Here - Stuff

TABLE 1Comparison of peak ground accelerations recorded at strong-motion sites near the city during the 2010 Darfield earthquakeand 2011 Christchurch earthquake. Data are from GeoNet strong motion FTP site. The unit of acceleration is g (1 g = 9.80 m/s 2 ).SeismicStationsSite Name2010 Darfield Earthquake 2011 Christchurch EarthquakeEp. Dist.(km) Vert Hor-1 Hor-2MaxHorEp. Dist.(km) Vert Hor-1 Hor-2HVSC Heathcote Valley Primary School 43 0.28 0.56 0.62 0.66 1 1.47 1.46 1.19 1.50LPCC Lyttelton Port Company 45 0.16 0.33 0.23 0.37 4 0.41 0.78 0.88 1.00CCCC Chch Cathedral College 38 0.16 0.23 0.20 0.24 6 0.69 0.48 0.37 0.49CMHS ChCh Cashmere High School 36 0.25 0.25 0.24 0.26 6 0.80 0.35 0.38 0.42PRPC Pages Road Pumping Station 41 0.31 0.20 0.23 0.23 6 1.63 0.66 0.59 0.73CHHC Christchurch Hospital 36 0.16 0.20 0.15 0.20 8 0.51 0.34 0.36 0.46REHS Christchurch Resthaven 37 0.21 0.24 0.25 0.33 8 0.53 0.71 0.37 0.73CBGS Christchurch Botanic Gardens 36 0.11 0.15 0.17 0.18 9 0.27 0.53 0.43 0.64HPSC Hulverstone Dr Pumping Station 43 0.13 0.16 0.11 0.16 9 0.86 0.15 0.24 0.25SHLC Shirley Library 39 0.12 0.18 0.18 0.19 9 0.50 0.31 0.34 0.34Note: Ep. Dist – Epicentral distance; Vert – vertical acceleration; Hor-1 and Hor-2 – horizontal components of acceleration;Max. Hor – calculated maximum resultant acceleration of horizontal components. Unit of acceleration is g (1 g = 980 cm/s 2 ). Source:GeoNet 2011.MaxHormotion FTP site, the maximum recorded acceleration was onthe order of 0.95 g near the earthquake epicenter (GeoNet2010); however, no serious damage was reported in the area.In the city of Christchurch, the recorded peak ground accelerations(PGA) were on the order of 0.15–0.30 g, as shown inTable 1. The seismic stations indicated in the table correspondto those shown in Figure 3. Figure 4 shows a typical accelerationrecord obtained during the 2010 earthquake. Note thatthe duration of significant shaking at Christchurch Hospital(CHHC), located at the southwest edge of the CBD, is on theorder of about 25–30 sec.2011 Christchurch EarthquakeThe 2011 Christchurch earthquake occurred more than fivemonths after the 2010 Darfield earthquake, with an epicenterlocated on an unmapped fault different from the Greendalefault. Yet it is considered an aftershock because it was causedby a fault rupture within the zone of aftershocks that followedthe September 2010 mainshock (National Hazards ResearchPlatform 2011). Because the M 6.3 aftershock was much closerto the Christchurch CBD than the M 7.1 mainshock, theground accelerations experienced in the CBD as a result of the2011 earthquake were three to four times greater than during2010 event (see Table 1); in the eastern suburbs, they wereabout five times greater. The vertical PGA recorded was 1.47g at Heathcote Valley primary school (about midway betweenthe CBD and the epicenter) while in the CBD the PGA was0.5–0.7 g and in the eastern suburbs the maximum recordedvertical PGA was 1.63 g (GeoNet 2011). A feature of this earthquakewas the very strong vertical component of PGA, whichin general was greater than the horizontal components. Figure4 also illustrates the time histories of acceleration recorded atChristchurch Hospital on 22 February 2011. Because of theshorter distance to the epicenter, the acceleration records inthis earthquake have a higher frequency and shorter durationtime, as well as larger amplitude, in comparison with the onesrecorded on 4 September 2010.COMPARISON OF OBSERVED LIQUEFACTIONDAMAGEAlthough structural failure of commercial buildings led to thegreatest casualties in the M 6.3 Christchurch earthquake, byfar the most significant damage to residential buildings andlifelines in both Canterbury earthquakes was the result of liquefactionand associated ground deformations. Liquefactionoccurred in areas that are known to have high potential forliquefaction—former river channels, abandoned meanders,wetlands, and ponds. Immediately following some of the largestaftershocks from the M 7.1 earthquake, liquefaction reoccurredin some of these areas. During the M 6.3 earthquake,liquefaction was more widespread and vents continued to surgeduring the aftershocks immediately following this event. Theimpact of sand boils and cracks caused by lateral spreading wasthat parts of the eastern suburbs were inundated with sand andsilt—in places there were layers of ejected soil that were manytens of centimeters thick.As mentioned earlier, two large aftershocks, measuringM 5.6 and M 6.3, shook the city on 13 June 2011 and causedextensive re-liquefaction in many parts of the city. Streets wereagain flooded with water and ejected sands, reminiscent ofwhat happened immediately after the 22 February 2011 earthquake.Such reoccurrence of liquefaction indicates that the soildeposits in the area were still loose even after the intense shakingthey had been subjected to over the last nine months.908 Seismological Research Letters Volume 82, Number 6 November/December 2011