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Magnetovariational measurements in the Cook Strait region of New Zealand
182
Physics of the Earth and Planetary Interiors, 39 (1985) 182—193 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
Magnetovariational measurements in the Cook Strait region of New Zealand Malcolm R. Ingham Physics Department and Institute of Geophysics, Victoria University of Wellington (New Zealand) (Received December 18, 1984; revision accepted February 18, 1985)
Ingham, M.R., 1985. Magnetovariational measurements in the Cook Strait region of New Zealand. Phys. Earth Planet. Inter., 39: 182—193. Magnetovariational measurements have been made at 10 sites on the northern side of the Cook Strait, New Zealand. Single-station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the Cook Strait is most important at around 1000 s period. At longer periods the effect of induced currents in the Pacific Ocean predominates. A two-dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60—80 km distance from the strait. Closer to the strait the inductive process is strongly three-dimensional. A simple d.c. line current model of current flow has been shown to reproduce some of the features of the observed responses. Induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region.
1. Introduction In recent years much attention has been given in the literature to the problem of the “channelling” of currents through regions of high electrical conductivity. In an extensive review of the topic Jones (1983) suggested that the necessity to invoke current channelling to explain particular sets of field data arises out of the inadequacy of representing three,dimensional structures by one or two-dimensional models. For a given situation the important criterion is how far removed from the three-dimensional structure it is necessary to be before a two dimensional model is adequate. The Cook Strait region of New Zealand (Fig. 1) is perhaps an ideal situation for the investigation of these ideas. The Cook Strait links the Pacific Ocean with the Tasman Sea and is a highly threedimensional structure superimposed on the predominantly two-dimensional tectonics of New 0031-9201/85/$03.30
~ 1985 Elsevier Science Publishers B.V.
Zealand. This paper presents the results of a magnetovariational study on the north side of the strait and indicates how the measured field responses are influenced by the three-dimensional land—sea boundary. How adequately simple twodimensional models can fit the observed responses at three sites reasonably removed from the strait has also been investigated. A subsequent paper will present results of three-dimensional modelling of the region. A second important aim of the study was to identify any conductivity anomalies associated with the tectonic structure of the region. To date few geomagnetic induction studies have been carried out in New Zealand, the principal ones being those of Hurst (1974) and Midha (1979). Of these most have concentrated on measurement in the Central Volcanic Plateau and the Taupo Volcanic Zone. However, there are several features further south than these two areas which beg investigation.
183
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184
Figure 1 shows the geological and geographic structure of central New Zealand and the locations of the sites used in this study. New Zealand is situated at the boundary of the Pacific and Indian plates. The Tonga—Kermadec subduction zone lies to the northeast and the subducted Pacific Plate passes beneath the North Island. Two main faults pass through the lower North Island—the Wellington Fault and further to the east the Wairarapa Fault. The sediments to the east of the Wairarapa Fault are generally assumed to be part of the accretionary border arising from the subduction (Cole and Lewis, 1981). There are two main geophysical features in the area. A broad negative gravity anomaly cuts across the region from northeast to southwest with its minimum centered on the bend in the coastline close to the site TUR. This anomaly has been interpreted by Hatherton (1970) as being the surface manifestation of the Bemoff zone. In the north of the region a line running roughly west—east from Mount Egmont to Mount Ruapehu marks a transition from a seismic region showing high frequency transmitting characteristics to one showing attenuating characteristics (Mooney, 1970). From Mount Ruapehu this boundary bends northeastwards. The positioning of sites was such that, as well as studying the three-dimensional effect of the Cook Strait, any induction anomaly associated with either of these two features or the two faults would be detected. Each site was occupied for a period of around 10 days. The three components of the magnetic field were measured using fbuxgate magnetometers and recorded on three-channel chart recorders. The subsequently digitized data have been analyzed using standard spectral techniques.
2. Induction arrows Single-station transfer functions (Schmucker, 1970) have been calculated for each of the sites. Although a more detailed analysis would be preferable, involving the separation of the normal and anomalous magnetic fields, the problem of defimng a normal site makes this difficult. The only geomagnetic observatory in New Zealand is close
to the coast and the region of the central North Island furthest from the coasts is highly anomabus itself due to the proximity of the Central Volcanic Plateau and the Taupo Volcanic Zone. As New Zealand is situated in mid-geomagnetic latitudes the standard assumptions associated with the use of single-station transfer functions are reasonably valid although close to the Cook Strait three-dimensional effects may mean that, locally, the anomalous horizontal fields are not small. Despite this reservation the use of single-station transfer functions will be shown to give results which are consistent with relatively simple interpretations. Real (in-phase) and imaginary (quadraturephase) induction arrows have been calculated from the transfer functions. These are shown in Fig. 2 for six periods of variation. The directions of the real arrows have been reversed, as is conventional, so as to point towards high conductivities. The imaginary arrows are unreversed, however, the nature of the time dependence assumed in the calculations is such that these too point towards good conductors (Lilley and Arora, 1982). For the seven southernmost sites the contrast in the directions of the arrows at different periods is striking. At periods of 5011 and 3162 s the real arrows, with one exception, point towards the deep waters of the Pacific Ocean exhibiting an essentially nonnal coast effect (Parkinson and Jones, 1979). The imaginary arrows tend to be rotated 45 and 90°clockwise from their real counterparts and indicate the presence of quadraturephase currents in the Cook Strait. At periods of 1259 and 794 s the effect of induction in the Cook Strait is apparent in the directions of both the real and imaginary arrows. The real arrows at NGA, MOR, REI, KAH, and WAI are clearly influenced by the presence of the strait and are rotated south from their directions at longer periods. The imaginary arrows still bear the same relationship to the real arrows (rotated 45—90°clockwise) and though rotated from their directions at longer periods still point towards the Cook Strait. At the two shortest periods, 316 and 200 s, the arrows at the sites closest to the Strait are still clearly influenced by it. However, across the line of sites REI, KAI, GOL local induction appears to
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186
be important and at 200 s there is a reversal of the real arrows between REI and GOL. Real arrows at periods 70 s also exhibit this reversal. Note that at 200 s period the magnitudes of the arrows at KAI are very small and the uncertainties in the directions comparatively large. The only induction arrows at the seven southem sites which display what might be termed anomalous behaviour are the real arrows at REI at 5011 s and at KAI at 1259 s. The former is small in magnitude and points westwards to the broader region of Cook Strait and to the Tasman Sea. One possible explanation of this is that as the continental slope to the west of New Zealand is further away than that to the east, induced currents flowing in the deeper water of the Tasman Sea only start to have a comparable effect to those in the Pacific Ocean at very long periods. The northeasterly direction of the real arrow at KAI at 1259 s may be indicative of conductive structure between KAI and HOP as there is, at this period, a hint of a reversal of real arrows between these sites. Indeed, given the relatively small magnitudes, the directions of both the real and imaginary arrows at HOP are remarkably consistent at all the periods shown in Fig. 2. The behaviour of the induction arrows at KAR and TUR suggests that further investigation is required. At KAR the real arrow points persistently northeastwards despite the presence of highly conducting seawater to the southwest of the site. At the longest periods the real arrow at TUR also points in this direction. This may well indicate that there is indeed a geomagnetic anomaly associated with the seismic transmitting/attenuating boundary mentioned above. At TUR, for periods <1000 s, both arrows become very small in magnitude and show very scattered directions. TUR is situated at the centre of the major gravity low in the North Island of New Zealand and the short period behaviour of the induction arrows may be associated with either this or the superficial sediments in the region. —
3. Hypothetical event analysis In an attempt to clarify the manner in which the three-dimensional structure of the Cook Strait
affects the distribution of induced currents in the region a hypothetical event analysis (Bailey et a!., 1974) has been applied to the single-station transfer functions for two of the periods of variation. Contour plots of the magnitude (x 10) of the in-phase and quadrature-phase vertical field produced by a unit primary horizontal field of 3162 s period are shown in Fig. 3 for two polarizations of the inducing field. Similar plots for a period of 794 s are shown in Fig. 4. The two polarizations of the inducing field chosen are N35°Eand N125°Ewhich correspond to regional electric current flows, respectively, perpendicular and parallel to the principal tectonic strike of New Zealand. In a numerical model study of a conducting channel between two oceans McKirdy and Weaver (1983) showed that the former case leads to a concentration of the regional current flow through the channel. In the latter case although the regional current flow is perpendicular to the channel the model results indicate that there is a leakage of current between the oceans through the channel. The geometry and dimensions of the Cook Strait region are not directly comparable with those of the simple model chosen by McKirdy and Weaver and the period of variation is also different. Nevertheless the contour plots of Fig. 3 show results which are in considerable agreement with those of the model study. For the approximately northeastward polarization of the inducing field both the in-phase and quadrature-phase contours indicate the flow of the regionally induced currents through the Cook Strait. The change in the sign of the in-phase part of the vertical field to the northeast of the Strait may indicate, however, that the currents are not confined to the seawater channel but also flow in the less conducting land. The contour plot of in-phase Z for an inducing field of approximately southeastward polarization indicates that the Cook Strait still has a marked effect when the regional current flow is northeastwards. Even taking into account that the value of the vertical field at NGA has a relatively large uncertainty the contours do not parallel the tectomc stnke as would be the case without current flow through the Cook Strait. The sign of the in-phase field at NGA indicates that
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189
currents flow in the Cook Strait from northwest to southeast. For this polarization the induced vertical field has only a very small quadrature-phase component. As discussed below, this makes it possible to model the distribution of current flow by a simple line current model. A further feature which is evident in Fig. 3 is that for the southeastward inducing field an anomaly associated with TUR is evident as a local high in the in-phase vertical field. In this area the direction of the contours, though ill-determined, appears to be perpendicular rather than parallel to the coast. The corresponding plots for a period of 794 s, shown in Fig. 4, are harder to interpret. For the N35°E direction of polarization of the inducing field the in-phase contours still suggest the concentration of regional currents through Cook Strait, In this plot, the line of zero vertical field bends northwards, possibly due to the deviation of currents around the bend in the Strait. The quadrature-phase values are all small except for that at WA!, the only site where the quadrature-phase induction arrow at this period points towards the southeastern end of the Cook Strait rather than the narrowest portion of the channel. Both contour plots for the second polarization direction differ markedly from the plots for the longer period. At 3162 s the in-phase field mdicates the leakage of regional currents through the Strait from the Tasman Sea to the Pacific Ocean. At 794 s, the change in sign of the in-phase response between NGA and MOR is not inconsistent with currents in the Cook Strait and Pacific Ocean flowing in directions such as to produce vertical fields which are in opposite directions, that is, the currents in the Cook Strait flow southeast to northwest. However, the low in the negative contours in the centre of the region is hard to reconcile with this effect and is probably due to induction in more localized structures paralleling the tectonic strike. An anomaly, of similar shape stretching from the southwest coast to north of KA!, is also present in the quadraturephase plot, The anomaly close to TUR is also apparent in the plots for 794 s period. A concentration of contours between KAR and TUR appears in the
in-phase field for N35°E polarization and the quadrature-phase field for N125°Epolarization.
4. Modelling Modelling has been carried out to investigate how far away from the three-dimensional structure of Cook Strait it is necessary to be before the observed response can be modelled by a two-dimensional structure. Qualitatively it is apparent from Figs. 3 and 4 that even when the regional current flow is parallel to the tectonic strike there is leakage of current through the Cook Strait. The alignment of contours in Fig. 3 suggests that, for a period of 3162 s, the effect of this leakage of current is still important at a considerable distance from the strait. At a period of 794 s the effect appears to be much more localized to the immediate vicinity of the strait. The approximate line of sites REI, KAI and GOL is almost halfway between the southeastern end of Cook Strait and the bend in the coastline close to TUR. To try and obtain a feel for the relative magnitudes of three- and two-dimensional effects the vertical field response induced at these sites by a unit horizontal field with N125°Epolarization has been compared to that calculated numerically for two simple two-dimensional models. The two models and the results of the cornparison are shown in Fig. 5. Model A investigates how well the vertical field response can be modelled purely by a two-dimensional coast effect. The only lateral conductivity contrasts in the model are those between land and sea. To the northwest of RE! the average depth of seawater is 200 m until the continental slope is reached at around 300 km distance. This transition is represented in the model by a change in conductivity of a 1 km thick layer such as to preserve the contrast in the integrated conductivity. Thus 200 rn of seawater of resistivity 0.25 ~lm is modelled by 1 km of 1.25 ~ m resistivity. The change in depth from 200 m to 1 km in the Pacific Ocean has been represented in the same manner. Model B has the added feature of a 3 km thick surface layer of resistivity 1 ~m stretching for 20 km southeast from the Wairarapa Fault. The value —
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of resistivity and the thickness of the layer are suggested by preliminary magnetotelluric measurements which will be reported elsewhere. Geologically this highly conducting block corresponds to the superficial sands and gravels shown in Fig. 1. The results are presented in the form of the amplitude and phase of the ratio of the vertical to horizontal field (Z/H) at periods of 3162 and 794 s. The uncertainty bars in the field data represent one standard deviation. The model responses have been calculated using the finite difference technique of Jones and Pascoe (1971). As the program used does not calculate absolute values of phase, the phases of Z/H are all relative to that at GOL. The results suggest that across the line of sites REI, KAI, GOL, the response to an inducing field of N125°Epolarization can be quite well represented by a two-dimensional structure. However,
the observed response cannot be modelled particularly successfully solely in terms of the coast effect. The response for Model A does not fit the observed phase at either period and at a period of 794 s gives too high a value for the amplitude of Z/H at GOL. In contrast Model B, containing only a simple conductive structure, produces a much better fit to the observed responses. Although the failure of this model to give an exact fit to the amplitude at REI at 3162 s and the phase at the same site at 794 s may be indicative of threedimensionality it is likely than an improved fit could be obtained by introducing more structure into the model. This is supported by the fact that for a period of 200 s, at which local induction mught be expected to be more important, the response calculated for Model B does not give such a good fit to the observed responses.
191
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192
Closer to the Cook Strait the observed responses are clearly not two-dimensional and three-dimensional modelling is necessary. As a precursor to full three-dimensional modelling a simple line current model of the current flow through Cook Strait has been used to model the in-phase vertical field response to an inducing field of N125°Epolarization. Such a model is not physically realistic and is only appropriate to this single polarization of the inducing field. Nevertheless it serves to give an indication of the geometry of the current flow through Cook Strait and how full three-dimensional modelling might be expected to give a fit to the field data. At a period of 3162 s, as was remarked above, there is little quadrature-phase response to an inducing field of N125°Epolarization and hence a d.c. line current model is slightly more valid for this period than for 794 s period at which there is considerable quadrature-phase response. As the regional current flow is parallel to the coastline for this polarization the line current models include a current along the continental shelf of the Pacific Ocean. To fit the in-phase responses of Figs. 3 and 4 this current is required to either divide so that current flows northwestwards through Cook Strait (794 s) or be joined by a southeastwards current flow through the strait (3162 s). If the geometry of the current flow is fixed then the magnitudes of the currents through the strait and along the coast can be calculated analytically so as to give the best least-squares fit to the observed vertical field responses at the seven sites NGA, MOR, KAH, WAI, REI, KAI and GOL. This is the procedure which has been adopted and the assumed geometry of the current flow and the calculated responses are shown in Fig. 6. Comparison of Fig. 6 with the relevant parts of Figs. 3 and 4 shows that at least some of the main features of the observed responses have been reproduced. The failure to reproduce some features, for example the elongated anomaly at 794 s, however, gives support to the argument that local conductivity contrasts on the land are also required in any full three-dimensional model. As indicated above the line current modelling requires a difference in the direction of current flow through the Cook Strait for the two periods.
At the longer period the current flow is in the opposite direction to that which might be expected from the direction of the regional current flow. A possible explanation of this is the effect at this period of the detailed bathymetry of the Pacific Ocean. In particular the Chathan Rise to the southeast of the Cook Strait may play a role in the deflection of induced currents.
5. Sununary and discussion This paper has reported the first of a series of magnetovariational studies in New Zealand. The results which have been obtained and how they pertain to the objectives of the study can be summarized as follows. At periods of variation of 3000 s and above the principal factor influencing geomagnetic variations at sites close to the Cook Strait appears to be the flow of induced currents in the Pacific Ocean. Even at these long periods there is some channelling or leakage of current through the Cook Strait. Induction in the three-dimensional structure of the strait is much more important at periods around 1000 s. For shorter periods the effect of the Cook Strait is still important at sites close to it but induction in more localized structures also has a significant effect on the observed responses. At a distance of around 60 km from the southeastern end of the strait the field data can be quite well modelled by a two-dimensional structure contaming both local conductivity variations and changes in the depths of the oceans. Closer to the Cook Strait three-dimensional modelling is required and even a simple three-dimensional d.c. line current model can reproduce some of the features of the observed field responses. Apparent conductivity anomalies are related to several of the tectonic and geophysical features in the region. A reversal of induction arrows between GOL and REI at 200 s period suggests a significant conductivity contrast associated with one of the two main faults. There is also some indication of an anomaly between sites HOP and KAI possibly striking perpendicular to the principal tectonic trend. The persistently northward pointing real induction arrow at KAR suggests that there may
193
well be a conductivity anomaly connected with the west—east seismic boundary between Mounts Egmont and Raupehu. The very small response at TUR may be associated either with this or with an anomaly linked to the gravity low in the area.
Acknowledgements This work was partly supported by University Grants Committee grant No. 83/153. The author thanks Messrs Dalzell, O’Hara, Wyeth, Bosch, McLeod, Redmayne, Hooper and Armistead for their willing help and use of their facilities in the field,
References Bailey, R.C., Edwards, R.N., Garland, G.D., Kurtz, R. and Pitcher, D.H., 1974. Electrical conductivity studies over a tectonically active area in eastern Canada. J. Geomagn. Geoelectr., 26: 125—146. Cole, J.W. and Lewis, K.B., 1981. Evolution of the Taupo—Hikurangi subduction system. Tectonophysics, 72: 1-21.
Hatherton, T., 1970. Upper mantle iñhomogeneity beneath New Zealand: surface manifestations. J. Geophys. Res., 75: 269284. Hurst, A.W., 1974. Magnetic effects in volcanic regions. Ph.D. thesis, Victoria University of Wellington, New Zealand, 369 pp. Jones, A.G., 1983. The problem of current channelling: a critical review. Geophys. Surv., 6: 79—122. Jones, F.W. and Pascoe, U., 1971. A general computer program to determine the perturbation of alternating electric currents in a two-dimensional model of a region of uniform conductivity with an embedded inhomogeneity. Geophys. J.R. Astron. Soc., 24: 3—30. Lilley, F.E.M. and Arora, B.R., 1982. sign convention for quadrature Parkinson arrows in The geomagnetic induction studies. Rev. Geophys. Space Phys., 20: 513—518. McKirdy, D.McA. and Weaver, J.T., 1983. A numerical study of the channelling of induced currents between two oceans. J. Geomagn. Geoelectr., 35: 623—641. Midha, R.K., 1979. Geoelectromagnetic induction studies in the North Island Volcanic Region, New Zealand. Ph.D. Thesis, Victoria University of Wellington, New Zealand, 342 pp. Mooney, H.M., 1970. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75:
285—309. Parkinson, W.D. and Jones, F.W., 1979. The geomagnetic coast effect. Rev. Geophys. Space Phys., 17: 1999—2015. Schmucker, U., 1970. Anomalies of geomagnetic variations in the southwestern United States. Bull. Scripps Inst. Ocean., Univ. of California, 13.
Physics of the Earth and Planetary Interiors, 39 (1985) 182—193 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
Magnetovariational measurements in the Cook Strait region of New Zealand Malcolm R. Ingham Physics Department and Institute of Geophysics, Victoria University of Wellington (New Zealand) (Received December 18, 1984; revision accepted February 18, 1985)
Ingham, M.R., 1985. Magnetovariational measurements in the Cook Strait region of New Zealand. Phys. Earth Planet. Inter., 39: 182—193. Magnetovariational measurements have been made at 10 sites on the northern side of the Cook Strait, New Zealand. Single-station transfer functions have been calculated for the sites and indicate that the effect of induction in the shallow water of the Cook Strait is most important at around 1000 s period. At longer periods the effect of induced currents in the Pacific Ocean predominates. A two-dimensional electrical conductivity model including local conductivity structure has been shown to satisfy the measured responses at sites about 60—80 km distance from the strait. Closer to the strait the inductive process is strongly three-dimensional. A simple d.c. line current model of current flow has been shown to reproduce some of the features of the observed responses. Induction arrows indicate the existence of conductivity anomalies associated with a known lateral seismic boundary and with one of the two principal faults in the region.
1. Introduction In recent years much attention has been given in the literature to the problem of the “channelling” of currents through regions of high electrical conductivity. In an extensive review of the topic Jones (1983) suggested that the necessity to invoke current channelling to explain particular sets of field data arises out of the inadequacy of representing three,dimensional structures by one or two-dimensional models. For a given situation the important criterion is how far removed from the three-dimensional structure it is necessary to be before a two dimensional model is adequate. The Cook Strait region of New Zealand (Fig. 1) is perhaps an ideal situation for the investigation of these ideas. The Cook Strait links the Pacific Ocean with the Tasman Sea and is a highly threedimensional structure superimposed on the predominantly two-dimensional tectonics of New 0031-9201/85/$03.30
~ 1985 Elsevier Science Publishers B.V.
Zealand. This paper presents the results of a magnetovariational study on the north side of the strait and indicates how the measured field responses are influenced by the three-dimensional land—sea boundary. How adequately simple twodimensional models can fit the observed responses at three sites reasonably removed from the strait has also been investigated. A subsequent paper will present results of three-dimensional modelling of the region. A second important aim of the study was to identify any conductivity anomalies associated with the tectonic structure of the region. To date few geomagnetic induction studies have been carried out in New Zealand, the principal ones being those of Hurst (1974) and Midha (1979). Of these most have concentrated on measurement in the Central Volcanic Plateau and the Taupo Volcanic Zone. However, there are several features further south than these two areas which beg investigation.
183
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Fig. 1. The geological and geographical structure of the Cook Strait region of New Zealand showing the location of geomagnetic sites.
184
Figure 1 shows the geological and geographic structure of central New Zealand and the locations of the sites used in this study. New Zealand is situated at the boundary of the Pacific and Indian plates. The Tonga—Kermadec subduction zone lies to the northeast and the subducted Pacific Plate passes beneath the North Island. Two main faults pass through the lower North Island—the Wellington Fault and further to the east the Wairarapa Fault. The sediments to the east of the Wairarapa Fault are generally assumed to be part of the accretionary border arising from the subduction (Cole and Lewis, 1981). There are two main geophysical features in the area. A broad negative gravity anomaly cuts across the region from northeast to southwest with its minimum centered on the bend in the coastline close to the site TUR. This anomaly has been interpreted by Hatherton (1970) as being the surface manifestation of the Bemoff zone. In the north of the region a line running roughly west—east from Mount Egmont to Mount Ruapehu marks a transition from a seismic region showing high frequency transmitting characteristics to one showing attenuating characteristics (Mooney, 1970). From Mount Ruapehu this boundary bends northeastwards. The positioning of sites was such that, as well as studying the three-dimensional effect of the Cook Strait, any induction anomaly associated with either of these two features or the two faults would be detected. Each site was occupied for a period of around 10 days. The three components of the magnetic field were measured using fbuxgate magnetometers and recorded on three-channel chart recorders. The subsequently digitized data have been analyzed using standard spectral techniques.
2. Induction arrows Single-station transfer functions (Schmucker, 1970) have been calculated for each of the sites. Although a more detailed analysis would be preferable, involving the separation of the normal and anomalous magnetic fields, the problem of defimng a normal site makes this difficult. The only geomagnetic observatory in New Zealand is close
to the coast and the region of the central North Island furthest from the coasts is highly anomabus itself due to the proximity of the Central Volcanic Plateau and the Taupo Volcanic Zone. As New Zealand is situated in mid-geomagnetic latitudes the standard assumptions associated with the use of single-station transfer functions are reasonably valid although close to the Cook Strait three-dimensional effects may mean that, locally, the anomalous horizontal fields are not small. Despite this reservation the use of single-station transfer functions will be shown to give results which are consistent with relatively simple interpretations. Real (in-phase) and imaginary (quadraturephase) induction arrows have been calculated from the transfer functions. These are shown in Fig. 2 for six periods of variation. The directions of the real arrows have been reversed, as is conventional, so as to point towards high conductivities. The imaginary arrows are unreversed, however, the nature of the time dependence assumed in the calculations is such that these too point towards good conductors (Lilley and Arora, 1982). For the seven southernmost sites the contrast in the directions of the arrows at different periods is striking. At periods of 5011 and 3162 s the real arrows, with one exception, point towards the deep waters of the Pacific Ocean exhibiting an essentially nonnal coast effect (Parkinson and Jones, 1979). The imaginary arrows tend to be rotated 45 and 90°clockwise from their real counterparts and indicate the presence of quadraturephase currents in the Cook Strait. At periods of 1259 and 794 s the effect of induction in the Cook Strait is apparent in the directions of both the real and imaginary arrows. The real arrows at NGA, MOR, REI, KAH, and WAI are clearly influenced by the presence of the strait and are rotated south from their directions at longer periods. The imaginary arrows still bear the same relationship to the real arrows (rotated 45—90°clockwise) and though rotated from their directions at longer periods still point towards the Cook Strait. At the two shortest periods, 316 and 200 s, the arrows at the sites closest to the Strait are still clearly influenced by it. However, across the line of sites REI, KAI, GOL local induction appears to
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186
be important and at 200 s there is a reversal of the real arrows between REI and GOL. Real arrows at periods 70 s also exhibit this reversal. Note that at 200 s period the magnitudes of the arrows at KAI are very small and the uncertainties in the directions comparatively large. The only induction arrows at the seven southem sites which display what might be termed anomalous behaviour are the real arrows at REI at 5011 s and at KAI at 1259 s. The former is small in magnitude and points westwards to the broader region of Cook Strait and to the Tasman Sea. One possible explanation of this is that as the continental slope to the west of New Zealand is further away than that to the east, induced currents flowing in the deeper water of the Tasman Sea only start to have a comparable effect to those in the Pacific Ocean at very long periods. The northeasterly direction of the real arrow at KAI at 1259 s may be indicative of conductive structure between KAI and HOP as there is, at this period, a hint of a reversal of real arrows between these sites. Indeed, given the relatively small magnitudes, the directions of both the real and imaginary arrows at HOP are remarkably consistent at all the periods shown in Fig. 2. The behaviour of the induction arrows at KAR and TUR suggests that further investigation is required. At KAR the real arrow points persistently northeastwards despite the presence of highly conducting seawater to the southwest of the site. At the longest periods the real arrow at TUR also points in this direction. This may well indicate that there is indeed a geomagnetic anomaly associated with the seismic transmitting/attenuating boundary mentioned above. At TUR, for periods <1000 s, both arrows become very small in magnitude and show very scattered directions. TUR is situated at the centre of the major gravity low in the North Island of New Zealand and the short period behaviour of the induction arrows may be associated with either this or the superficial sediments in the region. —
3. Hypothetical event analysis In an attempt to clarify the manner in which the three-dimensional structure of the Cook Strait
affects the distribution of induced currents in the region a hypothetical event analysis (Bailey et a!., 1974) has been applied to the single-station transfer functions for two of the periods of variation. Contour plots of the magnitude (x 10) of the in-phase and quadrature-phase vertical field produced by a unit primary horizontal field of 3162 s period are shown in Fig. 3 for two polarizations of the inducing field. Similar plots for a period of 794 s are shown in Fig. 4. The two polarizations of the inducing field chosen are N35°Eand N125°Ewhich correspond to regional electric current flows, respectively, perpendicular and parallel to the principal tectonic strike of New Zealand. In a numerical model study of a conducting channel between two oceans McKirdy and Weaver (1983) showed that the former case leads to a concentration of the regional current flow through the channel. In the latter case although the regional current flow is perpendicular to the channel the model results indicate that there is a leakage of current between the oceans through the channel. The geometry and dimensions of the Cook Strait region are not directly comparable with those of the simple model chosen by McKirdy and Weaver and the period of variation is also different. Nevertheless the contour plots of Fig. 3 show results which are in considerable agreement with those of the model study. For the approximately northeastward polarization of the inducing field both the in-phase and quadrature-phase contours indicate the flow of the regionally induced currents through the Cook Strait. The change in the sign of the in-phase part of the vertical field to the northeast of the Strait may indicate, however, that the currents are not confined to the seawater channel but also flow in the less conducting land. The contour plot of in-phase Z for an inducing field of approximately southeastward polarization indicates that the Cook Strait still has a marked effect when the regional current flow is northeastwards. Even taking into account that the value of the vertical field at NGA has a relatively large uncertainty the contours do not parallel the tectomc stnke as would be the case without current flow through the Cook Strait. The sign of the in-phase field at NGA indicates that
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189
currents flow in the Cook Strait from northwest to southeast. For this polarization the induced vertical field has only a very small quadrature-phase component. As discussed below, this makes it possible to model the distribution of current flow by a simple line current model. A further feature which is evident in Fig. 3 is that for the southeastward inducing field an anomaly associated with TUR is evident as a local high in the in-phase vertical field. In this area the direction of the contours, though ill-determined, appears to be perpendicular rather than parallel to the coast. The corresponding plots for a period of 794 s, shown in Fig. 4, are harder to interpret. For the N35°E direction of polarization of the inducing field the in-phase contours still suggest the concentration of regional currents through Cook Strait, In this plot, the line of zero vertical field bends northwards, possibly due to the deviation of currents around the bend in the Strait. The quadrature-phase values are all small except for that at WA!, the only site where the quadrature-phase induction arrow at this period points towards the southeastern end of the Cook Strait rather than the narrowest portion of the channel. Both contour plots for the second polarization direction differ markedly from the plots for the longer period. At 3162 s the in-phase field mdicates the leakage of regional currents through the Strait from the Tasman Sea to the Pacific Ocean. At 794 s, the change in sign of the in-phase response between NGA and MOR is not inconsistent with currents in the Cook Strait and Pacific Ocean flowing in directions such as to produce vertical fields which are in opposite directions, that is, the currents in the Cook Strait flow southeast to northwest. However, the low in the negative contours in the centre of the region is hard to reconcile with this effect and is probably due to induction in more localized structures paralleling the tectonic strike. An anomaly, of similar shape stretching from the southwest coast to north of KA!, is also present in the quadraturephase plot, The anomaly close to TUR is also apparent in the plots for 794 s period. A concentration of contours between KAR and TUR appears in the
in-phase field for N35°E polarization and the quadrature-phase field for N125°Epolarization.
4. Modelling Modelling has been carried out to investigate how far away from the three-dimensional structure of Cook Strait it is necessary to be before the observed response can be modelled by a two-dimensional structure. Qualitatively it is apparent from Figs. 3 and 4 that even when the regional current flow is parallel to the tectonic strike there is leakage of current through the Cook Strait. The alignment of contours in Fig. 3 suggests that, for a period of 3162 s, the effect of this leakage of current is still important at a considerable distance from the strait. At a period of 794 s the effect appears to be much more localized to the immediate vicinity of the strait. The approximate line of sites REI, KAI and GOL is almost halfway between the southeastern end of Cook Strait and the bend in the coastline close to TUR. To try and obtain a feel for the relative magnitudes of three- and two-dimensional effects the vertical field response induced at these sites by a unit horizontal field with N125°Epolarization has been compared to that calculated numerically for two simple two-dimensional models. The two models and the results of the cornparison are shown in Fig. 5. Model A investigates how well the vertical field response can be modelled purely by a two-dimensional coast effect. The only lateral conductivity contrasts in the model are those between land and sea. To the northwest of RE! the average depth of seawater is 200 m until the continental slope is reached at around 300 km distance. This transition is represented in the model by a change in conductivity of a 1 km thick layer such as to preserve the contrast in the integrated conductivity. Thus 200 rn of seawater of resistivity 0.25 ~lm is modelled by 1 km of 1.25 ~ m resistivity. The change in depth from 200 m to 1 km in the Pacific Ocean has been represented in the same manner. Model B has the added feature of a 3 km thick surface layer of resistivity 1 ~m stretching for 20 km southeast from the Wairarapa Fault. The value —
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of resistivity and the thickness of the layer are suggested by preliminary magnetotelluric measurements which will be reported elsewhere. Geologically this highly conducting block corresponds to the superficial sands and gravels shown in Fig. 1. The results are presented in the form of the amplitude and phase of the ratio of the vertical to horizontal field (Z/H) at periods of 3162 and 794 s. The uncertainty bars in the field data represent one standard deviation. The model responses have been calculated using the finite difference technique of Jones and Pascoe (1971). As the program used does not calculate absolute values of phase, the phases of Z/H are all relative to that at GOL. The results suggest that across the line of sites REI, KAI, GOL, the response to an inducing field of N125°Epolarization can be quite well represented by a two-dimensional structure. However,
the observed response cannot be modelled particularly successfully solely in terms of the coast effect. The response for Model A does not fit the observed phase at either period and at a period of 794 s gives too high a value for the amplitude of Z/H at GOL. In contrast Model B, containing only a simple conductive structure, produces a much better fit to the observed responses. Although the failure of this model to give an exact fit to the amplitude at REI at 3162 s and the phase at the same site at 794 s may be indicative of threedimensionality it is likely than an improved fit could be obtained by introducing more structure into the model. This is supported by the fact that for a period of 200 s, at which local induction mught be expected to be more important, the response calculated for Model B does not give such a good fit to the observed responses.
191
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192
Closer to the Cook Strait the observed responses are clearly not two-dimensional and three-dimensional modelling is necessary. As a precursor to full three-dimensional modelling a simple line current model of the current flow through Cook Strait has been used to model the in-phase vertical field response to an inducing field of N125°Epolarization. Such a model is not physically realistic and is only appropriate to this single polarization of the inducing field. Nevertheless it serves to give an indication of the geometry of the current flow through Cook Strait and how full three-dimensional modelling might be expected to give a fit to the field data. At a period of 3162 s, as was remarked above, there is little quadrature-phase response to an inducing field of N125°Epolarization and hence a d.c. line current model is slightly more valid for this period than for 794 s period at which there is considerable quadrature-phase response. As the regional current flow is parallel to the coastline for this polarization the line current models include a current along the continental shelf of the Pacific Ocean. To fit the in-phase responses of Figs. 3 and 4 this current is required to either divide so that current flows northwestwards through Cook Strait (794 s) or be joined by a southeastwards current flow through the strait (3162 s). If the geometry of the current flow is fixed then the magnitudes of the currents through the strait and along the coast can be calculated analytically so as to give the best least-squares fit to the observed vertical field responses at the seven sites NGA, MOR, KAH, WAI, REI, KAI and GOL. This is the procedure which has been adopted and the assumed geometry of the current flow and the calculated responses are shown in Fig. 6. Comparison of Fig. 6 with the relevant parts of Figs. 3 and 4 shows that at least some of the main features of the observed responses have been reproduced. The failure to reproduce some features, for example the elongated anomaly at 794 s, however, gives support to the argument that local conductivity contrasts on the land are also required in any full three-dimensional model. As indicated above the line current modelling requires a difference in the direction of current flow through the Cook Strait for the two periods.
At the longer period the current flow is in the opposite direction to that which might be expected from the direction of the regional current flow. A possible explanation of this is the effect at this period of the detailed bathymetry of the Pacific Ocean. In particular the Chathan Rise to the southeast of the Cook Strait may play a role in the deflection of induced currents.
5. Sununary and discussion This paper has reported the first of a series of magnetovariational studies in New Zealand. The results which have been obtained and how they pertain to the objectives of the study can be summarized as follows. At periods of variation of 3000 s and above the principal factor influencing geomagnetic variations at sites close to the Cook Strait appears to be the flow of induced currents in the Pacific Ocean. Even at these long periods there is some channelling or leakage of current through the Cook Strait. Induction in the three-dimensional structure of the strait is much more important at periods around 1000 s. For shorter periods the effect of the Cook Strait is still important at sites close to it but induction in more localized structures also has a significant effect on the observed responses. At a distance of around 60 km from the southeastern end of the strait the field data can be quite well modelled by a two-dimensional structure contaming both local conductivity variations and changes in the depths of the oceans. Closer to the Cook Strait three-dimensional modelling is required and even a simple three-dimensional d.c. line current model can reproduce some of the features of the observed field responses. Apparent conductivity anomalies are related to several of the tectonic and geophysical features in the region. A reversal of induction arrows between GOL and REI at 200 s period suggests a significant conductivity contrast associated with one of the two main faults. There is also some indication of an anomaly between sites HOP and KAI possibly striking perpendicular to the principal tectonic trend. The persistently northward pointing real induction arrow at KAR suggests that there may
193
well be a conductivity anomaly connected with the west—east seismic boundary between Mounts Egmont and Raupehu. The very small response at TUR may be associated either with this or with an anomaly linked to the gravity low in the area.
Acknowledgements This work was partly supported by University Grants Committee grant No. 83/153. The author thanks Messrs Dalzell, O’Hara, Wyeth, Bosch, McLeod, Redmayne, Hooper and Armistead for their willing help and use of their facilities in the field,
References Bailey, R.C., Edwards, R.N., Garland, G.D., Kurtz, R. and Pitcher, D.H., 1974. Electrical conductivity studies over a tectonically active area in eastern Canada. J. Geomagn. Geoelectr., 26: 125—146. Cole, J.W. and Lewis, K.B., 1981. Evolution of the Taupo—Hikurangi subduction system. Tectonophysics, 72: 1-21.
Hatherton, T., 1970. Upper mantle iñhomogeneity beneath New Zealand: surface manifestations. J. Geophys. Res., 75: 269284. Hurst, A.W., 1974. Magnetic effects in volcanic regions. Ph.D. thesis, Victoria University of Wellington, New Zealand, 369 pp. Jones, A.G., 1983. The problem of current channelling: a critical review. Geophys. Surv., 6: 79—122. Jones, F.W. and Pascoe, U., 1971. A general computer program to determine the perturbation of alternating electric currents in a two-dimensional model of a region of uniform conductivity with an embedded inhomogeneity. Geophys. J.R. Astron. Soc., 24: 3—30. Lilley, F.E.M. and Arora, B.R., 1982. sign convention for quadrature Parkinson arrows in The geomagnetic induction studies. Rev. Geophys. Space Phys., 20: 513—518. McKirdy, D.McA. and Weaver, J.T., 1983. A numerical study of the channelling of induced currents between two oceans. J. Geomagn. Geoelectr., 35: 623—641. Midha, R.K., 1979. Geoelectromagnetic induction studies in the North Island Volcanic Region, New Zealand. Ph.D. Thesis, Victoria University of Wellington, New Zealand, 342 pp. Mooney, H.M., 1970. Upper mantle inhomogeneity beneath New Zealand: seismic evidence. J. Geophys. Res., 75:
285—309. Parkinson, W.D. and Jones, F.W., 1979. The geomagnetic coast effect. Rev. Geophys. Space Phys., 17: 1999—2015. Schmucker, U., 1970. Anomalies of geomagnetic variations in the southwestern United States. Bull. Scripps Inst. Ocean., Univ. of California, 13.