of high precision satellite gravity data as well as manipulate this satellite gravity data alongside topographic data to produce a variety of gravity anomalies which illuminate the crustal structure throughout the Hauraki Rift. Using newly released industry aeromagnetic data will allow even further precision as it will provide an accurate estimate of the thickness of the sediment layer in the Hauraki Lowlands. A clearer picture of the rift's structure will assist in resolving the way this rift has and continues to evolve.

2. Background 2.1. Geological Evolution of Northern New Zealand

2.1.1. Gondwana Origins and Inherited Basement Fabrics

The opening of the Tasman Sea ~60-80 Mya rifted the now largely submerged landmass Zealandia eastward away from the Australian/Antarctic sector of Gondwanaland (Sutherland, 1999; Bache et al., 2014). This micro-continental block was comprised of fragments of continental Gondwana that now make up the Western Province of New Zealand (Mortimer, 2004) and terranes of marine sedimentary and igneous material that had been accreted to the Gondwana Margin which now form the > 110 Ma indurated greywackes, argillites, cherts and volcanic rocks underlying most of Zealandia (Sutherland, 1999; Mortimer, 2003). This includes the Maitai Terrane and the Dunite Mountain Ophiolite Belt within it that features the continuous Junction Magnetic Anomaly which traverses 2400 km through most of Zealandia and passes just west of the Hauraki Rift, roughly parallel to its bounding faults (Hunt, 1979; Sutherland, 1999; Bache et al., 2014, Mortimer, 2014). This region of parallel accreted terranes, repeatedly cobbled together since it was part of Gondwana, may have influenced the Hauraki Rift's spreading direction by providing a path of least resistance via ripping its sutures apart.

2.1.2. Onset and Evolution of the Modern Plate Boundary

Around ~55-45 Mya the Australian-Pacific plate boundary changed from a lateral strike-slip relationship to one where the Pacific plate began subducting northwestwards beneath the Australian plate (Schellart, 2007; Reyners, 2013). At 45 Mya this proto-Tonga-Kermadec trench (Sdrolias et al., 2003) begins and propagates southward through the non-submerged area of Zealandia, henceforth simply New Zealand. The strain associated with the rollback of the subducting Pacific slab possibly explains the subsequent opening of the South Fiji and Norfolk basins just north of New Zealand (Schellart et al., 2006).

As the Pacific plate continued to subduct westward, the Hikurangi Plateau travelled with it until colliding with the Australian plate boundary ~40 Mya, resisting subduction due to its thickness and buoyancy (Reyners, 2013). This is supported by a change in character from convergence north of the Hikurangi Plateau to extension south of it, with the opening of the Emerald Basin to the south of New Zealand ~ 45 Mya (Sutherland, 1995). In addition, from ~40-30 Mya the clockwise pole of rotation between the Pacific and Australian plates was near the western tip of this plateau (Sutherland, 1995), turning the western tip of the Hikurangi Plateau into a pivot point near the plate boundary.

Possibly as a result of this new torque, the Taranaki Fault began 43-40 Mya near the plate boundary and extended N-NW into the Australian Plate (Stagpoole and Nicol, 2008). The Taranaki Fault is a back thrust fault opposite to the plate boundary, which implies it accommodated westward strain in the overlying Australian plate from the subduction process, shortening it. Additionally the length of active areas on the fault has decreased with time between 34-20 Mya from north to south, further suggesting that the subducting Pacific plate has rotated clockwise with slab rollback a larger factor in the north than south (Stagpoole and Nicol, 2008). This coincides with the observation at ~25 Mya of a large increase in land area between the plate boundary and Taranaki Fault caused by the high strain in the region, as modelled via rock volume balances by Wood and Stagpoole (2007).

At 23 Mya, the strain associated with the western edge of the Hikurangi Plateau leads to the creation of the Alpine Fault (Kamp, 1986; Cooper et al., 1987; Schellart et al., 2006) via a Subduction-Transform Edge Propagator model (Govers and Wortel, 2005; Reyners, 2013). The trench rollback associated with this model as well as the resulting strike-slip orientation of the Alpine Fault implies extension in the overlying Australian Plate perpendicular to the plate boundary.

By 22-21 Mya the stage for the current plate boundary configuration had been set with Hikurangi subduction zone arriving at its southernmost location obstructed against the shallow Chatham Rise crustal region (King, 2000). Shortly thereafter (21-20 Mya) the Norfolk and South Fiji basins re-open north of New Zealand as the joint Colville arc and Havre Trough system migrates eastward into the Kemadec Trench along the Vening Meinesz Fracture Zone (King, 2000; Mortimer et al., 2007). Between these two stresses to the North and South, the subducting Pacific slab rotates clockwise beneath the Australian and the first arc volcanism associated with it begins to show in the Northland circa 25 Mya (Nicholson et al., 2004).

2.1.3 North Island Volcanism

As the Pacific slab subducts beneath the Australian plate it hydrates the asthenospheric mantle wedge, facilitating melting which subsequently rises and erupts at surface. The geometry of slab subduction in relation to surface arc-type volcanism is remarkably consistent worldwide (Syracuse and Abers, 2006; Hayes et al., 2012) and thus volcanism can be traced SE through time according to the location and depth of the slab, here resulting in three distinct volcanic eras in the North Island as the plate boundary and subduction angle rotated clockwise relative to the Australian Plate.

Figure 1. Distribution of subducting Pacific slab, from Seebeck et al. (2014). Red dots show dated volcanic material. Dark area indicates inferred location of Pacific slab.

These three regions, shown in Figure 1 above, are the Northland Volcanic Arc (25-15 Mya), the Coromandel Volcanic Zone (16-4 Mya), and the Taupo Volcanic Zone (2 Mya to present) (Nicholson et al., 2004). This rapid SE migration of volcanism would indicate that the subducting slab is rolling back vs steepening, pointing towards a system of trench migration and high crustal extension in the Australian plate (Seebeck et al, 2013).

2.2. The Hauraki Rift Zone

The Hauraki Rift (Figure 2) currently resides ~50 km east of Auckland, is about 20-30 km wide, and extends nearly 300 km from Whangarei, through the Hauraki Gulf to the Firth of Thames and into the Hauraki Lowlands, becoming progressively narrower in width. These dimensions are typical of most continental rifts (Bosworth, 1985). From west to east within the Firth of Thames it is comprised of a fault angle depression tracing Firth of Thames Fault, a median horst tracing the Kerepehi Fault, and a bounding fault on the eastern shore tracing the Hauraki Fault (Hochstein and Nixon, 1979; Hochstein et al., 1986; Hochstein and Ballance, 1993). The Kerepehi Fault, which bisects the rift, also runs south of the Firth of Thames into the swampy Hauraki Lowlands. Of these three faults, only the Kerepehi Fault is in the New Zealand Active Faults Database maintained by GNS Science (http://data.gns.cri.nz/af/index.jsp). Onshore to the west of the Firth of Thames lies the Wairoa North Fault, striking North-South through the greywacke-dominated Hunua Ranges which is also included in the Active Faults Database. Further west and parallel to the Hauraki Rift's axis is the Junction Magnetic Anomaly (Figure 6) associated with the Dun Mountain Ophiolite belt within the basement (Sutherland, 1999; Mortimer, 2003), and east of the Hauraki Fault are the Coromandel volcanics which make up the majority of the Coromandel Peninsula.

Figure 2. The geology and faults of the Hauraki Rift region, from Hochstein and Nixon (1979).

2.2.1. Initial Activity in the Hauraki Rift

The Hauraki Rift became active during the period of Coromandel Volcanic Zone volcanism 16-4 Mya (Nicholson et al., 2004) but its age is poorly constrained. Hochstein and Ballance (1993) speculate it is as old as 10 Ma, and Skinner (1986) noted arc volcanics 11.5-5.6 Ma on either side of the rift.

The nearest source of tectonic stress circa the rift at this time was N-NE at the junction of the Colville and Keremedec ridge systems along the Vening Meinesz Fracture Zone (VMFZ) (Figure 3). The evolution of the Colville arc/ridge into the Keremedec and subsequent dissolution of the VMFZ may have stretched the Australian plate perpendicular to the Hauraki Rift's axis as the plate boundary moved eastward.

Figure 3. Plate reconstruction modified from King (2000). Hauraki Rift region is shown in the red square at 12 Ma. The Colville system merges with Kermadec Arc/Ridge system over the course of the presumed beginning of the Hauraki Rift, perhaps driving initial rifting.

The orientation of the rift axis is roughly parallel with that of the Junction Magnetic Anomaly and therefore the strikes of the accreted Gondwana terranes (Eccles et al., 2005). This may have guided the direction of incipient rifting offering a path of least resistance (Twiss and Moores, 1992), a notion supported by the orientation of seismic anisotropy at basement levels in this area (Reyners and Eberhart-Phillips, 2006; Eberhart-Phillips and Bannister, 2015).

2.2.2 Current Activity in the Hauraki Rift

Movement in the Hauraki Rift is currently accommodated by slip along the known active Kerepehi and Wairoa North faults, any unknown faults, and possibly faults currently considered inactive. A component may also be accommodated via broad crustal deformation caused by long wavelength tectonic stresses (Hochstein and Ballance, 1993). Past major offsets have been found on the Kerepehi Fault dating back 9000 years with an average occurrence interval of 2500 years producing a subsidence of ~13 mm/yr averaged over the course of several large displacements found in tephra cores on the Kerepehi Fault (De Lange and Lowe, 1990). Some combination of active crustal thinning and hot asthenospheric upwelling is likely occurring as well, causing a high concentration of thermal springs with water discharge temperatures > 20C and as high as 80C within the Hauraki Lowlands (Reyes, 2015).

Figure 4. Horizontal velocity solutions for North Island continuous GPS stations relative in the ITRF08 Australian fixed reference frame, courtesy of Sigrun Hreinsdottir (2015). Active faults are overlain in green. Rifting is clearly occurring somewhere between the western and eastern stations, presumably via the Kerepehei Fault bisecting the Hauraki Rift.

A few permanent and continuous GPS stations exist in the region (Figure 4) providing high precision velocity data relative to the ITRF08 Australian reference frame (from Hreinsdottir,

personal communication). These indicate that the Hauraki Rift is still widening, though perhaps further oblique to the rift axis given the sharp S-SE vectors of the continuous Coromandel (CORM) and Tauranga (TRNG) stations. Thus the current driver for this widening may have changed since rifting began, influenced more now by stress from the clockwise motion and rollback of the plate boundary and subduction to the south as shown in Figure 4.

Hochstein et al. (1986) characterize the seismicity in the rift as higher than the surrounding region and seismicity from the GeoNet (geonet.org.nz) catalogue (Figure 6) shows a small cluster of events on the southern end of the Kerepehi Fault as it nears the Taupo Volcanic Zone but otherwise no distinct pattern is discernible. This is potentially due to the strict detection requirements for the GeoNet autolocation algorithms, intrinsically small events in the slow-moving Hauraki Rift and the relative dearth of permanent seismic stations in the area. GeoNet's catalogue goes back to the 1940's under various automatic schema (some anecdotally) but as of 2012 events are automatically located using the SeisComP3 algorithm and a 3-D New Zealand-wide model designed by Eberhart-Phillips et al. (2010) and require an event to be observed on 5 stations before it is located and catalogued. A re-assessment of archived event data under more lenient detection rules is likely unveil a clearer pattern of seismicity along the Kerepehi and Wairoa North faults.

Figure 5. GPS velocity estimates from Wallace et al. (2004), relative to a fixed Australian plate. A clear clockwise angular trend is present south of our study region.

Figure 6. Topography of greater Hauraki Rift zone. Catalogue GeoNet seismicity is shown by small blue stars, continuous GPS stations shown in large purple squares, and the approximate location of the Junction Magnetic Anomaly in black with black squares. Faults from the GNS Science Active Faults database are in red, labelled as the Wairoa North Fault and the Kerepehi Fault.

2.3. Satellite Geodesy in NZ

New Zealand has a ~100 year history of geodetic surveying, with thousands of established and maintained trig beacons allowing line of sight surveying from topographic highs distributed across the country (LINZ; www.linz.govt.nz). With the advent of the satellite global positioning system (GPS) in 1978 the scientific utility of geodetic surveys in geophysics has made a dramatic leap forward providing nearly three orders of magnitude improvement in positional accuracy (Seever, 2003).

With current technologies it is now possible to determine 3-D locations to a millimetre or less, and therefore possible to determine crustal strain rates through repeat surveying of the same locations (Hackl et al., 2009). It is imperative to use a consistent, durable, and easily locatable point for repeat measurements, but that is largely solved in New Zealand by the network of geodetic beacons already put in place by LINZ.

GPS surveying in and around the Hauraki Rift, in comparison to the rest of New Zealand, is lacking but not completely absent. Aside from the small cluster of permanent stations near our study area (Figure 4) only a handful of sites were visited between 1994 and 2013 and the data was rarely re-surveyed with most ending up archived or used as a frame of reference for studies elsewhere. This is understandable in the early days of NZ satellite geodesy as most notable tectonic features e.g. the Alpine Fault, the TVZ, and the Hikurangi margin were studied first and more rigorously (e.g. Beavan and Haines, 2001; Wallace et al., 2004; Wallace et al., 2007). The other limiting factor for GPS geodesy in the Hauraki Rift was that, until recently, the error in GPS accuracy was usually greater than its average strain rate of ~1 mm/year (Hreinsdottir, personal communication), diminishing demand for regular campaigns. Both of these issues are now moot with additional geodetic surveys to the south becoming redundant as velocities become well defined, and GPS technology evolving to precisions capable of measuring slow deformation within reasonable year to year timeframes. Thus a logical new target should be a geodetic campaign in the active Hauraki Rift, in close proximity to Auckland and Hamilton, which is what this research proposes to accomplish.

3. Methodology 3.1. Geodetic Station Planning

To constrain the dynamics of the Hauraki Rift, we would like to have several linear transects of stations across the Hauraki Rift at different latitudes; with some lines orthogonal to the structure and some NE-SW orientated to capture oblique rifting as hypothesised by Hochstein et al (1986). In addition, a widely spaced grid of stations across the broader region captures the background velocity field. A uniform and wide station distribution grants a high quality interpolation when constructing a strain field map and higher density deployments striking across the presumed spreading axes ensures we will capture the spreading dynamic as well as possible.

The campaign geodetic network is shown in Figure 6 below. It includes 35 sites in addition to permanent continuous GPS/GNSS stations shown as purple squares. Sites where we have obtained historic geodetic survey data for (shown in Figure 7 as blue triangles) will provide higher precision velocity solutions than otherwise possible as the additional data point from 3-20 years ago will have allowed much more plate deformation to occur and thus every attempt to re-occupy all sites in which historic data exists has been made.

Figure 7. GPS station coverage in the greater Hauraki Gulf Region. Continuous and permanent reference stations are drawn in purple squares, campaign stations are drawn as green diamonds, and campaign stations where historic survey data has been located are drawn as blue squares. Active faults are drawn in red.

3.2. Geodetic Station Deployment

Eight Trimble 5700 GPS receivers and Trimble Zephyr 1 antennas as well as all the required accoutrements (batteries, tripods, etc.) were borrowed from GNS Science. These receivers and

antennas are high-specification and proven capable for geodetic work. Field training was gained by participating in the GNS Science February 2015 TVZ geodetic deployment.

Each station in the Hauraki network is occupied for a minimum of a week during each campaign. Given adequate sky coverage, a good setup and good ionospheric and atmospheric conditions, a 2-4 day occupation could technically achieve the required precision. However, with 8 pieces of equipment, there should be little to no bottlenecks with a superfluous 7+ day rotating deployment plan.

It takes ~2 hours to deploy or retrieve each site. Whilst 2015 has been especially time consuming due to the selection and permitting of appropriate sites, in 2016 and 2017 it is estimated that geodetic fieldwork can be comfortably completed in a 2 month period.

3.3. Derivation of Crustal Strain Rates

The GAMIT/GLOBK software suite maintained by Dept. of Atmospheric and Planetary Sciences at MIT (http://www-gpsg.mit.edu/~simon/gtgk/index.htm) will be used to convert satellite phase data into three dimensional relative positions of the stations. This software is available free of charge and its source code can be reviewed, but requires a license granted by its authors on request. It is well established in the GPS/GNSS community and is the preferred software of co-supervisor Sigrun Hreinsdottir at GNS Science. Comparable software does exist (e.g. GIPSY-OASIS from JPL or the commercial software Bermese) but is licensed restrictively or at prohibitive costs. The initial setup for data processing is time consuming but once in place the processing of successive surveys is straightforward.

Velocity vectors, relative to some fixed reference frame, are subsequently determined by comparing the discrepancy between the geospatial locations of the same site at different points in time. This PhD will include a total of three field campaigns a year apart. To get the analysis underway we do however have current velocity solutions of all the continuous stations in the North Island as well as historic data at 15 of our 35 campaign stations (Figure 7, blue triangles) dating back from 1993-2013. The continuous stations will give an accurate velocity estimate and the data from the past surveys will provide a reference point needed to compute a velocity. While the geospatial precision of these past surveys may be much lower than our current one, with usual occupations on the order of hours instead of days, the extended amount of time between measurements allows for a larger degree of crustal deformation to occur relative to instrument uncertainty.

In addition to our campaign sites and the permanent continuous GPS stations in the Hauraki Rift, we will analyse data from continuous stations in the IGS Network (igs.org) located around the greater NZ region to help constrain orbits and reference frame, including several station on the Australian continent. Using spatially diverse sites encompassing our study area offers a check on our campaign solutions and expedites the convergence and accuracy of our solutions (Seeber, 2003).

A repeated geodetic campaign survey such as planed here assumes that crustal velocities are constant over the duration of the study, with no significant events causing a large offset in the time series. Given that the tectonic stresses involved act and change at scales on the order of thousands of years we consider this a reasonable assumption. It also assumes that there have

been no significant events causing a single large discontinuous offset, which could be caused by an earthquake, or landslide, or other large scale disruption. In the case of such an event we can examine data from the nearby continuous stations to adjust our survey data. This is a common correction in other geodetic surveys worldwide (Seeber, 2003).

3.4. Derivation of a General 2-D Crustal Strain Model

Given the relatively uniform and high density of our station distribution, we can interpolate between stations to get a 2-D velocity map of our study region at using a general curvature spline algorithm (Smith and Wessel, 1990).

From there, strain within the crust can be calculated via the strain rate tensor (Equation 1) = 12 + (Equation 1)

As well as the antisymmetric rotation rate tensor (Equation 2)

= 12 (Equation 2) It is then possible to derive the second order invariant from these tensors (Equation 3) which gives values of absolute, scalar crustal strain using Equation 1 (Figure 7). This provides insight to where high-strain, high-risk, active faults may lie without relying on any assumptions about the underlying geology. Similarly solving for the anti-rotational tensor (Equation 2) illuminates the degree and velocity the region is rotating about an axis, if any.

2 = 1122 1221 (Equation 3) 3.5. The Problem with Slow Deformation

Measuring the slow (< 2 mm/yr) crustal strain rates in the Hauraki Rift over the relatively short amount of time afforded to this PhD is the biggest obstacle in this project, but it is not insurmountable (e.g. Newman et al, 1999; Hreinsdottir and Bennett, 2009; Baryla et al., 2014). Diligent planning and an extended 7-day station occupation repeated every year at the same time should produce the best possible precision with a campaign mode survey.

GPS precision is dependent on propagation errors through the troposphere and ionosphere, site-dependent signal multipaths, time synchronization errors, and station equipment error. Propagation and time errors are largely voided by incorporating simultaneous data from nearby, well-constrained continuous stations when solving for our campaign sites, but additional steps can be taken. To further minimize signal propagation errors we plan to survey only in the dry summer months to avoid atmospheric disturbances caused by moisture, and avoid the active solar storm seasons around the equinoxes when the earth is nearest the ecliptic (Seever, 2003). Surveying during the same dry months each year also reduces station height variability due to soil saturation and loading. Multipath and environment errors, usually caused by obstructed views between the station and the sky, can be reduced by careful site selection and extended occupation times which increase the ratio of high quality data recorded to low quality reflected or interrupted signals.

Equipment and human errors are largely unavoidable, but additional surveys beyond the minimum requirement help bind velocity calculations. For this project, this means three yearly surveys instead of two. Assuming a constant velocity or strain rate, the third occupation grants an opportunity to average out the noise and interpolate a best fit measurement rather than be forced to connect two points (e.g. Figure 8). Finally, while Hauraki Rift has been largely un-surveyed relative to the rest of NZ, there are ~15 evenly distributed sites in which some historic

data has been collected (Figure 7) dating back as far as 1993. This data should be old enough to allow a well-defined strain rate to be calculated regardless of its quality and the slow-deforming nature of our study area. It should also be recognized that the three yearly surveys from this campaign will serve as a vital cluster of data points for all future GPS geodetic surveys in the greater Hauraki Gulf region regardless of any immediate issues with precision.

3.6. Seismic Station Planning and Deployment

The GeoNet catalogue of earthquake hypocentres in the Hauraki Rift dating back to 1960 are shown in Figure 6 above and Figure 9 below. Despite the presence of both the Kerepehi and Wairoa North active faults, there seems to be little grouping of events or a general pattern of seismicity anywhere within greater Hauraki Rift region. This is either due to a legitimate lack of focused seismicity in the area, or a failure of automatic detection and location algorithms. Assuming there is in fact seismicity within the rift (Hochstein and Nixon, 1979; Hochstein et al.; 1986; Hochstein and Ballance, 1993; Chick et al., 2001) additional seismic receivers closer to the median axis of the rift should help detect and accurately locate them.

I plan to deploy 2 temporary stations comprised of RefTek 130 digitizers/loggers and Geospace GS-ONE 3-C sensors, jointly owned by the University of Auckland and Auckland Uniservices Ltd, on either side of the rift south of the Firth of Thames (shown as green triangles in Figure 9). The close proximity of these additional receivers to the Kerepehi Fault (~20 km) should increase sensitivity to small earthquakes occurring on the onshore portion of the rift. Analysis of the data from these stations in conjunction with waveform data available online for the permanent GeoNet stations (www.geonet.ord.nz) will allow improved earthquake location and lower the resolution threshold. Ideally we could locate the stations much closer to the fault, but the soft, swampy, sediment-heavy surface geology of the Hauraki Lowlands has an adverse attenuating and site specific time delay effect on the signal. These two stations will be deployed

Figure 8. While a velocity measurement (change in distance over change in time) is possible with only two surveys, each additional survey averages the instrument and human error, increasing accuracy.

for a year and will require a routine 3-month visitation cycle to quickly download data and perform an equipment check.

Figure 9. Current locations of GeoNet seismic station deploy (blue diamonds) and proposed site locations of my temporary deploy (green triangles). Catalogue events from GeoNet are in small blue stars. Active faults are drawn in red.

In addition to recording and locating new events, I plan to reprocess existing waveform data available via GeoNet to both expand the catalogue and improve upon hypocentre locations. Both P- and S-wave arrival times will be manually picked via an independent algorithm using the free and open source SEISAN (Havskov and Ottemoller, 1999) seismic processing software. Data will be retriggered unveiling events that had not met the automatic location requirement of being observed on any 5 of the Auckland network GeoNet stations denoted by blue diamonds in Figure 9 (C. Little at GNS, personal communication). 3D earthquake locations will be mapped, magnitudes calculated and patterns in space and time interrogated to improve understanding of the activity of tectonic faults in the region (e.g. van Eck and Hofstetter, 1990; Bonjer, 1997; Lyakhovsky et al., 2001).

3.7. Analysis of Gravity and Magnetic Anomalies

Satellite gravity data (Sandwell and Smith, 2009; Sandwell et al., 2014; Garcia et al., 2014) can be analysed to determine the subsurface density distribution and hence geology around the Hauraki Rift. The Bouguer Gravity Anomaly (Figure 10) has had the measured gravity corrected for non-geological factors that can otherwise be accounted for such as latitude, elevation and 3-D topography and should be representative only of subsurface density. Techniques for interpreting the gravitational effect of the topography vary greatly, but the one I will utilise takes advantage of a 2D Fourier analysis numerical method (Parker, 1995) which allows almost the complete removal of all topographic features from our gravity data, leaving only the gravitational signal produced by subsurface density contrasts. This provides the highest theoretical resolution of gravity anomalies possible without conducting additional field measurements.

Figure 10. The gravity Bouguer Anomaly, calculated by removing the topographic gravitational effect from the satellite Free Air Anomaly data (Garcia et al., 2014)

The Bouguer Gravity Anomaly (BGA) in our study region highlights regions of geologic interest, including locations where the crust may be especially thin, or thick, comprised of a large amount of low-density (e.g. sediments), or high-density (e.g. volcanics) material. It is also possible to use sediment thicknesses within the Firth of Thames derived from seismic

refraction (Ferguson et al, 1980), borehole data archived by Environment Waikato (http://www.waikatoregion.govt.nz), and gravity (Hochstein et al., 1986; Hochstein and Ballance, 1993) and subtract the gravitational effect of the sediment from this BGA to gather a clearer picture of all three faults within the greater Hauraki Rift.

Further, we can look for areas where unknown faults may exist by exploring the horizontal derivative of the BGA. This highlights areas in the crust where there is a lateral change in density, a common feature as faults are intrinsically the boundary area where two discrete sections of different rock meet. Averaging all directional derivatives gives a scalar quantity (Figure 11) which can be loosely interpreted in terms of the probability that a fault exists nearby if the region coincides with elevated crustal strain and earthquake activity.

Figure 11. The horizontal gradient of the Bouguer Anomaly (Figure 10). This highlights subsurface areas with large changes in lateral density.

The satellite gravity data can also be used in conjunction with topographic data to calculate the admittance between them in the frequency domain using Forsyth's method (1985), which allows modelling regions of lithosphere in isostatic equilibrium, or regions dynamically compensated or buoyed by upwelling asthenosphere brought about by the subducting Pacific slab. Areas within the Hauraki Rift exhibiting signs of compensation or flexural rigidity could explain any stresses currently acting on the region.

Figure 12, from Mule and Stenning (2010). A, Left: Magnetic anomaly of the southern Hauraki Rift region, obtained via airplane. B, Right: Inferred depth to the magnetic basement, in this case the Greywacke basement of New Zealand.

Finally, I plan to use new aeromagnetic data collected for Chartwell Energy NZ Pty Ltd which has been recently made publically available on the New Zealand Petroleum and Minerals database (http://www.nzpam.govt.nz/cms) to help interpret crustal structure in the Hauraki Lowlands (Figure 12A). This high-precision data, previously obtained by private industry to find potential coal deposits, has also been used to derive a predicted depth to magnetic basement (Figure 12B) which in this instance is almost certainly the basement Greywacke. This knowledge allows for the removal of the gravity signature of the sediment and provides a good estimate of crustal thickness and structure, since both sediment and Greywacke densities are well-documented in the GNS Science PETLAB database (http://pet.gns.cri.nz/index.jsp).

4. Facilities to be Used 4.1. Hardware

4.1.1. GPS Survey Gear. Free of charge, on loan from GNS Science Lower Hutt, NZ

- 8 x Trimble 5700 receivers, Trimble Zephyr 1 antennas, Tripods, Batteries w/ chargers, Optical plummets, Etc. Tools

4.1.2. School of Environment Toyota Hilux 4WD. Rentable from the University of Auckland

4.1.3. Seismic Equipment. Free of charge on loan from Auckland Uniservices Ltd.

- 2 x Geospace GS-ONE 3-C sensors, Reftek 130 Digitizers / Data Loggers, Storage Units, Solar Panels, Batteries, Etc.

4.2. Software (free / open source unless noted)

4.2.1. Generic Mapping Tools (Wessel et al., 2013, SOEST)

4.2.2. GAMIT/GLOBK 10.6 (Chandler et al., 2015, MIT)

4.2.3. dMODELS (Battaglia et al., 2013)

4.2.4. TEQC (Estey & Meertens, 1999)

4.2.5. SEISAN (Havskov and Ottemoller, 1999)

5. Budget 5.1. Geodetic Survey Costs

5.1.1. Equipment

~ All the GPS equipment required has been generously lent to us by GNS Science in Lower Hutt free of charge.

5.1.2. Ground + Air Transportation

~ 24 days car rental x $60 per day + fuel = $2100 PER YEAR

~ 4 flights x $200 = $800 PER YEAR

5.1.5. Etc. Equipment

~ $300 PER YEAR

5.1.6. PROJECT TOTAL = $9600

5.2. Seismic Survey Costs

5.2.1. Equipment: The majority of the core seismic equipment required has been generously lent to us by Auckland Uniservices Ltd free of charge.

5.2.2. Ground Transportation: 3 days car rental x $60 per day + fuel = $360 PER YEAR

5.2.3. Etc. Equipment: 2 stations x $500 = $1000 ONE TIME

5.2.4. PROJECT TOTAL = $1360

5.3. Personal Travel Costs

5.3.1 GNS Science / Wellington: 3-5 RT airfare + car rentals = $2000 PER YEAR

5.3.2 AGU: 1 RT airfare x $2500 = $2500 TOTAL

5.3.3 PROJECT TOTAL = $8500

5.4 Total Costs and Analysis

All together I expect the totality of this research to cost under $20,000 NZD, with the majority of expenditures in the first year alone.

Funding for this research for the first two years is already secured by Dr Jennifer Eccles in the form of a project grant # 9856-3706774. In addition, my personal yearly stipend for the duration of this work has been awarded by an EQC postgraduate scholarship and all three years of University of Auckland fees have been paid by DEVORA. Research related expenses in year three should be less than either the first or second years as by then the initial campaign reconnaissance and setup efforts have been completed. Regardless, assuming an even expense distribution of $7k a year, my PRESS account alone could theoretically cover the third year of surveying. We will also apply for the John Beavan Memorial Scholarship administered by the Geoscience Society of New Zealand that specifically funds student geodetic fieldwork in New Zealand.

6. Schedule

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