New Zealand s Long Term Tide Gauge Record and the effect of Seismically Induced Vertical Land Motion

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1 New Zealand s Long Term Tide Gauge Record and the effect of Seismically Induced Vertical Land Motion Presented at the FIG Working Week 217, May 29 - June 2, 217 in Helsinki, Finland Paul Denys 1, John Beavan 2, John Hannah 1, Neville Palmer 2, Mike Denham 1, Chris Pearson 1 and Sigrum Hreinsdottir 2 1 School of Surveying, Otago University, Dunedin 2 GNS Science, Lower Hutt

2 The IPCC Assessment Snow cover Ice extent Ocean heat content Sea level change Source: IPCC (213)

3 Rising global sea levels Globally distributed tide gauge data Average since 188 : +1.7 ±.3mm/yr GSL has risen ~2cm during the C2 th Global Network of Tide Gauges Source: Church & White (211)

4 Tide gauge measurement system Source: sealevel.colorado.edu/content/tide-gauge-sea-level The global network of tide gauges is a poorly distributed sea level measurement system But, it offers the only source of historical, precise, long-term sea level data

5 Tide gauge stability uplift uplift subsidence subsidence Source: sealevel.colorado.edu/content/tide-gauge-sea-level Source: sealevel.colorado.edu/content/tide-gauge-sea-level Levelling : cgps : local stability of the tide gauge structure regional stability

6 Benchmark Trend (mm) Lyttelton Local stability levelling network Year DHAN DHAP DHAQ DHAT DHAR DJMF B4V BF1U DHAM B418 Reference Marks GPS +.3 TGBM mm/yr

7 TG Trend (mm) Relative sea level trends - NZ 3 25 Auckland Vel: mm/yr rms: +-34mm RSL Trends - NZ Average: +1.7 mm/yr 2 New Plymouth Vel: mm/yr rms: +-57mm 15 Wellington Vel: mm/yr rms: +-37mm 1 Lyttelton Vel: mm/yr rms: +-3mm 5 Dunedin Vel: mm/yr rms: +-34mm Year

8 Port environments - challenging GPS/GNSS often established in poor environments High rise building Topography e.g. volcanic crater Reclaimed land compaction Wharf stability e.g. wooden Port operations Port construction Auckland CDB Conflicting requirements Unobstructed sky visibility and stability/bedrock Port Lyttelton

9 GPS height time series Height Trend [mm] 3 25 AUCK TAKL WARK Auckland -.6 mm/yr 2 WGTN WGTT Wellington -3. mm/yr 15 1 AVLN LYTT MQZG Lyttelton -.5 mm/yr -1.4 mm/yr OUSD 5 OUS2 DUNT DUND Dunedin -1. mm/yr Time [years]

10 Height (mm) Auckland : cgps replaced TAKL: v H = -.54 mm/yr AUKT: v H = -.63 mm/yr TAKL + AUKT: v H = -.62 mm/yr

11 Wellington Height connection TG - cgps

12 Height Difference (metres) Wellington Height connection TG cgps cgps Hgt Diff TG -3 mm/yr +1 mm/yr -2 mm/yr mm rms = ±1.15 mm relative to a -.98 mm/yr linear fit Year

13 Height (mm) Height (mm) Wellington Vertical trend WGTN v H = -2.9 mm/yr WGTT v H = -3.1 mm/yr

14 Height (mm) Height (mm) Wellington Slow Slip Events Kapiti Coast slow slip events WGTN mm cgps Hgt Diff SSE TG mm/yr 2 +1 mm/yr +1 mm/yr -1 mm/yr WGTT +13 mm

15 Height (mm) Height (mm) Lyttelton Christchurch earthquake events 1 5 LYTT v H = -.5 mm/yr MQZG v H = -1.6 mm/yr

16 Height (mm) Height (mm) Lyttelton Post-seismic decay LYTT v H = -.5 mm/yr 2 MQZG v H = -1.6 mm/yr

17 Height (mm) Height (mm) Dunedin Far-field earthquake events Macquaire Island 24 Dusky Sound Secretary Island Uplift: +5-8 mm

18 GPS Velocity Trends Height Trend [mm] 3 AUCK TAKL Auckland 25 WARK 2 WGTN WGTT Wellington AVLN 15 LYTT Lyttelton 1 MQZG OUSD 5 OUS2 DUNT Dunedin DUND Time [years]

19 Vertical Land Motion Velocity (mm/yr) GPS SSE/EQ Local Sum Auckland Wellington Lyttelton Dunedin SSE velocity averaged over 15 years Earthquake coseismic displacement averaged over time series

20 Sea Level Velocity (mm/yr) RSL GPS SSE/EQ Local Sum Corrected Auckland ±.9 Wellington ±.19 Lyttelton ±.19 Dunedin ±.2 Mean : ±.16

21 Summary New Zealand s RSL is consistent with global trends: +1.7 mm/yr VLM needs to be taken into account Local motion levelling Regional motion continuous GPS/GNSS NZ s earthquake events appear to have a significant effect WGTT : uplift is periodically caused by SSEs LYTT : post-seismic deformation caused by ChCh events DUNT : post-seismic deformation caused by (far-field) Fiordland events Evidence sites are uplifting/subsiding <~1 mm/yr Unclear what the long term effects of seismic activity are