Wellington Whose Fault?

Transcription

1 Wellington Whose Fault?

2 It s Our Fault Major Sponsors A 7-year applied research project funded by EQC, WCC, ACC, Greater Wellington, Wellington Region CDEM Group, and FRST

3 It s Our Fault The Goal To see Wellington positioned to become a more resilient city through a comprehensive study of the likelihood of large Wellington earthquakes, the effects of these earthquakes, and their impacts on humans and the built environment

4 New Zealand s Tectonic Setting Australian Plate Pacific Plate A cricket pitch of displacement every 500 years 40 mm/yr

5 New Zealand Shallow Seismicity Depths < 40 km (a ten year snap-shot) New Zealand Deep Seismicity Depths 40 km (a ten year snap-shot)

6 Large Historical Earthquakes Largest historical earthquake 1855, Mag ~

7 New Zealand s Active Faults

8 Time averaged general hazard

9 A more specific question Which faults contribute most to losses?

10 It s Our Fault motivation (an example) 6 Damage to NZ Houses in earthquakes Likely Losses ($b) Year (plot courtesy of W. Smith, GNS Science)

11 It s Our Fault motivation (an example) 6 Damage to NZ Houses Wellington region faults Other sources 5 Likely Losses ($b) Year (plot courtesy of W. Smith, GNS Science)

12 Importance of characterising Likelihood Relative contribution to loss by specific faults But how much did we actually know about earthquake timing on the Wellington Fault?

13 It s Our Fault Wellington Fault Knowledge Prior to It s Our Fault 2 nd rupture Most recent rupture Limited knowledge made it difficult to estimate timing of next rupture

14 It s Our Fault Wellington Fault Possible new knowledge scenarios

15 It s Our Fault Wellington Fault Possible new knowledge scenarios So, which scenario is most correct?

16 It s Our Fault - Likelihood Phase Geological Investigations Wellington Fault paleoearthquake characterization Wellington Fault single-event displacement characterization Wellington Fault slip-rate characterization Wairarapa Fault paleoearthquake characterization Wairarapa Fault slip-rate characterization Ohariu Fault paleoearthquake characterization Detailed mapping offshore Cook Strait faults Past Subduction Zone ruptures Wellington Geodetic & GPS Studies Synthetic Seismicity Modelling Wellington Fault Conditional Probability of Rupture

17 It s Our Fault Likelihood team Geological Investigations R. Van Dissen, U. Cochran, M. Hemphill-Haley, R. Langridge, N. Litchfield, W. Ries, H. Seebeck, P. Villamor, K. Wilson P. Barnes, G. Lamarche, J. Mountjoy, N. Pondard T. Little, R. Carne, D. Ninis, U. Rieser, E. Schermer, E. Smith Wellington Geodetic & GPS Studies L. Wallace, J. Beavan, N. Palmer Synthetic Seismicity Modelling R. Robinson, N. Litchfield, R. Van Dissen Wellington Fault Conditional Probability of Rupture D. Rhoades, R. Langridge, R. Robinson, R. Van Dissen T. Little, D. Ninis, E. Smith Project Facilitation H. Brackley, K. Berryman, A. King, T. Webb

18 Wellington Fault Conditional Probability of Rupture Objective: Calculate conditional probability of rupture of the Wellington Fault, accounting for all relevant parameter and data uncertainties, and encompassing: new and existing earthquake geology data: elapsed time since most recent rupture, timing of older ruptures, single event displacement size, and slip rate; rupture statistics of the Wellington-Wairarapa fault pair in a synthetic earthquake catalogue derived from a physics-based numerical model; various recurrence-time models.

19 Wellington-Hutt Valley segment of the Wellington Fault

20 Wellington Fault IOF Investigation Sites TK-LG EH-TM K

21 Input data timing of most recent rupture & timing of older rupture events Timing of most recent rupture is slightly younger than previously thought Inter-event times between ruptures are longer than previously suspected Datum is AD 2010 (Langridge et al.)

22 Input data size of single-event displacements Young River Terraces at Te Marua Photograph by Lloyd Homer Important site previous single-event estimate at site: (last EQ only) was 4.2 ± 0.5 m (Little et al., JGR, 2010)

23 Summary of Displacements (Four Groupings) Emerald Hill 4 Single-Event Displacements Last EQ 9.7 ± 1.6 m EQ 4 Mean Single-Event Displacement 5.0 ± 1.5 m (1σ RMS scatter of slips about the mean) Penult. EQ EQ 3 EQ 5? This mean per-event slip is ~20% higher than previous estimate of singleevent slip on Wellington Fault (Little et al., JGR 2010)

24 Input data Holocene slip-rate (Emerald Hill) (work in progress, D. Ninis et al.) Displacement This rate is ~15 % slower than the previous estimate (Berryman, 1990) Holocene Slip-Rate 5.7 ± 0.9 mm/yr (1σ 1σ) OSL ages 9.0 ± 0.6 ka 9.4 ± 0.6 ka G J Te Marua Emerald Hill Birchville Park

25 Combined effect of new data is a LONGER Average Recurrence Interval Mean: 860 years (cf. 640 years before IOF)

26 Input Consideration - Fault Inteactions Synthetic Seismicity Modelling 1855 rupture When one tugs at a single thing in nature, he finds it attached to the rest of the world. John Muir

27 Synthetic Seismicity Modelling Fault Interaction Investigations Effect of fault rupture on other nearby faults 1855 earthquake de-stressed Wellington & Ohariu faults

28 How it was modelled 15 years ago Seven faults with very simplified geometries (Robinson & Benites, JGR 1996)

29 Fault Interaction model Large region to encompass Wellington and Wairarapa faults (Robinson et al.) Over 50 large faults with realistic geometries, and 3000 randomly placed small faults

30 Cumulative moment release vs. time (for time period 55,000-56,000 years) Times when you wish you had yrs Times to build resilience (Robinson et al.)

31 Wellington-Wairarapa fault-pair rupture statistics Wellington Fault rupture (Robinson et al.)

32 Wellington-Wairarapa fault-pair rupture statistics Mag >7.3 Wairarapa Fault rupture lengthens time between large Wellington Fault ruptures (Robinson et al.)

33 It s Our Fault Wellington Fault data Current knowledge, after It s Our Fault Event 4 Event 3 Event 2 Event 1 Less frequent Wellington Fault ruptures plus a recent Wairarapa But, Fault by rupture how pushes much? probability of next Wellington Fault rupture out into the future

34 Recurrence Time Models Exponential Weibull Weibull Log-normal Inverse Gaussian Inverse Gaussian Log-normal Exponential Time (normalised to recurrence interval)

35 Methodology Rhoades, D.A., Van Dissen, R.J., Dowrick, D.J., 1994, On the handling of uncertainties in estimating the hazard of rupture on a fault segment. Journal of Geophysical Research 99: 13,701-13,712. Rhoades, D.A., Van Dissen, R.J., 2003, Estimates of the time-varying hazard of rupture of the Alpine Fault, New Zealand, allowing for uncertainties. New Zealand Journal of Geology & Geophysics 46: Rhoades, D.A., Stirling, M.W., Schweig, E., Van Dissen, R.J., 2004, Time-varying earthquake hazard in the Wellington region. Institute of Geological & Nuclear Sciences Client Report 2004/141 (prepared for the Earthquake Commission). allows hazard (probability of rupture) to be expressed as a single value that accounts for both data and parameter uncertainties

36 Wellington Fault estimated probability of rupture in 100 years Recurrence-time distribution Exponential Lognormal Weibull Inverse Gaussian 100 yr probability 10% 12% 11% 11% % change from previous estimate ~ 40% reduction ~ 70% reduction ~ 60% reduction ~ 60% reduction

37 One way of looking at new Wellington Fault results

38 Another way of looking at it Probability of rupture of Wellington Fault within next 100 years Pre-It s Our Fault: ~ 30% New: ~ 10%

39 Sensitivity runs Favour the older end of the age range of the most recent event Exclusion of occurrence of a more speculative Event 5 Higher coefficient of variation of single-event displacement (0.57 cf. 0.30) Elevated and reduced slip rate

40

41 0.20 Preferred dates Event 1 modified Event 4 modified Small symbols: Event V excluded Sensitivity study results: Probability of rupture of the Wellington Fault in the next 100 years from AD yr probability of rupture Colour codes: Black: Preferred data Red: Elapsed time reduced to compensate for 1848 and 1855 earthquakes Green: CoV of single-event displacement increased to 0.57 Blue: Reduced slip rate Orange: Elevated slip rate Exponential Lognormal Weibull BPT Recurrence time distribution

42 after sensitivity runs, still looks like this Probability of rupture of Wellington Fault within next 100 years Pre-It s Our Fault: ~ 30% New: ~ 10%

43 It s Our Fault some key conclusions Significant reduction in the likelihood of rupture of the Wellington Fault over the next hundred years, or so. This is Good. However, no room for complacency as there are other earthquake sources in, and around, the region that can produce significant damage and loss. This highlights the need, and benefits, of continued pursuit of resilience.

44 More than a few large earthquake sources in and around Wellington

45 The Wellington Fault Parting Shot The Region

46 3D Geological & Geotechnical Characterisation of Central Wellington

47 Thorndon overbridge Study Area Central Wellington (Semmens et al., 2010) Wellington Hospital

48 Study Area Central Wellington Greywacke Rock Shallow Soil Very Soft Soil Deep/Soft Soil Wellington Fault Greywacke Rock Wellington Hospital

49 Project objectives & outputs 3D Geological model for central Wellington Derivative Maps Surficial geological Depth to bedrock Site period V s 30 Subsoil class (based on NZS )

50 Inputs 1025 Boreholes Microtremor Sites shear-wave velocities low amplitude natural period unit thicknesses

51 Database(s) From the borehole and microtremor information, databases have been compiled that include: lithology (as recorded on the log), geotechnical properties, SPT N count, shear-wave velocity, model lithology These data have been uploaded into the web-accessible GNS PetLab

52 Data Modelling Each layer in a borehole was assigned one of 17 units based on: grain size, SPT N count, weathering condition & depositional setting The 17 units were then assigned to one of four larger groups and were successfully modelled: Hydraulic Fill Loose/Soft Deposits Stiff/Hard Deposits Greywacke Sandstone/Mudstone Bedrock

53 Data Modelling

54 Vs summary for Wellington City

55

56 (Semmens et al., 2010)

57

58 Map units include: Results Geological Map (1:5000 scale) Greywacke, alluvium, colluvium, marginal marine and swamp deposits Compiled using information from boreholes, test pits and existing maps with limited field mapping (Semmens et al., 2010)

59 Results Depth to Bedrock (red contours are at 50 m intervals) (Semmens et al., 2010)

60 Results Bedrock Surface

61 Subsoil Class Old-fashioned microzoning for earthquake effects - as in 1974 and 1992 Assessment of relative earthquake hazard components (e.g. shaking amplification) Simple, qualitative, designation for amplification potential (e.g. high-medium-low) Perhaps useful for urban planning, but not much use for engineering design

62 Subsoil Class NZS : 2004 The earthquake loading standard for New Zealand Attempts to quantify the shaking amplification across differing subsoil classes (i.e. near surface ground conditions), and prescribes design actions Defines subsoil classes and methods to establish them Fundamental components in determining subsoil class include: natural period & shear wave velocity

63 NZS Subsoil Class Definitions: Subsoil Class Class: Description: Definition: A Strong Rock UCS > 50MPa...& V s 30m > 1500m/s...& Not underlain by <18MPa or V s 600m/s materials B Rock 1< UCS <50MPa...& V s 30m >360 m/s...& Not underlain by UCS< 0.8MPa or V s 300m/s materials A surface layer no more than 3m depth (HW-CW rock/soil) C Shallow Soil Not class A, B or E Low Amplitude Natural Period 0.6s, or Have depths of soils not exceeding those in Table 2 D Deep or Soft Soil Not class A, B or E Low Amplitude Natural Period > 0.6s, or Have depths of soils exceeding those in Table 2, or Underlain by < 10m soils with undrained shear strength < 12.5 KPa or SPT N < 6 E Very Soft Soil > 10m soils with undrained shear strength < 12.5 KPa or SPT N < 6 or Vs 150m/s or >10m combined (previous)

64 Results (contour interval = 0.2 s; red contour is 0.6 s) Site Period Based on: Modelled 3D distribution of geological units Hydraulic Fill Loose/Soft Deposits Stiff/Hard Deposits Bedrock Vs determinations for these units 4 shear-wave velocity travel time Ground-truthed against known, but limited, site period determinations (Semmens et al., 2010)

65 Subsoil Class B NZS map = a more usable microzoning C D D C B (Semmens et al., 2010)

66 It s Our Fault Publications Project Overview Van Dissen, R., Barnes, P., Beavan, J., Cousins, J., Dellow, G., Francois-Holden, C., Fry, B., Langridge, R., Litchfield, N., Little, T., McVerry, G., Ninis, D., Rhoades, D., Robinson, R., Saunders, W., Villamor, P., Wilson, K., Barker, P., Berryman, K., Benites, R., Brackley, H., Bradley, B., Carne, R., Cochran, U., Hemphill-Haley, M., King, A., Lamarche, G., Palmer, N., Perrin, N., Pondard, N., Rattenbury, M., Read, S., Semmens, S., Smith, E., Stephenson, W., Wallace, L., Webb, T., Zhao, J., 2010, It s Our Fault: better defining earthquake risk in Wellington. in proceedings, 11 th IAEG Congress, Auckland, New Zealand, 5-10 September, in press, 8p. Van Dissen, R., Berryman, K., King, A., Webb, T., Brackley, H., Barnes, P., Beavan, J., Benites, R., Barker, P., Carne, R., Cochran, U., Dellow, G., Fry, B., Hemphill-Haley, M., Francois-Holden, C., Lamarche, G., Langridge, R., Litchfield, N., Little, T., McVerry, G., Ninis, D., Palmer, N., Perrin, N., Pondard, N., Semmens, S., Stephenson, W., Robinson, R., Villamor, P., Wallace, L., Wilson, K., 2009, It s Our Fault: Better Defining the Earthquake Risk in Wellington - Results to Date & a Look to the Future. in proceedings, New Zealand Society for Earthquake Engineering Technical Conference, Christchurch, New Zealand, 3-5 April, Paper No. 48, 8p. Likelihood Phase Wellington Fault Langridge, R.M., Van Dissen, R., Rhoades, D., Villamor, P., Litchfield, N., Clark, K., Little, T., Clark, D., in prep, Four thousand years of surface ruptures on the Wellington Fault, New Zealand: implications for recurrence and fault segmentation. To be submitted to Bulletin of the Seismological Society of America. Langridge, R.M., Van Dissen, R., Villamor, P., Little, T., 2009, It s Our Fault Wellington Fault paleoearthquake investigations: final report. GNS Science Consultancy Report 2008/344. Little, T.A., Van Dissen, R., Rieser, U., Smith, E.G.C., Langridge, R., 2010, Co-seismic strike-slip at a point during the last four earthquakes on the Wellington fault near Wellington, New Zealand. Journal of Geophysical Research 115. B doi: /2009jb Ninis, D., Little, T., Van Dissen, R., Smith, E., Langridge, R., 2010, The Wellington Fault - Holocene displacements and slip rates at Emerald Hill, Wellington, New Zealand: Progress Report. VUW Research Report No p. Rhoades, D.A., Van Dissen, R.J., Langridge, R.M., Little, T.A., Ninis, D., Smith, E.G.C., Robinson, R., in press, Re-evaluation of conditional probability of rupture of the Wellington-Hutt Valley segment of the Wellington Fault. Bulletin of the New Zealand Society for Earthquake Engineering 43(4). Rhoades, D.A., Van Dissen, R.J., Langridge, R.M., Little, T.A., Ninis, D., Smith, E.G.C., Robinson, R., 2010, It s Our Fault: Re-evaluation of Wellington Fault conditional probability of rupture. in proceedings, New Zealand Society for Earthquake Engineering Technical Conference, Wellington, New Zealand, March, Paper No. 23, 8p. Rhoades, D.A., Van Dissen, R.J., Langridge, R.M., Little, T.A., Ninis, D., Smith, E.G.C., Robinson, R., 2010, It s Our Fault Conditional probability of rupture of the Wellington-Hutt Valley segment of the Wellington Fault. GNS Science Consultancy Report 2010/16. 17p.

67 It s Our Fault Publications Wairarapa Fault Carne, R.C., Little, T.A., Rieser, U., in review, Using displaced river terraces to determine Late Quaternary slip rate for the central Wairarapa Fault at Waiohine River, New Zealand. Submitted to New Zealand Journal of Geology and Geophysics. Little, T.A., Van Dissen, R., Schermer, E., Carne, R., 2009, Late Holocene surface ruptures on the southern Wairarapa fault, New Zealand: link between earthquakes and the raising of beach ridges on a rocky coast. Lithosphere 1(1): doi: /l7.1. Schermer, E.R., Little, T.A., Rieser, U., 2009, Quaternary deformation along the Wharekauhau fault system, North Island, New Zealand: Implications for an unstable linkage between active strike-slip and thrust faults. Tectonics 28: TC6008, doi: /2008tc Villamor, P., Langridge, R.M., Ries, W., Carne, R., Wilson, K., Seebeck, H. & Cowan, L., 2008, It s Our Fault Wairarapa Fault slip rate investigations: completion report - Is the Wairarapa Fault slip rate decreasing to the north? GNS Science Consultancy Report 2008/170. Ohariu Fault Litchfield, N., Van Dissen, R., Hemphill-Haley, M., Townsend, D., Heron, D., 2010, Post c. 300 year rupture of the Ohariu Fault in Ohariu Valley, New Zealand. New Zealand Journal of Geology and Geophysics 53: doi: / Litchfield, N., Van Dissen, R., Hemphill-Haley, M., Townsend, D., Heron, D., 2010, It s Our Fault Ohariu Fault paleoearthquake investigations: final report. GNS Science Consultancy Report 2010/17: 31p. Cook Strait active faulting Barnes, P., Pondard, N., in review, Derivation of direct on-fault submarine paleoearthquake records from high-resolution seismic reflection profiles: Wairau Fault, New Zealand. Submitted to Geochemistry Geophysics Geosystems. Barnes, P.M., Pondard, N., Lamarche, G., Mountjoy, J., Van Dissen, R., Litchfield, N., 2008, It s Our Fault: active faults and earthquake sources in Cook Strait. NIWA Client Report WLG : 36 p. Pondard, N., Barnes, P., in review, Structure and paleoearthquake records of active submarine faults, Cook Strait, New Zealand: Implications for fault interactions, stress loading, and seismic hazard. Submitted to Journal of Geophysical Research. Subduction zone earthquake geology & geodesy Beavan, J., Wallace, L., 2008, It s Our Fault Wellington geodetic and GPS studies: task completion report Fault coupling results from inversion of new and reprocessed GPS campaign data from the Wellington region. GNS Science Consultancy Report 2008/172. Clark, K., Hayward, B., Cochran, U., Grenfell, H., Hemphill-Haley, E., Mildenhall, D., Hemphill-Haley, M., Wallace, L., in review, Investigating subduction earthquake geology along the southern Hikurangi margin using paleoenvironmental histories of intertidal inlets. Submitted to New Zealand Journal of Geology and Geophysics. Wallace, L.M., Barnes, P., Beavan, R.J., Van Dissen, R., Pondard, N., Litchfield, N.J., Lamarche, G., Little, T.A., in prep, Kinematics of transition from subduction to strike-slip: an example from the New Zealand plate boundary. To be submitted to Journal of Geophysical Research. Wilson, K., Cochran, U., 2008, It s Our Fault Past subduction zone ruptures: task progress report GNS Science Consultancy Report 2008/173.

68 It s Our Fault Publications Wellington region synthetic seismicity Robinson, R., Van Dissen, R., Litchfield, N., in prep, Using synthetic seismicity to evaluate seismic hazard in the Wellington region, New Zealand. To be submitted to Geophysical Journal International. Robinson, R., Van Dissen, R., Litchfield, N., 2009, It s Our Fault synthetic seismicity of the Wellington region: final report. GNS Science Consultancy Report 2009/192: 36 p. Effects Phase Wellington & Lower Hutt geological & geotechnical characterisation Boon, D.P., and others, in prep, Geological, geotechnical and subsoil class (NZS ) characterisation of Lower Hutt, New Zealand. To be submitted to New Zealand Journal of Geology and Geophysics. Boon, D.P., Perrin, N., Dellow, G., Lukovic, B., 2010, It s Our Fault geological and geotechnical characterisation and site class revision of the Lower Hutt Valley. GNS Science Consultancy Report 2010/163. Fry, B., in prep, Passive seismic noise-based imaging of the Wellington Basin, New Zealand. To be submitted to Pure and Applied Geophysics. Fry, B., Stephenson, B., Benites, R., Barker, P., 2010, Seismic instrumentation and inversion for physical parameters of Wellington and the Hutt Valley. GNS Science Consultancy Report 2010/18: 43p. Perrin, N.D., Stephenson, W.R., Semmens, S., 2010, Site class determinations (NZS ) in Wellington using borehole data and microtremor techniques. in proceedings, New Zealand Society for Earthquake Engineering Technical Conference, Wellington, New Zealand, March, Paper No. 22, 8p. Semmens, S., and others, in prep, Geological, geotechnical and subsoil class (NZS ) characterisation of central Wellington, New Zealand. To be submitted to New Zealand Journal of Geology and Geophysics. Semmens, S., Perrin, N., Barker, P., 2010, What lies beneath: Geological and geotechnical characterisation of the Wellington central commercial area. in proceedings, 11 th IAEG Congress, Auckland, New Zealand, 5-10 September, in press, 8p. Semmens, S., Perrin, N.D., Dellow, G., 2010, It s Our Fault geological and geotechnical characterisation of Wellington City. GNS Science Consultancy Report 2010/176: 48p. Impacts Phase Social Science component Johnston, D.M., McClure, J., Becker, J.S., Paton, D., Finnis, K., Leonard, G.L., Wright, K., in review, Community understanding of earthquake and tsunami risk in Wellington, New Zealand. Cities at Risk: Living with Perils in the 21 st Century, Risk Perceptions and Behaviours: submitted.