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(A)Magnitude, M w876542009 NZdatabase04/09/201022/02/201110 0 10 1 10 2Distance, R rup(km)(B)Number of exceedances2009 NZdatabase04/09/201022/02/2011Total10 110 00.2 0.4 0.6 0.8 1 1.2 1.4102Peak ground acceleration, PGA (g)▲ ▲ Figure 3. Significance of the 22 February 2011 Christchurch and 4 September 2010 Darfield earthquakes in relation to previouslyrecorded ground motions in New Zealand: A) magnitude-distance distribution; B) exceeded values of peak ground acceleration.While New Zealand can be considered as a region of highseismicity in a global context, prior to the 4 September 2010Darfield and 22 February 2011 Christchurch earthquakesthere was a paucity of high-amplitude recorded strong groundmotions, primarily as a result of a sparse instrumentation networkbefore the commencement of GeoNet in 2001. Figure3 illustrates the magnitude-distance distribution of recordedground motions from active shallow crustal earthquakes up to2009 as complied by Zhao and Gerstenberger (2010). Also illustratedin Figure 3A are the ground motions recorded in both the4 September 2010 Darfield and 22 February 2011 Christchurchearthquakes. The significance of the recorded ground motionsin these two earthquakes is even more apparent if the groundmotions in Figure 3A are plotted in terms of their geometricmean horizontal PGA. Figure 3B illustrates the number ofground motions exceeding specific values of PGA. It can beseen that up to 2009, the maximum PGA recorded in NewZealand was 0.39 g, with only seven observed ground motionsexceeding 0.2 g PGA. Figure 3B also illustrates the exceedancevalues observed in the Darfield and Christchurch earthquakes.With the addition of these two events (not to mention recordsobtained from numerous significant aftershocks, which arenot discussed herein) it can be seen that ground motions of upto 1.41 g have now been recorded, with 12 observed groundmotions exceeding 0.4 g and 39 exceeding 0.2 g.OBSERVED NEAR-SOURCE GROUND MOTIONSTable 1 presents a summary of the ground motions in the widerChristchurch region that were recorded within a source-to-sitedistance of R rup = 20 km, including: station site class accordingto the New Zealand loading standard, New Zealand Standards1170.5 (2004); PGA, peak ground velocity (PGV); 5–95%significant duration, (D s5–95 ) (Bommer and Martinez-Pereira1999) for geometric mean horizontal component; and peakvertical ground acceleration (PGA v ).Figures 4–6 illustrate the spatial distribution of accelerationtime histories recorded at the aforementioned strongmotion stations in the form of fault-normal, fault-parallel,and vertical components, respectively. The aforementioned“trimmed” finite fault model of Beavan et al. (2011, page 789of this issue) is also shown. The following sections discuss variousaspects of the ground motions observed in Figures 4–6.Ground Motion on Rock and Soil SitesIn interpreting the observed ground motions in Figures 4–6, itis first worth noting that only the Lyttelton Port (LPCC) stationto the southeast of Christchurch is located on engineeringbedrock (i.e., site class B). Stations HVSC and LPOC locatednear the edge of the Port Hills rock outcrop are site class C,while all remaining stations are situated on the Christchurchsedimentary basin and are predominantly site class D (theexceptions being HPSC, NNBS, PRPC, and KPOC, which aresite class E). Unfortunately at present the site characterizationof strong motion stations in the Christchurch region, and NewZealand in general, is relatively poor with the above site classesdetermined from geological maps and direct surface inspection(N. Perrin, personal communication 2011), and details suchas P- and S-wave velocity, SPT, and CPT data not available.Clearly, obtaining such information is necessary for a rigorousanalysis of the observed ground motions, and is the focus ofimmediate studies. Nonetheless, a wealth of insight can still beobtained from inspection and analysis of the observed groundmotions.A direct comparison of the effect of soil and rock site canbe made by comparing the ground motions observed at LPCCand LPOC located at Lyttelton Port approximately 1 km apart.The LPCC instrument is located on engineering bedrock, andwhile detailed information of the site conditions at LPOC arepresently unavailable, it is said to be a relatively thin (~30 m)colluvium layer comprised primarily of silt and clay (J. Berrill,personal communication 2011). In addition to a comparison ofSeismological Research Letters Volume 82, Number 6 November/December 2011 855