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TMSVTTPGCRETTTQ-GTM-PPGCRETBPV▲▲Figure 2. Synoptic tectonostratigraphic column for theCanterbury Plains (not to scale). T = Torlesse basement assemblage;CRET = Late Cretaceous terrestrial sequence; MSV = Mt.Somers volcanics; PG = Paleogene marine sequence;M-P = Miocene-Pliocene marine-terrestrial sequence;BPV = Miocene Banks Peninsula volcanics; Q-G = Quaternarygravels.Tfrom geodetic retriangulation and more modern GPS studies.Congruence between the two sources suggests a remarkablyuniform regional stress field throughout the Canterburyregion with maximum compressive stress σ 1 horizontal andoriented WNW-ESE (115° ± 5°). The apparent parallelismbetween σ 1 stress trajectories and maximum contractionalstrain-rate is explicable if the latter can be treated as a measureof maximum incremental shortening subparallel to σ 1 (cf.Keiding et al. 2009). This σ 1 orientation is also consistent withpredominantly reverse-slip reactivation of structures trendingNNE-NE along the Southern Alps range front. However, thedominance of strike-slip faulting in the Canterbury earthquakesequence suggests that the regional stress field is thatof an “Andersonian” wrench (strike-slip) regime (Anderson1905, 1951) with principal compressive stresses oriented:σ 1 : 0°/115° ± 5°; σ 2 : vertical; σ 3 : 0°/025° ± 5°. Some local stressheterogeneity is, of course, to be expected, perhaps throughbuttressing by the Banks Peninsula volcanic edifice.From the combination of strike-slip and reverse faultingthroughout the region the likelihood is that the stress field isof the form σ 1 > σ v = σ 2 ~ σ 3 , with some local variance betweenσ v = σ 2 and σ v = σ 3 . Brittle structures developing within thecurrent “wrench” regime would be expected to have the orientationsshown in the Figure 3 inset. Newly forming strike-slipfaults should be vertical, lying at 25°–35° to σ 1 in accordancewith the Coulomb criterion (Anderson 1905). Note that thisalso reflects the optimal attitude for reactivating existing faultsover a broad range of sliding friction (0.8 > μ s > 0.4) (Sibson1985). Grain-scale microcracks and macroscopic extensionfractures would be statistically aligned perpendicular to σ 3with an orientation 115° ± 5°/V. Planes of maximum shearstress controlling the orientation of subvertical ductile shearzones at depth below the seismogenic zone should lie at ±45° toσ 1 along trends of 070° (dextral) and 160° (sinistral).PRINCIPAL COMPONENTS OF THE SEQUENCEThe principal components of the 2010–2011 Canterburyearthquake sequence are summarized in a seismotectonic cartoonillustrating the relationship of the different structures tothe inferred σ 1 stress trajectories (Figure 3). Regional CMTmechanisms for M w ≥ 4.0 shocks within the sequence are illustratedin Figure 4. The sequential development and structuralcharacteristics of the larger ruptures are discussed below.TABLE 1Stress Indicators in Canterbury, Central South Island (see also Sibson et al., forthcoming).Region Method σ 1 azimuth ReferenceStress DeterminationsNorth Canterbury-Marlborough Inversion of focal mechanisms 115 ± 16° Balfour et al. 2005Epicentral area 1994 Arthur’s Pass M w 6.7 Inversion of aftershock focal mechanisms 118 ± 4° Robinson and McGinty2000Central S. Alps Inversion of focal mechanisms 110 – 120° Leitner et al. 2001Offshore S. Canterbury Borehole breakouts 114 ± 9° Sibson et al. 2012Maximum Contractional Strain RatesOkarito west of Alpine F. Determination from repeated triangulation 117 ± 6° Pearson 1994Godley Valley east of Alpine F. Determination from repeated triangulation 116 ± 14° Pearson 1994Canterbury Plains NW of ChCh Determination from GPS campaigns 116 ± 9° Pearson et al. 1995Central Southern Alps Determination from GPS campaigns 105 – 115° Beavan and Haines 2001Canterbury-Otago block Rotating elastic block model 110 ± 8° Wallace et al. 2007826 Seismological Research Letters Volume 82, Number 6 November/December 2011
σ 1dσ 3Nuσ 3CM w 7.1dw 6.2u70°M w55°6.0MBPV10 kmσ 1σ 1115 ± 5°σ 3145 ± 5°▲▲Figure 3. Seismotectonic cartoon of the 2010–2011 Canterbury earthquake sequence in relation to the surface outcrop of BanksPeninsula volcanics (BPV), central Christchurch city (C), and the inferred regional stress field. Expected orientations of newlyformed structures (ellipse = extension fracture; solid lines = Coulomb shears; dashed lines = ductile shears) shown in inset at lowerleft. Epicenters of major shocks denoted by stars; thick bold line = Greendale fault surface rupture; thinner dash-dot lines with filledteeth indicating dip direction = subsurface fault models for the M w 6.2 and M w 6.0 aftershocks (Beavan et al. 2011, page 789 of thisissue); dotted line = microearthquake lineament; dark shading = area of intense aftershock activity in dilational stepover; gray-shadedbands = aftershock lineaments; hollow-toothed lines = belt of reverse-slip aftershocks (dip unconstrained).▲▲Figure 4. Regional CMT focal mechanisms for M w > 4.0 shocks within the 2010–2011 Canterbury earthquake sequence.Seismological Research Letters Volume 82, Number 6 November/December 2011 827
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Volume 82, Number 6 November/Decemb
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News and Notes (continued)Nominatio
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Preface to the Focused Issue on the
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TABLE 1Peak ground acceleration (PG
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▲▲Figure 2. A) Sketch of the
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▲▲Figure 4. A) Adopted moment r
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▲▲Figure 7. As in Figure 6 but
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Page 64 and 65: TABLE 1Pairs of SAR imagery used in
Page 67 and 68: Depth (km)Coulomb Stress Change(bar
Page 69 and 70: Crippen, R. E. (1992). Measurement
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Page 77 and 78: DISCUSSIONThe 2010-2011 Canterbury
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Page 81 and 82: ▲ ▲ Figure 2. Observed vs. pred
Page 83 and 84: 10Obs SA(1s)AS1AS+SDAB 2006AB+SDSA(
Page 85 and 86: Fine-scale Relocation of Aftershock
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Page 91 and 92: REFERENCESAvery, H. R., J. B. Berri
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Page 95 and 96: ▲ ▲ Figure 4. Vertical accelera
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Page 99 and 100: Near-source Strong Ground MotionsOb
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Page 103 and 104: Scale0.5 g5 seconds▲▲Figure 4.
Page 105 and 106: (A)(B)Spectral Acc, Sa (g)North/Wes
Page 107 and 108: Vertical-to-horizontal PGA ratio543
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Page 111 and 112: REFERENCESAagaard, B. T., J. F. Hal
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Page 119 and 120: TABLE 1Mean (μ LLH ) and standard
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ings and the Modified Takeda-Slip M
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high, but there were no buildings d
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REFERENCES▲▲Figure 8. Heavily d
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(A)(B)(C)(D)(E)▲▲Figure 1. A) M
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(A) (B) (C)▲ ▲ Figure 3. A) Typ
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(A) (B) (C)▲ ▲ Figure 4. A) Typ
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Case StudyKey ParametersTABLE 1Key
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▲ ▲ Figure 9. Representative bu
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Soil Liquefaction Effects in the Ce
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▲ ▲ Figure 2. Representative su
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Location of structures illustrated
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Shading indicates areaover which pr
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1.8 deg15 cmGround cracking due to
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30 cm17 cm30 cmFoundation beam▲
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Comparison of Liquefaction Features
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(A)(B)▲▲Figure 2. A) Simplified
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(A)Acceleration (Gal)6004002000-200
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(A)(B)▲▲Figure 7. Distribution
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(A)(B)▲▲Figure 10. Damage to a
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(A)(B)▲ ▲ Figure 14. A) Subside
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▲▲Figure 17. A trench in a resi
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Ambient Noise Measurements followin
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▲▲Figure 1. Location of the noi
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▲▲Figure 5. Site N20 showing HV
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▲▲Figure 8. Comparison between
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Use of DCP and SASW Tests to Evalua
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▲ ▲ Figure 2. Aerial image of C
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(A)(B)▲▲Figure 4. DCP test bein
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▲▲Figure 7. SASW setup at a sit
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where X ~ N(μ X , σ X 2 ) is shor
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Using the same critical layers as s
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Performance of Levees (Stopbanks) d
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▲▲Figure 3. Typical geometry an
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TABLE 1Damage severity categories (
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(A)(B)▲▲Figure 6. A) Large sand
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(A)(B)▲▲Figure 8. A) Representa
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each of the Waimakariri River and a
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▲ ▲ Figure 2. Horizontal peak g
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only minor damage, mostly to their
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(A)(C)(B)▲▲Figure 5. Ferrymead
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(A)(B)▲▲Figure 7. Damage to sou
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(A)(B)▲▲Figure 11. Settlement o
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(A)(C)(B)▲▲Figure 14. Railway B
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Events Reconnaissance (GEER) Associ
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New PublicationsCanGeoRefThe Americ
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Wednesday, 18 AprilTechnical Sessio
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Verification of a Spectral-Element
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EASTERN SECTIONRESEARCH LETTERSReas
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(A)70°N100°W 60°W70°N(B)100°E1
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Mongolia SCRThe presence or absence
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the small horizontal relative motio
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80°100°120°140°EXPLANATIONBorde
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Chang, K. H. (1997). Korean peninsu
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Wheeler, R. L. (2008). Paleoseismic
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A significant outcome of this study
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TABLE 1 (continued)Earthquakes for
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▲▲Figure 2. Earthquakes used in
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Meeting CalendarM E E T I N GC A L
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201 Plaza Professional Bldg. • El
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Seismological Research Letters (SRL
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Christa von Hillebrandt-Andrade, Pr
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