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▲ ▲ Figure 2. Observed vs. predicted ground motions from McVerry at al. (2006) for the 22 February 2011, Mw 6.3 earthquake. In allpanels, the prediction is shown with stress drop scaling (green solid line) and without (gray solid line). For both, the 16th and 84th percentilemotions are also shown in the same color. (A) PGA—both raw (black circles) and processed (open triangles) PGA are shown;(B) 0.5-s spectral accelerations; (C) 1.0-s spectral accelerations; and (D) 2.0-s spectral accelerations.find average apparent stresses to be around 0.2–0.3 MPa.Shallow events worldwide have a spread from about 6 MPa to0.04 MPa, with an average of about 0.5 MPa. While the datafor shallow, strike-slip events is limited, strike-slip mechanismsnear subduction zones show much higher τ a ; these events havean average of around 3 MPa (Choy et al. 2001). Typically, highτ a is indicative of spatially small yet strong asperities yieldingshort rise times and strong, high-frequency waves in the nearfield. Pervasive Cretaceous-age high-angle EW-trending faultzones that are hundreds of kilometers long are present offshoreof Christchurch (Wood and Herzer 1993). It is likelythat these faults continue in the crust beneath the CanterburyPlains and are capable of being reactivated as strike-slip faults.In this case, the fault strength would likely come from healingof the fault over time. Another possible fault origin for theFebruary event is reactivation of faults generated during the12 Ma–6 Ma emplacement of the Banks Peninsula volcanicrocks to the south of the city (see Figure 1 of Bannister et al.2011, page 839 of this issue). The relative immaturity of faultsin this hypothesis would contribute to the observed high τ a . Itis also possible that the faults that were active in the Februaryand June events were cross-cut by volcanic intrusions. Even ifthe faults were originally weak, cross-cutting dikes would yieldhigh stresses when broken. Precise locations of aftershocksfall along the northern and western flanks of the extinct volcano(Bannister et al. 2011, page 839 of this issue) and subsequentlydo not discriminate these competing hypotheses forthe origin of fault strength.OBSERVED GROUND MOTIONSIf the τ a of the Canterbury earthquakes is larger than the averageτ a of earthquakes used in generating the New Zealandground motion prediction equation (GMPE) (McVerry et al.2006), measured ground motions should be larger than predictedones.Figure 2 shows plots of observed, maximum vector groundmotions from the 22 February 2011 earthquake comparedto predictions for Class D, deep or soft soil (Standards NewZealand, 2004) and for an oblique mechanism; this is thestandard GMPE used in the New Zealand National SeismicHazard Model (Stirling et al. 2002). Distances are calculatedusing the closest distance to the current estimated fault plane(Beavan et al. 2011, page 789 of this issue). For comparison,we have applied a stress drop scaling term as proposed byAtkinson and Boore (2006). Stress drop is typically estimatedSeismological Research Letters Volume 82, Number 6 November/December 2011 835