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stations less than 150 km from Christchurch, using the manuallypicked P- and S-arrival times. Cross-correlation andbispectrum (BS) (Du et al. 2004) methods were then used tocalculate the waveform-based differential times (WBDT) forall event-station pairs, after pre-filtering, following the techniqueof Du et al. (2004). These derived differential times wereweighted based on the quality of the arrival time measurements.Absolute travel times and the two types of differentialtimes (CBDT and WBDT) were then combined and simultaneouslyinverted using the approach of Zhang et al. (2009)in an iterative least-squares procedure that utilizes the LSQRminimization method (Paige and Saunders 1982).In the first stage of the relocation analysis, the best 1,008events were used primarily to refine the velocity model in thevicinity of Christchurch. These events were selected fromthose earthquakes recorded in the six weeks following theDarfield M w 7.1 earthquake, as well as from the three weeksimmediately following the February M w 6.2 earthquake, whennew temporary seismometer stations were deployed south andnorth of Christchurch. Initial event selection was based onthe number of phases and initial standard errors. The datasetformed for this first stage of inversion contained 22,062 absolutephase times and 271,910 catalog-based differential times.The initial 3D velocity model was based on the most recent versionof the 3D New Zealand velocity model (Eberhart-Phillipset al. 2010). We interpolated the New Zealand–wide velocitymodel to a denser rectilinear grid centered on Christchurchcity using Delaunay triangulation and then carried out a seriesof inversion runs using the tomoDDPS approach of Zhang etal. (2009), slowly decreasing the inversion node spacing as theevent and station density allowed, with appropriate smoothingand weighting constraints (Zhang et al. 2009). The densestrong-motion array across Christchurch city, and the highnumber of events, led to high ray path coverage immediatelysouth of the city, and enabled quite a fine node spacing to beused close to the aftershocks. The velocity model after thisfirst stage had a minimum grid spacing of 1.5 km along the x-and y-axes in the section of the volume nearest Christchurchcity, and vertical nodes at 1, 2, 3, 4, 5.5, 8, 12, 24, and 30km depth, fully encompassing the aftershock volume for theChristchurch M w 6.2 earthquake. We expect further modificationand improvement of this velocity model in the next fewmonths as further waveform data is collected and analyzed(Yoshihisa Iio, personal communication 2011). Detailed resolutionanalysis, for example using checkerboard approaches,has yet to be carried out. However, we expect that the resolutionwill be quite spatially variable, reflecting the station andevent distribution, with the best resolution just south and westof Christchurch city, where the ray path coverage is highest.In a subsequent second stage we located 4,660 events,all with more than eight phase arrival observations, using thetomoDDPS software of Zhang et al (2009). This dataset consistedof 43,980 absolute phase times and 534,281 differentialtimes. Relocation was carried out using the velocity modeldeveloped in the first phase described above. Final epicentersare shown in Figure 3, and details of the relocated hypocentersfor February and March 2011 are provided in supplementarytable S1.DISCUSSIONFigure 3 shows the epicenters of the relocated events, whileFigures 4 and 5 show projections of the events onto verticalplanes AA′ (Figure 4) and BB′ (Figure 5).A feature of the relocated aftershocks is that they do notclearly define the fault plane of the 22 February earthquakeas determined from the centroid moment tensor (Sibson etal. 2011, page 824 of this issue) and geodetically (Beavan etal. 2011, page 789 of this issue); the projection of the geodeticallypreferred fault plane is overlain on cross-section AA′(Figure 4). Some vertical alignments of the aftershocks are visible,although they are not directly on the inferred M w 6.2 faultplane. One of these alignments, for example at X = 6–7 km onFigure 4, may be associated with a secondary strike-slip faultseveral kms south of the M w 6.2 earthquake, which has alsobeen inferred geodetically (Beavan et al. 2011, page 789 ofthis issue).The refined velocity model highlights P-wave velocities(V p ) greater than 6 km/s beneath part of the Banks Peninsula,as shallow as 5–8 km depth (Figures 4 and 5), at the samedepth range as the M w 6.2 February earthquake and the subsequentaftershocks. These high velocities are indicative ofschist basement, consistent with the interpretation of Wood etal. (1989). Previous passive- and active-source seismic work inthe Canterbury region by Reyners and Cowan (1993) and VanAvendonk et al. (2004) has also found V p greater than 6.0 km/sat 5–10 km depth, but with strong spatial variation. Many ofthe aftershocks appear to be located close to, or just below, thetop of this high-velocity basement.A possible reason for the lack of correspondence betweenaftershocks and the inferred fault plane for the M w 6.2 eventis that there may have been very little post-seismic slip on thefault. This explanation would be consistent with the high stressdrop of the mainshock (Fry and Gerstenberger 2011, page833 of this issue), which implies high fault friction. Lack oflarge-scale post-seismic deformation carries implications forthe mechanisms and rate of static stress transfer, and ultimatelythe longevity and the potential of the sequence to produce temporallyclustered aftershocks. This would help to explain therelatively high energy release observed for the September M w7.1 mainshock and aftershocks. In this scenario, the lack ofcontinual, shallow viscous deformation would encourage highlevels of regional stress build up, followed by large events withrelatively long aftershock sequences.ACKNOWLEDGMENTSWe thank GeoNet for providing seismological data, StaceyMartin and GeoNet staff for assisting with phase picking,and Banks Peninsula farmers for hosting temporary seismometers.We used GMT (Wessel and Smith 1998) and ObsPy(Beyreuther et al. 2010) for some figures.842 Seismological Research Letters Volume 82, Number 6 November/December 2011
A’0 km 4 8−43.5°B’B−43.6°A172.6° 172.8°Mag 2Mag 3Mag 4A’−43.5°Mag 5B’B−43.6°A172.6° 172.8°▲ ▲ Figure 3. Top: Epicenters for events occurring between 1 September 2010 and 31 January 2011. The locations of cross-sections AA’(Figure 4) and BB’ (Figure 5) are marked. Bottom: Epicenters of events occurring between 1 February 2011 and 30 April 2011, inclusive.The February M w 6.2 earthquake is marked as a solid star.Seismological Research Letters Volume 82, Number 6 November/December 2011 843
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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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▲ ▲ Figure 8. Misfit parameters
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▲ ▲ Figure 10. Spatial variabil
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▲ ▲ Figure 12. Standard spectra
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Quigley, M., R. Van Dissen, P. Vill
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slip on a 59-degree striking fault
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▲▲Figure 4. Convergence of inve
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observations and other source studi
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-42. 5-43. 0-43. 5-44. 0-44. 5-43.2
Page 38 and 39: “Product CSK © ASI, (ItalianSpac
Page 40 and 41: TABLE 2Solutions for fault location
Page 42 and 43: -43.45(A)degrees N-43.50-43.552.52.
Page 44 and 45: is still a good fit to the horizont
Page 46 and 47: Coulomb Stress Change Sensitivity d
Page 48 and 49: mation takes on a larger strike-sli
Page 50 and 51: P 9.4267BLDU45P 20.1213CASY39P 2.62
Page 52 and 53: ERMJNUMAJOINUJHJ2CBIJMIDWJOWYHNBTPU
Page 54 and 55: (A)6.146.13(B)6.246.36Misfit6.156.1
Page 56 and 57: (A)(B)(C)(D)▲▲Figure 10. The co
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Page 62 and 63: −44˚00' −43˚00'4-Sep-2010Mw 7
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
Page 71 and 72: AlpineFaultHope Fault38 mm/yr0+ +-1
Page 73 and 74: σ 1dσ 3Nuσ 3CM w 7.1dw 6.2u70°M
Page 75 and 76: Right-lateral Faults(A) Range Front
Page 77 and 78: DISCUSSIONThe 2010-2011 Canterbury
Page 79 and 80: Large Apparent Stresses from the Ca
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
Page 93 and 94: ▲ ▲ Figure 2. A) shows three-co
Page 95 and 96: ▲ ▲ Figure 4. Vertical accelera
Page 97 and 98: 0.8PRPC Z0.40Normalized (Max PGA +
Page 99 and 100: Near-source Strong Ground MotionsOb
Page 101 and 102: (A)Magnitude, M w876542009 NZdataba
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
Page 109 and 110: (A)(B)Station:CCCCSolid:AvgHorizDas
Page 111 and 112: REFERENCESAagaard, B. T., J. F. Hal
Page 113 and 114: ▲ ▲ Figure 1. Shear-wave veloci
Page 115 and 116: Spectral Acceleration (0.3 s), (g)I
Page 117 and 118: Spectral Acceleration (3 s), (g)In[
Page 119 and 120: TABLE 1Mean (μ LLH ) and standard
Page 121 and 122: Strong Ground Motions and Damage Co
Page 123 and 124: ings and the Modified Takeda-Slip M
Page 125 and 126: high, but there were no buildings d
Page 127 and 128: REFERENCES▲▲Figure 8. Heavily d
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Page 131 and 132: (A) (B) (C)▲ ▲ Figure 3. A) Typ
Page 133 and 134: (A) (B) (C)▲ ▲ Figure 4. A) Typ
Page 135 and 136: Case StudyKey ParametersTABLE 1Key
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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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