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The Character of Accelerations in the Mw 6.2Christchurch EarthquakeB. Fry, R. Benites and A. KaiserB. Fry, R. Benites and A. KaiserGNS ScienceINTRODUCTIONThe Canterbury earthquakes of 2010 and 2011 have producedsome of the strongest ground motions ever measuredin New Zealand. Many of the highest acceleration recordingsarose from seismic stations within the city of Christchurch(population ~377,000). A dense array of strong-motion seismometerswas in place prior to the mainshock of 4 September2010. Subsequent to the mainshock, numerous rapid responseaccelerometers were installed in the Canterbury Plains, BanksPeninsula, and in the city itself (Gledhill et al. 2011; Cochranet al. 2011). Many of the strongest aftershocks were recordedby this dense amalgamation of permanent and temporaryarrays and provide a detailed record of variable ground motionthroughout the region during the aftershock sequence.The most extreme ground motions were recorded duringthe Mw 6.2 earthquake of 22 February 2011 that struck a fewkilometers to the south of Christchurch (Beavan et al. 2011,this issue; Holden et al. 2011, this issue; Bannister et al. 2011,this issue), generating severe damage throughout the city. Infact, damage to the built environment and ground liquefactionwas much more widespread in the February event than in theSeptember Mw 7.1 mainshock (Kaiser et al. 2011). This event isone of the best-recorded shallow thrust earthquakes in the nearfield. Recorded peak ground acceleration (PGA) in Februaryexceeded 2 g near the epicenter and was greater than 0.6 g overmuch of the central and eastern suburbs (Figure 1). At thesenear-source stations, vertical accelerations were generally markedlyhigher than horizontal accelerations. The large accelerationscan be reasonably well explained by the combination ofHPSCCHHCPRPCHVSCCCCC▲ ▲ Figure 1. Vertical and horizontal acceleration vectors and their location relative to the February 22 epicenter (green star). Greenhorizontal lines on each waveform show the baseline.846 Seismological Research Letters Volume 82, Number 6 November/December 2011 doi: 10.1785/gssrl.82.6.846
▲ ▲ Figure 2. A) shows three-component accelerogram from station PRPC (see Figure 1 for location) for the 22 February event, scaledto units g. B) shows their corresponding spectra.the proximity of the February event to Christchurch and theeffects of strong source directivity. However, not all featurescan be explained by source effects alone. The dense near-sourcedata from the 22 February earthquake have provided us with avaluable opportunity to study the response of the shallow subsurfaceto extreme ground motions in very fine detail.The way ground responds to an earthquake is a result ofthe earthquake rupture process, the path that the waves takebetween the source and the surface, and the response of theshallow materials below the ground. We know that the topfew meters of the ground in Christchurch played an importantrole in the shaking. This role is evident in low-frequencysignals resulting from liquefaction. Many of the poorly consolidated,low shear-wave velocity soils liquefied at shallowdepths with less than 0.1 g peak horizontal accelerations andexperienced deep liquefaction at around 0.2 to 0.3 g accelerations.The influence of the shallow subsurface is also exhibitedby the existence of energetic high-frequency signals resultingfrom the interaction of the waves with both the water table andunconsolidated soils prior to liquefaction. A marked featureof the strong-motion seismograms recorded at several nearsourcesites in Christchurch during the earthquake sequence isthe much higher frequency content of the vertical componentcompared to the corresponding horizontal recordings (Figure2). We believe that this phenomenon is due to the presence ofa shallow water table dramatically attenuating the propagationof high-frequency shear waves. A rigorous numerical demonstrationof such water effect would require the calculation ofseismic wave propagation in layered media in which one of thelayers is a porous, elastic solid containing an incompressible,inviscous fluid (Biot 1956a, Biot 1956b). However, for thiswork, we simulate such an effect in an indirect way by model-Seismological Research Letters Volume 82, Number 6 November/December 2011 847
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The Character of Accelerations in the Mw 6.2Christchurch EarthquakeB. Fry, R. Benites and A. KaiserB. Fry, R. Benites and A. KaiserGNS ScienceINTRODUCTIONThe Canterbury earthquakes of 2010 and 2011 have producedsome of the strongest ground motions ever measuredin New Zealand. Many of the highest acceleration recordingsarose from seismic stations within the city of Christchurch(population ~377,000). A dense array of strong-motion seismometerswas in place prior to the mainshock of 4 September2010. Subsequent to the mainshock, numerous rapid responseaccelerometers were installed in the Canterbury Plains, BanksPeninsula, and in the city itself (Gledhill et al. 2011; Cochranet al. 2011). Many of the strongest aftershocks were recordedby this dense amalgamation of permanent and temporaryarrays and provide a detailed record of variable ground motionthroughout the region during the aftershock sequence.The most extreme ground motions were recorded duringthe Mw 6.2 earthquake of 22 February 2011 that struck a fewkilometers to the south of Christchurch (Beavan et al. 2011,this issue; Holden et al. 2011, this issue; Bannister et al. 2011,this issue), generating severe damage throughout the city. Infact, damage to the built environment and ground liquefactionwas much more widespread in the February event than in theSeptember Mw 7.1 mainshock (Kaiser et al. 2011). This event isone of the best-recorded shallow thrust earthquakes in the nearfield. Recorded peak ground acceleration (PGA) in Februaryexceeded 2 g near the epicenter and was greater than 0.6 g overmuch of the central and eastern suburbs (Figure 1). At thesenear-source stations, vertical accelerations were generally markedlyhigher than horizontal accelerations. The large accelerationscan be reasonably well explained by the combination ofHPSCCHHCPRPCHVSCCCCC▲ ▲ Figure 1. Vertical and horizontal acceleration vectors and their location relative to the February 22 epicenter (green star). Greenhorizontal lines on each waveform show the baseline.846 Seismological Research Letters Volume 82, Number 6 November/December 2011 doi: 10.1785/gssrl.82.6.846
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