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Measuring Contemporary Deformation in the Taupo Volcanic Zone, New Zealand, Using Sar Interferometry

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Measuring Contemporary Deformation in the Taupo Volcanic Zone, New Zealand, Using Sar Interferometry MEASURING CONTEMPORARY DEFORMATION IN THE TAUPO VOLCANIC ZONE, NEW ZEALAND, USING SAR INTERFEROMETRY J. K. Hole(1), A. Hooper(2), G. Wadge(1) and N. F. Stevens(3) (1) ESSC, University of Reading, UK (2) Stanford University, USA (3) Institute of Geological and Nuclear Sciences, NZ ABSTRACT The Taupo Volcanic Zone (TVZ) is an area of active back-arc extension in the North Island, New Zealand that represents the most productive area of rhyolitic magmatism on earth. The relationship between the magmatism and active rifting has produced a complex system where it is hard to distinguish between cause and effect, and how tectonic and volcanic deformation signals are distributed across the TVZ. There are also localised areas of subsidence related to geothermal fluid extraction. At present, deformation in the TVZ is measured by differential GPS and at widely spaced continuous stations. Campaign GPS has shown that the TVZ is widening by 8mm/yr but so far the network has been too sparse to identify how the extension is distributed. In this paper we investigate the use of C-band dInSAR for the measurement of TVZ deformation. Archived descending-pass ERS SAR images are available from 1996-2003 and ascending and descending Envisat ASAR images have been collected since 2003. We have shown that C-band can be used to measure contemporary deformation in the TVZ. Using short-term, low baseline pairs, it is possible to map the extent of geothermal subsidence with a high spatial resolution. Extracting tectonic information is difficult due to large temporal decorrelation in the interferograms but by stacking ERS descending interferograms we have identified an area of large-scale uplift of 10mm/year (line of sight) north of Lake Taupo. Persistent Scatterer analysis has detected a similar sense of motion across the TVZ that compares well with Lake Levelling data. Fig. 1 Summary of TVZ structure. Geothermal areas [4] (blue); active Fault traces of the TFB [6] (green);the caldera systems active in the last 2 million years [1] (red);TVZ boundary (inset) [1] 1 INTRODUCTION The Taupo volcanic zone (TVZ) is an area of back arc spreading associated with the subduction of the Pacific plate beneath the Australian plate. The TVZ has been described as the most intense region of magmatism on this planet [1]. The main geological features, including the locations of the known TVZ calderas active in the last 2 Ma years, are shown in Fig. 1. Two of these caldera systems, Taupo and Okataina, have been active in the last two thousand years; the c. A.D. 186 Taupo eruption was thought to be one of the most explosive eruptions ever documented [2], and the last Okataina eruption was from Mount Tarawera in 1886 AD [3]. The TVZ also has a very high heat output, 4200 W [4], which drives 23 geothermal systems. The extents of these fields are marked on Fig. 1. Many have been exploited to provide heat and electricity. The TVZ is also undergoing active extension of about 8mm/year [5]. The surface expression of this is manifested as a band of mainly normal faulting (Taupo Fault Belt, TFB) [6], and is associated with a high level of background siesmicity [7]. Fig. 1 demonstrates how complex and interactive the deformation sources in the TVZ are. They act on very different length and time scales. The aims of this work are to better understand the surface deformation in the TVZ; to distinguish the contributions from tectonic, magmatic and geothermal components of deformation; and to explore the ability of C- band InSAR to measure the various deformation signals and compare them with other geodetic sources of deformation such as GPS and levelling. After describing the expected deformation signals, we discuss the available radar data. Section 4 presents the deformation measured by dInSAR. We conclude with a discussion of the value of using InSAR and future prospects. 2 EXPECTED DEFORMATION GPS data from 1990 to 2001 show an opening of the TVZ region in the order of 8mm /year [6]. Horizontal velocity data, calculated from continuous and campaign GPS data from 1990 to 2004 [8] was interpolated onto a 5 km grid and projected into the line of sight (LOS) for the 3 satellite configurations. Fig. 2 shows the relative LOS motion across each simulated interferogram. The different viewing angles have different sensitivities to measuring horizontal motion across the TVZ. In all cases, the relative LOS change is very much less that one fringe (28mm). Over short time periods (less than one year) these signals will be hard to differentiate from the interferogram noise. Campaign velocity data within the TVZ is sparse and local deviations from the simple tectonic model of Fig. 2 are expected: motions of faults in the TFB, for example, the 1987 Edgecumbe earthquake [9] and the 1983 Taupo earthquake swarm [10]; deformation caused by magma intrusion into the crust beneath Lake Taupo [11], which may act on a larger scale; and subsidence at geothermal fields due to pressure draw-down, on a smaller scale [12]. ERS Descending θ = 23° Envisat Descending θ = 41° Envisat Ascending θ = 23° 2 mm/year4 mm/year 1 mm/year Fig. 2 Relative line of sight motion projected from GPS horizontal velocity, in the period 1990 to 2004, using the method of Beavan and Haines [8]. Dashed line shows the TVZ outline 3 AVAILABLE DATA 24 ERS data are available from 1996 to 2003 and, at time of this study, there are 15 ascending and 16 descending Envisat data (Fig. 3). The ascending pass was collected on swath 2 for continuity with the ERS data. The main component of the tectonic signal is horizontal and is in a NNE-NSW direction, so swath 5 was chosen for the descending pass because it is more sensitive to horizontal motion. There are only two periods with a good time series: 1998 to 2000 and 2003 to 2005. The descending pass satellite direction is perpendicular to the expected extension direction of the TVZ, and although this is optimum for measuring the extension, orbital errors, which are most common in the azimuth direction, may be mistaken for deformation. 4 RESULTS When studying distributed interferograms, only image pairs with orbital baselines of less than 200m and with time periods of less than one year have been used in order to minimise topographic errors and maximise coherence. Fig. 4a shows a 140 day ERS interferogram with a baseline of 161 m. There is good coherence in the urban areas but across the rest of the terrain the coherence is patchy. There is large area of native New Zealand forest, east of Lake Taupo, which is completely decorrelated. The rest of the image, mainly farmland and planted forest, is also affected by temporal decorrelation. There is also evidence of small scale atmospheric variability that could be mistaken for deformation. 4.1 Geothermal deformation Figure 4a shows that due to temporal decorrelation, not all of the fields are coherent in all the interferograms; however, four of the coherent fields (marked in red) show deformation signals in more than one independent interferogram. Fluid extraction is occurring at all of these fields. At Wairakei–Tauhara (Wa-Ta), development began in the 1950’s and over 16m of subsidence has been recorded. Fluid re-injection began in the 1990s and subsidence rates have since slowed but are still continuing. Rates of up to 80 mm/year have been measured using InSAR. Subsidence has also been observed at Rotorua (Ro) and Ohaaki (Oh) geothermal fields. Conversely, an inflation signal has been observed at Mokai (Mo) from 1999 to October 2000. This period corresponds to the date of commissioning of the power station. This looks to be fault controlled, but we are still investigating the cause of this signal. 1500 1500 1000 1000 500 500 0 0 -500 -500 -1000 -1000 ERS perpendicular baseline (m) Envisat perpendicular baseline (m) ERS-2 Ascending -1500 ERS-1 Descending -1500 1996 1997 1998 1999 2000 2001 2002 2003 2004 2003 2004 2005 Time (years) Time (years) Fig. 3 Perpendicular baselines ERS, track 358 frame 4383, referenced to ERS-1 orbit 25396 (green); Envisat, track 380 frame 6412, referenced to orbit 08542 (blue); and Envisat, track 272 frame 4397, referenced to orbit 07841 (red). a b Mokai Ro Wairakei Mo Oh Tauhara Lake Taupo Wa-Ta o 0 10 20 Km Fig. 4 (a) Wrapped ERS interferogram 05/03/99-23/07/99. Also shown are the lakes (white) and the outlines of the geothermal fields (black). Blue to red represents increased distance to the satellite (i.e. subsidence). The fields where deformation has been measured are outlined in Red: (Mo) Mokai, (Wa-Ta) Wairakei-Tauhara, (Oh) Ohaaki, (Ro) Rotorua; (b) Stack of four unwrapped independent ERS interferograms: 29/01/99-27/08/99, 05/03/99-23/07/99, 09/04/99-01/10/99 and 18/06/99-14/01/00. Red to blue represents increased distance to the satellite (i.e. subsidence); fault traces are shown in red; lakes and rivers in black; and the Wairakei-Tauhara and Mokai geothermal field boundaries in blue. 4.2 Tectonic/Magmatic Deformation The simulated interfeograms in Fig 2. show that the expected horizontal signals are less than 4mm/year. Periods of greater than one year will be needed to detect them; however, this is not possible because of temporal decorrelation. In order to reliably detect these signals, the images must be stacked. Fig. 4b shows a stack of four independent interferograms, covering the area North of Lake Taupo, from January 1999 to January 2000. They have a total time period of two years.
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