Record 2015/04 GeoCat 82325 Pacific Sea Level Monitoring Project CGPS Coordinate Time Series Analysis Report Jia, M., Dawson, J., Twilley, B. and Hu, G. APPLYING GEOSCIENCE TO AUSTRALIA S MOST IMPORTANT CHALLENGES www.ga.gov.au
Pacific Sea Level Monitoring Project CGPS Coordinate Time Series Analysis Report GEOSCIENCE AUSTRALIA RECORD 2015/04 Jia, M., Dawson, J., Twilley, B. and Hu, G.
Department of Industry and Science Minister for Industry and Science: The Hon Ian Macfarlane MP Parliamentary Secretary: The Hon Karen Andrews MP Secretary: Ms Glenys Beauchamp PSM Geoscience Australia Chief Executive Officer: Dr Chris Pigram This paper is published with the permission of the CEO, Geoscience Australia Commonwealth of Australia (Geoscience Australia) 2015 With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. (http://creativecommons.org/licenses/by/4.0/) Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision. Geoscience Australia is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please email clientservices@ga.gov.au. ISSN 2201-702X (PDF) ISBN 978-1-925124-58-3 (PDF) GeoCat 82325 Bibliographic reference: Jia, M., Dawson, J., Twilley, B. and Hu, G. 2015. Pacific Sea Level Monitoring Project: CGPS Coordinate Time Series Analysis Report. Record 2015/04. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/record.2015.004
Contents Executive Summary... 1 1 Introduction... 2 2 Methodology... 3 3 Results and Discussions... 4 4 Conclusions... 5 5 GPS Coordinate Time Series... 6 5.1 Cook Is (CKIS) Raw Residuals... 6 5.2 Cook Is (CKIS) Model Residuals... 7 5.3 Fiji (LAUT) Raw Residuals... 8 5.4 Fiji (LAUT) Model Residuals... 9 5.5 Kiribati (KIRI) Raw Residuals...10 5.6 Kiribati (KIRI) Model Residuals...11 5.7 Manus Is (PNGM) Raw Residuals...12 5.8 Manus Is (PNGM) Model Residuals...13 5.9 Marshall Is (MAJU) Raw Residuals...14 5.10 Marshall Is (MAJU) Model Residuals...15 5.11 Nauru (NAUR) Raw Residuals...16 5.12 Nauru (NARU) Model Residuals...17 5.13 Pohnpei (POHN) Raw Residuals...18 5.14 Pohnpei (POHN) Model Residuals...19 5.15 Samoa (SAMO) Raw Residuals...20 5.16 Samoa (SAMO) Model Residuals...21 5.17 Solomon Is (SOLO) Raw Residuals...22 5.18 Solomon Is (SOLO) Model Residuals...23 5.19 Tonga (TONG) Raw Residuals...24 5.20 Tonga (TONG) Model Residuals...25 5.21 Tuvalu (TUVA) Raw Residuals...26 5.22 Tuvalu (TUVA) Model Residuals...27 5.23 Vanuatu (VANU) Raw Residuals...28 5.24 Vanuatu (VANU) Model Residuals...29 References...30 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report iii
iv Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
Executive Summary The Pacific Sea Level Monitoring Project was initially developed in the early 1990 s (known at that time as the Pacific Sea Level and Climate Monitoring Project) as a response to concerns expressed by South Pacific Forum Leaders about the potential impacts of global warming on sea levels and climate in the South Pacific. This Australian aid project was established with the goal of providing an accurate, long term record of sea levels in the South Pacific both for Forum countries and for the international scientific community that need such information to better understand how the Pacific oceanographic and meteorological environment is changing. During the 1990 s a network of high resolution sea level and climate monitoring stations was established in the South West Pacific and observations from those stations were made available to stakeholders. In 2001, a Continuous Global Positioning System (CGPS) monitoring component was added and CGPS instruments were established near and linked to the sea level tide gauges stations in all partner countries so as to enable the measurement of vertical land movements. Vertical land motion at sea level stations can be equal or larger than the local absolute sea level signal, thus masking sea level change related information recorded at those stations. Therefore, the CGPS network is a crucial component for reliably determining absolute sea level change. This document reports the analysis results of CGPS coordinate time series from 2001 to 2013. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 1
1 Introduction This report presents the coordinate time series for the 12 Continuous GPS (CGPS) stations of the Pacific Sea Level Monitoring Project (SPMP). The CGPS stations are located in Cook Island, Fiji, Kiribati, Manus Island Papua New Guinea (PNG), Marshall Island, Nauru, Pohnpei Federated States of Micronesia (FSM), Samoa, Solomon Island, Tonga, Tuvalu and Vanuatu. The time series is generated from 13 years of weekly GPS solutions for the period 2001 to 2013 to obtain station velocities in a globally consistent geocentric reference frame, the International Terrestrial Reference Frame (ITRF) 2008 (Altamimi et al., 2011). The vertical velocities are important in sea level determinations, since they indicate the vertical crustal deformation of the land at the sea level monitoring stations, allowing correction for this motion when analysing the tide gauge data. Recent global estimates for sea level rise in the past century are 1.8+/-0.5 mm/yr (Bouin and Woppelmann, 2010) and 1.7+/-0.2 mm/yr (Church and White, 2011; The 5th IPCC report, 2014), which are consistent with earlier estimates by Tushinam and Peltier (1991) and Douglas (1997). Therefore, the precision of the net vertical velocity of the GPS station, corrected for differential motion relative to the tide gauge sensor, must be better than the 1 mm/yr to correctly determine absolute sea level changes. The GPS processing strategy has been implemented in accordance with the International Earth Rotation Service (IERS) conventions (IERS, 2010) and has adopted the IGb08 reference frame, the International GNSS Service (IGS) realisation of ITRF2008. The report is organised as follows: in Section 2 we describe the input weekly solutions and combination strategy for generating the time series; in Section 3, the vertical velocity estimates are presented and discussed; in Section 4 conclusions are given; in Section 5 the raw and modelled time series are shown. The raw time series are uncorrected for offsets due to equipment changes and earthquakes and are uncorrected for outliers whereas the modelled time series are corrected for these effects and outliers. 2 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
2 Methodology The input solutions for generating the station velocities are the weekly Solution Independent EXchange (SINEX) format solutions produced by Geoscience Australia (GA). These solutions are produced by combining daily solutions and aligning to the IGb08 reference frame using a minimum constraint approach based on a set of core stations. The solution contains a set of globally distributed IGb08 core stations which provide consistent alignment to the IGb08 reference frame. The strategy for the GPS analysis is summarised in a summary file accompanying each weekly SINEX solution, available at ftp://ftp.ga.gov.au/geodesy-outgoing/gnss/solutions/final/weekly. In the analysis, constraints are removed from the input SINEX solutions and minimum constraints are applied for all stations over all seven transformation parameters (Altamimi et al., 2011). The IGb08 core reference stations which have more than 80% of solutions available for the period are chosen to form the minimum constraint alignment to IGb08. This ensures stability of the combined solution with respect to IGb08, in terms of the transformation parameters. The time series combination is performed next, with the Combination and Analysis of Terrestrial Reference Frames (CATREF) software (Altamimi et al., 2011). The combination is iterated five times and outliers are removed, the rejection criteria being 0.01 m horizontal and 0.025 m vertical, in the last iteration. At the end of the combination, the station coordinates and velocities are available in the IGb08 reference frame. The residuals for the times series are generated for each site in a format acceptable for the Create and Analyse Time Series (CATS) software (Williams, 2008). The CATS software is used to determine realistic uncertainties for the station velocities, with appropriate handling of white noise plus random walk and flicker noise. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 3
3 Results and Discussions The vertical velocities, with uncertainties, determined in the analysis are given in Table 3.1. JPL solutions from http://sideshow.jpl.nasa.gov/post/series.html are also given for comparison. Table 3.1 Vertical velocities, with uncertainties (1 sigma) for the GA solution compared to the JPL solution. CGPS Location Installation date Vertical Velocity and Uncertainty of GA Solution (mm/yr) Vertical Velocity and Uncertainty of JPL Solution (mm/yr) Cook Is 10. Sep. 2001-0.9 ± 0.8 +0.0 ± 0.4 Fiji 25. Nov. 2001-0.7 ± 0.6-0.6 ± 0.6 Kiribati 04 Aug. 2002-0.7 ± 0.7 +0.5 ± 0.4 Manus Is (PNG) 01 May 2002-1.1 ± 1.1-0.5 ± 0.8 Marshall Is 22 May 2007 +0.7 ± 1.0 +1.4 ± 1.9 Nauru 30 Jun 2003-0.7 ± 0.6-0.9 ± 0.9 Pohnpei (FSM) 01 May 2003-0.5 ± 1.1 +0.8 ± 0.5 Samoa 01 Jul. 2001-2.1 ± 1.4-0.8 ± 0.4 Solomon Is 29 Jun. 2008 +1.0 ± 1.4 N/A Tonga 18 Feb. 2002 +2.9 ± 1.0 +4.3 ± 1.4 Tuvalu 02 Dec. 2001-0.8 ± 0.4-0.2 ± 0.4 Vanuatu 11 Sep. 2002-4.0 ± 1.5-1.7 ± 0.8 Six of 12 differences between the GA and the JPL vertical velocities are within 1mm/yr. Other differences are -2.3 mm/yr at Samoa, -1.4 mm/yr at Tonga, -1.3 mm/yr at Vanuatu, -1.3 mm/yr at Pohnpei and -1.2 mm/yr at Kiribati. Possible reasons for the differences at Samoa, Tonga and Vanuatu are that each time series is impacted by several significant earthquakes during the period. In some cases pure coseismic offets do not model the deformations caused by the earthquakes well and post-seismic deformations are evident in the modelled time series as shown in Figure 5.16, Figure 5.20 and Figure 5.24. The difference at Pohnpei may be caused by including an extra jump by JPL around February 2011 with an unknown reason. Other possible reasons include the use of different reference frames (e.g., the number of the core sites) by GA and JPL, as all differences are of a negative sign (Collilieux and Woppelmann 2011). In addition, different GPS data processing software and analysis strategies may also cause some velocity differences between GA and JPL. 4 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
4 Conclusions Time series combination and analysis was carried out to determine the vertical velocities and their uncertainties for 12 CGPS stations in the South Pacific. Comparison of the velocities with the recent JPL solutions shows discrepancies at the 1 mm/yr level, except for Kiribati, Pohnpei, Samoa, Tonga and Vanuatu. The larger velocity differences at these sites may be caused by the methods used at GA and JPL to deal with deformations caused by earthquakes. Some velocity difference may be also caused by the reference frames and software and analysis strategies used by GA and JPL. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 5
5 GPS Coordinate Time Series 5.1 Cook Is (CKIS) Raw Residuals Figure 5.1 De-trended Raw Time Series Plot Cook Is. 6 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.2 Cook Is (CKIS) Model Residuals Figure 5.2 De-trended Modelled Time Series Plot Cook Is. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 7
5.3 Fiji (LAUT) Raw Residuals Figure 5.3 De-trended Raw Time Series Plot Fiji. The offsets are caused by antenna and receiver changes. The offset time is indicated by the red lines. 8 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.4 Fiji (LAUT) Model Residuals Figure 5.4 De-trended Modelled Time Series Plot Fiji. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 9
5.5 Kiribati (KIRI) Raw Residuals Figure 5.5 De-trended Raw Time Series Plot Kiribati. 10 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.6 Kiribati (KIRI) Model Residuals Figure 5.6 De-trended Modelled Time Series Plot Kiribati. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 11
5.7 Manus Is (PNGM) Raw Residuals Figure 5.7 De-trended Raw Time Series Plot Manus Is. 12 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.8 Manus Is (PNGM) Model Residuals Figure 5.8 De-trended Modelled Time Series Plot Manus Is. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 13
5.9 Marshall Is (MAJU) Raw Residuals Figure 5.9 De-trended Raw Time Series Plot Marshall Is. 14 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.10 Marshall Is (MAJU) Model Residuals Figure 5.10 De-trended Modelled Time Series Plot Marshall Is. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 15
5.11 Nauru (NAUR) Raw Residuals Figure 5.11 De-trended Raw Time Series Plot Nauru. The offsets are caused by antenna and receiver changes. The offset time is indicated by the red lines. 16 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.12 Nauru (NARU) Model Residuals Figure 5.12 De-trended Modelled Time Series Plot Nauru. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 17
5.13 Pohnpei (POHN) Raw Residuals Figure 5.13 De-trended Raw Time Series Plot Pohnpei. 18 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.14 Pohnpei (POHN) Model Residuals Figure 5.14 De-trended Modelled Time Series Plot Pohnpei. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 19
5.15 Samoa (SAMO) Raw Residuals Figure 5.15 De-trended Raw Time Series Plot Samoa. The offsets are caused by an antenna change in July 2007 and the Mag. 8 earthquake in October 2009. The offset time is indicated by the red lines. 20 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.16 Samoa (SAMO) Model Residuals Figure 5.16 De-trended Modelled Time Series Plot Samoa. Significant post-seismic deformations are evident after co-seismic deformations caused by the Mag. 8 earthquake in October 2009, are modelled by jump functions. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 21
5.17 Solomon Is (SOLO) Raw Residuals Figure 5.17 De-trended Raw Time Series Plot Solomon Is. Reasons for the discontinuity need to be investigated when more data is available. 22 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.18 Solomon Is (SOLO) Model Residuals Figure 5.18 De-trended Modelled Time Series Plot Solomon Is. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 23
5.19 Tonga (TONG) Raw Residuals Figure 5.19 De-trended Raw Time Series Plot Tonga. The offsets are caused by the Mag. 8 earthquake in May 2006 and several other earthquakes in close proximity to the station. The offset time is indicated by the red lines. 24 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.20 Tonga (TONG) Model Residuals Figure 5.20 De-trended Modelled Time Series Plot Tonga. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 25
5.21 Tuvalu (TUVA) Raw Residuals Figure 5.21 De-trended Raw Time Series Plot Tuvalu. A small offset is caused by an unknown reason. The offset time is indicated by the red lines. 26 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.22 Tuvalu (TUVA) Model Residuals Figure 5.22 De-trended Modelled Time Series Plot Tuvalu. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 27
5.23 Vanuatu (VANU) Raw Residuals Figure 5.23 De-trended Raw Time Series Plot Vanuatu. The offsets are caused by several earthquakes in close proximity to the station. The offset time is indicated by the red lines. 28 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report
5.24 Vanuatu (VANU) Model Residuals Figure 5.24 De-trended Modelled Time Series Plot Vanuatu. The red lines indicate the offset time. Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report 29
References Altamimi, Z., Collilieux, X., Metivier, L., 2011. ITRF2008: an improved solution of the international terrestrial reference frame. Journal of Geodesy, open access, DOI 10.1007/s00190-011-0444-4. Bouin, M. N., Woppelmann, G., 2010. Land motion estimates from GPS at tide gauges: a geophysical evaluation. Geophys. J. Int., doi: 10.1093/gji/ggt131. Church, J.A. and White, J.W. 2011. Sea-Level Rise from the Late 19th to the Early 21 st Century. Surveys in Geophysics. doi:10.1007/s10712-011-9119-1. Collilieux, X., Woppelmann, G., 2011. Global sea-level rise and its relation to the terrestrial reference frame. J Geod. doi 10.1007/s00190-010-0412.4. Douglas, B.C., 1997. Global sea rise: A redetermination. Surv. Geophys. 18: 279-292. The 5 th IPCC report 2014. http://ipcc-wg2.gov/ar5/report/. Petit, G., Luzum, B., 2010. IERS Conventions (2010): IERS Technical Note No. 36, Verlag des Bundesamts fur Kartographie und Geodasie, Frankfurt, Germany, 179 pp. Tushinam, A. M., Peltier, W. R., 1991. Ice-3G: A new global model of late Pleistocene deglaciation based upon predictions of post-glacial relative sea level change. J. Geophys. Res. 96: 4497-4523. Williams, S. D. P., 2008. CATS: GPS coordinate time series analysis software. GPS Solutions. 12, pp 147-153, DOI 10.1007/s10291-007-0086-4. 30 Pacific Sea Level Monitoring Project: CGPS coordinate Time Series Analysis Report