The National Seismic Hazard Model, NZS1170, & the M7.1, 4 Sept 2010 Darfield Earthquake. Mark Stirling, Graeme McVerry, & Matt Gerstenberger

Transcription

1 The National Seismic Hazard Model, NZS1170, & the M7.1, 4 Sept 2010 Darfield Earthquake Mark Stirling, Graeme McVerry, & Matt Gerstenberger

2 Plate Tectonic Setting Christchurch Earthquake epicentre

3 Crustal Seismicity

4 Darfield EQ basic facts Magnitude 7.1 earthquake, 30 km west of Christchurch Saturday 4 th September at c. 4:36 a.m. Local time 0 fatalities, only 2 serious injuries Surface rupture of c. 29 km, with strike-slip displacements E-W striking fault, previously unmapped Shaking damage, liquefaction, lateral spreading Most damage confined to older brick and un-reinforced masonry structures and residential damage due to liquefaction and lateral spreading Cost: Estimate of $4 billion for direct and indirect Hundreds of ongoing aftershocks (M 3-5.9) Psychological impacts on community despite no loss of life

5 Regional Context

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7 Aftershocks in relation to epicentre and surface rupture

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9 Focal Mechanisms

10 Temporary GPS array MMI MMI from from public GeoNet response public to response GeoNet Temporary seismograph and strong motion array GeoNet and VUW

11 Preliminary interpretation of GPS network and INSAR (satellite radar differencing)

12 National Seismic Hazard Model

13 Source Model Background sources LogN/yr Fault sources Magnitude

14 2010 NSHM Over 200 new fault sources (mainly offshore) 11 years more seismicity data New regionalisation scheme for background seismicity New methods for parameterisation of fault and background seismicity sources

15 Active Fault Sources (prior to 4 Sept 2010) Christchurch

16 Background Seismicity Model 2010 Gutenberg-Richter modelled seismicity rates Avalue: Continuously -distributed/gaussian smoothed Bvalue: calculated for zones or combinations of zones Mcutoff: M7.2 everywhere except TVZ (M6.5) Source: Matt Gerstenberger

17 NSHM 2010

18 NSHM 2010

19 Christchurch Hazard Deaggregation model 100% Port2GreyS 80% Springbank Pegasus1nw Percentage 60% 40% Return period (years) Cust AlpineF2K Ashley Hope1888 HopeCW AlpineK2T HopeConw ay Kelly 20% Other faults Background Return period 0% MM Intensity 10

20 Ground Motion Records epicentral pgas Very strong motions in epicentral region Unfiltered pgas up to 1.26g vertical and 0.77g horizontal at Greendale, about 0.5g horizontally at Darfield and g vertically at Rolleston, Hororata, Templeton and Lincoln Crop and Food Research Filtering (25 Hz max frequency) reduces some vertical pgas 0.95g at Greendale, g at other sites above, little effect on horizontal pgas

21 Peak ground accelerations Vertical pgas > horizontal pgas near-source Unprocessed PGAS - Vertical and Horizontal Components Vertical pgas generally stronger than horizontal for distances to about 40 km 1g g PGA (mm/s^2) Vertical Horizontal_1 Horizontal_ Chch at about km epicentral distance Epicentral distance (km) Horizontal pgas similar to expected values for Mw 7.1 earthquake

22 Selection of recorded peak ground accelerations (unfiltered) Epicentral Distance (km) PGA (g) Site Code Vertical Horiz_1 Horiz_2 Name of Recording Site GDLC Greendale DFHS Darfield High School DSLC Dunsandel School ROLC Rolleston School HORC Hororata School TPLC Templeton School LINC Lincoln Crop and Food Research CACS Christchurch Canterbury Aero Club RHSC Riccarton High School CMHS Christchurch Cashmere High School KPOC Kaiapoi North School LPCC Lyttelton Port Company

23 Ground motion records epicentral PGVs and displacements Peak ground velocities about 0.95m/s from standard processing at Greendale, perhaps 1.5m/s from special processing that accommodates permanent displacements PGVs > 0.5m/s at Hororata, Lincoln, Rolleston Some near-source records show indications of permanent displacements require special processing Some polarisation, often two major velocity or displacement peaks polarised in different directions (NS, NW-SE) (source complexity, both strike-slip and reverse episodes within mainshock)

24 Selection of recorded peak ground velocities (PGVs > 500mm/s are very strong motions) Epicentral Distance (km) PGV (mm/s) Site Code Name of Recording Site Vertical Horiz_1 Horiz_ GDLC Greendale DFHS Darfield High School DSLC Dunsandel School ROLC Rolleston School HORC Hororata School TPLC Templeton School LINC Lincoln Crop and Food Research CACS Christchurch Botanic Gardens RHSC Christchurch Hospital CMHS Christchurch Cathedral College KPOC Kaiapoi North School

25 Epicentral motions vs NZS yr, 2500-yr (and 5000-yr) motions Epicentral Region vs NZS1170 Deep Soil Spectrum Z=0.3 R=1.0, R=1.8 and R=2.34 (Approx 500-, and 5000-year spectra) yr 2500-yr SA(T) (g) Greendale NZS1170 R=1 (500-year) NZS1170 R=1.8 (2500-year) 2.34*NZS1170 (approx year) Greendale_GDLC Hororata_HORC Darfield_DFHS 500-yr 0.5 Hororata Darfield Period T(s)

26 Acceleration records from Greendale note long-period pulses 1 0 Acceleration (g) ` VERT Time (s)

27 Ground motion records in Christchurch CBD Four sites about 1-2 km from CBD Stronger component (as recorded) PGAs g, average about 0.21g 500-year (R=1) design-motion pga for deep soil is 0.22*1.12g=0.25g Vertical pgas g Strong PGVs m/s Strong spectral peaks around 2.5s Velocities and displacements polarised approx NS (not so obvious in accelerations)

28 PGAs from four sites closest to CBD Epicentral Distance (km) PGA (g) Site Code Vertical Horiz_1 Horiz_2 Name of Recording Site CBGS Christchurch Botanic Gardens CHHC Christchurch Hospital REHS Christchurch Resthaven CCCC Christchurch Cathedral College Average stronger horizontal component pga about 0.21g

29 CBD Spectra Median PGA close to NZS1170 Deficient up to 0.3s Approx code-level 0.3s-1s Increasingly stronger than code for T>1.5s Site, source or rupture-propagation effect?

30 Approx NS polarisation of CBD velocities and displacements Cathedral College Christchurch Hospital

31 CBD accelerograms Christchurch Hospital Strong NS polarisation obvious in velocities and displacements

32 1/3 Current Design Spectra for ERBs CENTRAL CITY SPECTRA vs 1/3 NZS1170 CLASS D DEEP OR SOFT SOIL (ERBs) Larger Horizontal Component SA(T) (g) Current design levels NZS1170 Class D Deep or Soft Soil Botanic Gardens CBGS Cathedral College CCCC Hospital CHHC Resthaven REHS Median Larger Central City 1/3 NZS1170 Deep Soil (ERBs) ERBs Period T(s)

33 Christchurch urban area selection of records 2.5 CHRISTCHURCH SPECTRA vs NZS1170 CLASS D DEEP OR SOFT SOIL AND CLASS E VERY SOFT SOIL Larger horizontal component Head of Heathcote Valley High pga and short period peaks Weak at long periods NZS1170 Class D Deep or Soft Soil Aero Club CACS Botanic Gardens CBGS Cathedral College CCCC Hospital CHHC SA(T) (g) Heathcote Valley HVSC Kaiapoi KPOC 1 Papanui PPHS Resthaven REHS Riccarton RHSC 0.5 Median Larger NZS1170 Class E Very Soft Soil Period T(s)

34 Eastern Christchurch spectra EASTERN CHRISTCHURCH RECORDED SPECTRA vs NZS SA(t) (g) NZS1170 Class D Deep or Soft Soil NZS1170 Class E Very Soft Soil SHIRLEY_SHLC PagesRd_PRPC Hulverstone_HPSC Geometric Mean of 3 sites Period T(s) Note long-period components, pgas about g

35 Records at liquefied sites Hulverstone Road Pumping Station (by Anzac Drive bridge) Note change to long-period character High-frequency blips in acceleration Important for analysis of ground-motions inducing liquefaction was long-period character of Christchurch motions important?

36 Short-period site responses Softer soils do not necessarily produce stronger ground motions Material with low velocity tends to have low Q (fraction of critical damping =1/(2Q)) Amplifying effect of low-velocity material may be partly or totally offset by greater attenuation A cross-over from amplification to deamplification often occurs at frequencies above 5-10 Hz Nonlinear soil behaviour further diminishes highfrequency response Softer soils usually give stronger amplifications at long periods

37 Aftershock Motions Christchurch Hospital 7:49am Wednesday 8 Sep (M5.1 near Lyttelton) 0.25g H 0.31g V SAmax approx 1g in EW component

38 Summary year motions at Greendale year motions at Hororata Strong velocity pulses and permanent displacements near-source CBD spectra approx R=1 NZS1170 deep soil Deficient at very short-periods (except pgas), much stronger at long periods (and strong PGVs) Approx 0.65g at Heathcote Valley likely to be siteresponse related (short-period character) Records from liquefied sites

39 Materials Kindly Provided by: Kelvin Berryman, Natural Hazards Platform manager Mark Quigley, Geological Sciences, University of Canterbury (plus other staff & students) earthquake geology team, especially Pilar Villamor, Russ Van Dissen, Nicola Litchfield & Robert Langridge /GeoNet seismologists, especially John Ristau, Martin Reyners, Bill Fry & Caroline Holden /LINZ geodetic team Engineering seismologists from GNS & Canterbury University Matt Gerstenberger, Warwick Smith,